Podcasts about galaxy clusters

We're experimenting and would love to hear from you!In this episode of ‘Discover Daily', we explore groundbreaking developments across technology and space. OpenAI unveils its first comprehensive rebrand, introducing a refined blossom logo, custom OpenAI Sans typeface, and a nature-inspired color palette under the leadership of their design team. We also delve into Apple's exciting ELEGNT project, a revolutionary lamp-like robotic companion prototype that could hit markets by 2027.Our main story focuses on an astronomical breakthrough that's reshaping our understanding of the universe. The Quipu superstructure, discovered by Dr. Hans Böhringer and his team at the Max Planck Institute, spans an astounding 1.3 billion light-years and contains 200 quadrillion solar masses. This cosmic giant is part of a newly identified group of five superstructures that collectively house nearly half of all known galaxy clusters in their observed region. The massive discovery challenges existing cosmological models and could revolutionize our understanding of dark matter, gravity, and universal structure formation. Named after the ancient Incan record-keeping system, Quipu's vast size and complex shape are forcing scientists to reconsider theories about how matter clusters at the largest scales, potentially leading to groundbreaking revisions in our understanding of cosmic evolution and structure formation.From Perplexity's Discover Feed: https://www.perplexity.ai/page/openai-rebrands-olNMmsxrR2e8pTl74GelVA https://www.perplexity.ai/page/apple-prototypes-tabletop-robo-B0ohlEMNRXei655eYqfigg https://www.perplexity.ai/page/largest-structure-in-universe-kX97crgVQuKyHnGt6dR4MQ Perplexity is the fastest and most powerful way to search the web. Perplexity crawls the web and curates the most relevant and up-to-date sources (from academic papers to Reddit threads) to create the perfect response to any question or topic you're interested in. Take the world's knowledge with you anywhere. Available on iOS and Android Join our growing Discord community for the latest updates and exclusive content. Follow us on: Instagram Threads X (Twitter) YouTube Linkedin

What can gamma rays tell us about supernovae and galaxy formation? Neil deGrasse Tyson and co-host Chuck Nice sit down with astrophysicist Tim Paglione to explore high-energy cosmic phenomena, gamma rays, and the extreme events that create them.NOTE: StarTalk+ Patrons can listen to this entire episode commercial-free here:https://startalkmedia.com/show/the-extreme-universe-with-tim-paglione/Thanks to our Patrons Alexander Storts, Chris Henderson, Micheal Mayo, Jose Lotzin, Rebecca Noland, Scientific Panda, Sander Bergheim, Aubrey Loftus, John Leon, Jaquelin Butkovic, Jesse McIntyre, Kelly Sheffield, Kaseim カセイム, Bradley Westbrook, Chris Rassette, Aquahood, BA_MPH_JD_PhD-aspirant, Ravenwingfeather, Kaity Sturgell, Norma Bazan, Mickey Brumfield, lamar Gibson, Bong Bong, Andrew Hayes, Billy Madison, Bruce Muller, parker martindale, James Pope, Carrie Williams, Robert Lester, Mike Bundy, and My Pug is a Bug for supporting us this week. Subscribe to SiriusXM Podcasts+ on Apple Podcasts to listen to new episodes ad-free and a whole week early.

From May 24, 2023. We live at a time when technological advances are allowing us to explore ideas faster than ever before. So today, we bring you lab results on ice that affect how we see the outer solar system, and observations of galaxies that affect our understanding of the universe's formation. We go from things smaller than a proton – which we just learned is 0.73 femtometers across -- to galaxy clusters 10s of millions of lightyears across. It's all tied together, and we'll tell you how. (This episode originally aired on television February 11, 2023.)   We've added a new way to donate to 365 Days of Astronomy to support editing, hosting, and production costs.  Just visit: https://www.patreon.com/365DaysOfAstronomy and donate as much as you can! Share the podcast with your friends and send the Patreon link to them too!  Every bit helps! Thank you! ------------------------------------ Do go visit http://www.redbubble.com/people/CosmoQuestX/shop for cool Astronomy Cast and CosmoQuest t-shirts, coffee mugs and other awesomeness! http://cosmoquest.org/Donate This show is made possible through your donations.  Thank you! (Haven't donated? It's not too late! Just click!) ------------------------------------ The 365 Days of Astronomy Podcast is produced by the Planetary Science Institute. http://www.psi.edu Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org.

We live at a time when technological advances are allowing us to explore ideas faster than ever before. So today, we bring you lab results on ice that affect how we see the outer solar system, and observations of galaxies that affect our understanding of the universe's formation. We go from things smaller than a proton – which we just learned is 0.73 femtometers across -- to galaxy clusters 10s of millions of lightyears across. It's all tied together, and we'll tell you how. (This episode originally aired on television February 11, 2023)

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Astronomers have captured a spectacular, ongoing collision between at least three galaxy clusters. Data from Chandra, XMM-Newton, and a trio of radio telescopes are helping astronomers sort out what is happening in this jumbled scene.

The black holes found at the centres of most large galaxies are now found to be fundamental to galactic formation and evolution. Until recently, however, little was understood about how these massive bodies affect the behaviours of their host galaxies and beyond. Through their research, Dr Stefi Baum and Dr Christopher O'Dea at the University of Manitoba have made important strides towards untangling the many mysteries involved in this intriguing astronomical problem.

Shattering and growth of cold clouds in galaxy clusters: the role of radiative cooling, magnetic fields and thermal conduction by Fred Jennings et al. on Wednesday 30 November In galaxy clusters, the hot intracluster medium (ICM) can develop a striking multi-phase structure around the brightest cluster galaxy. Much work has been done on understanding the origin of this central nebula, but less work has studied its eventual fate after the originally filamentary structure is broken into individual cold clumps. In this paper we perform a suite of 30 (magneto-)hydrodynamical simulations of kpc-scale cold clouds with typical parameters as found by galaxy cluster simulations, to understand whether clouds are mixed back into the hot ICM or can persist. We investigate the effects of radiative cooling, small-scale heating, magnetic fields, and (anisotropic) thermal conduction on the long-term evolution of clouds. We find that filament fragments cool on timescales shorter than the crushing timescale, fall out of pressure equilibrium with the hot medium, and shatter, forming smaller clumplets. These act as nucleation sites for further condensation, and mixing via Kelvin-Helmholtz instability, causing cold gas mass to double within 75 Myr. Cloud growth depends on density, as well as on local heating processes, which determine whether clouds undergo ablation- or shattering-driven evolution. Magnetic fields slow down but don't prevent cloud growth, with the evolution of both cold and warm phase sensitive to the field topology. Counter-intuitively, anisotropic thermal conduction increases the cold gas growth rate compared to non-conductive clouds, leading to larger amounts of warm phase as well. We conclude that dense clumps on scales of $500$ pc or more cannot be ignored when studying the long-term cooling flow evolution of galaxy clusters. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.09183v2

Shattering and growth of cold clouds in galaxy clusters: the role of radiative cooling, magnetic fields and thermal conduction by Fred Jennings et al. on Wednesday 30 November In galaxy clusters, the hot intracluster medium (ICM) can develop a striking multi-phase structure around the brightest cluster galaxy. Much work has been done on understanding the origin of this central nebula, but less work has studied its eventual fate after the originally filamentary structure is broken into individual cold clumps. In this paper we perform a suite of 30 (magneto-)hydrodynamical simulations of kpc-scale cold clouds with typical parameters as found by galaxy cluster simulations, to understand whether clouds are mixed back into the hot ICM or can persist. We investigate the effects of radiative cooling, small-scale heating, magnetic fields, and (anisotropic) thermal conduction on the long-term evolution of clouds. We find that filament fragments cool on timescales shorter than the crushing timescale, fall out of pressure equilibrium with the hot medium, and shatter, forming smaller clumplets. These act as nucleation sites for further condensation, and mixing via Kelvin-Helmholtz instability, causing cold gas mass to double within 75 Myr. Cloud growth depends on density, as well as on local heating processes, which determine whether clouds undergo ablation- or shattering-driven evolution. Magnetic fields slow down but don't prevent cloud growth, with the evolution of both cold and warm phase sensitive to the field topology. Counter-intuitively, anisotropic thermal conduction increases the cold gas growth rate compared to non-conductive clouds, leading to larger amounts of warm phase as well. We conclude that dense clumps on scales of $500$ pc or more cannot be ignored when studying the long-term cooling flow evolution of galaxy clusters. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.09183v2

An interview with Dr. Yvette Perrott, Senior Lecturer at the School of Chemical and Physical Sciences at University of Victoria Wellington.  Yvette has degrees in BA, BSc and BSc( with Hons)majoring in Physics, Italian and Spanish. She completed her PhD in the Cavendish Astrophysics Group in 2013 on a Rutherford PhD Scholarship and was awarded a Trinity College Junior Research Fellowship in the same year. She was also awarded a Rutherford Discovery Fellowship in 2017. Her area of research is on galaxy clusters as cosmological probes for the origins of the universe.In this interview we will learn about how her research in galaxy clusters could unlock the origins of the universe.  She also talks about a new multidisciplinary space undergraduate degree programme at University of Victoria starting up next year.Resources:Open Stax astronomy - extensive repository of astronomy topicsNASA Skyview - SkyView is a Virtual Observatory on the Net generating images of any part of the sky at wavelengths in all regimes from Radio to Gamma-Ray.Hosted by: Emeline Paat-Dahlstrom, Co-Founder and CEO, SpaceBaseMusic: reCreation by airtone (c) copyright 2019 Licensed under a Creative Commons (3.0)If you like our work, please consider donating to SpaceBase through The Gift Trust or RSF Social Finance (for US charitable donations) and indicate "SpaceBase" gift account. Or be a SpaceBase Patreon sponsor!Support the showSupport the show

Euclid preparation XXVII Covariance model validation for the 2-point correlation function of galaxy clusters by Euclid Collaboration et al. on Friday 25 November Aims. We validate a semi-analytical model for the covariance of real-space 2-point correlation function of galaxy clusters. Methods. Using 1000 PINOCCHIO light cones mimicking the expected Euclid sample of galaxy clusters, we calibrate a simple model to accurately describe the clustering covariance. Then, we use such a model to quantify the likelihood analysis response to variations of the covariance, and investigate the impact of a cosmology-dependent matrix at the level of statistics expected for the Euclid survey of galaxy clusters. Results. We find that a Gaussian model with Poissonian shot-noise does not correctly predict the covariance of the 2-point correlation function of galaxy clusters. By introducing few additional parameters fitted from simulations, the proposed model reproduces the numerical covariance with 10 per cent accuracy, with differences of about 5 per cent on the figure of merit of the cosmological parameters $Omega_{rm m}$ and $sigma_8$. Also, we find that the cosmology-dependence of the covariance adds valuable information that is not contained in the mean value, significantly improving the constraining power of cluster clustering. Finally, we find that the cosmological figure of merit can be further improved by taking mass binning into account. Our results have significant implications for the derivation of cosmological constraints from the 2-point clustering statistics of the Euclid survey of galaxy clusters. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12965v1

Euclid preparation XXVII Covariance model validation for the 2-point correlation function of galaxy clusters by Euclid Collaboration et al. on Thursday 24 November Aims. We validate a semi-analytical model for the covariance of real-space 2-point correlation function of galaxy clusters. Methods. Using 1000 PINOCCHIO light cones mimicking the expected Euclid sample of galaxy clusters, we calibrate a simple model to accurately describe the clustering covariance. Then, we use such a model to quantify the likelihood analysis response to variations of the covariance, and investigate the impact of a cosmology-dependent matrix at the level of statistics expected for the Euclid survey of galaxy clusters. Results. We find that a Gaussian model with Poissonian shot-noise does not correctly predict the covariance of the 2-point correlation function of galaxy clusters. By introducing few additional parameters fitted from simulations, the proposed model reproduces the numerical covariance with 10 per cent accuracy, with differences of about 5 per cent on the figure of merit of the cosmological parameters $Omega_{rm m}$ and $sigma_8$. Also, we find that the cosmology-dependence of the covariance adds valuable information that is not contained in the mean value, significantly improving the constraining power of cluster clustering. Finally, we find that the cosmological figure of merit can be further improved by taking mass binning into account. Our results have significant implications for the derivation of cosmological constraints from the 2-point clustering statistics of the Euclid survey of galaxy clusters. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12965v1

The spatially resolved view of star formation in galaxy clusters by Bianca M. Poggianti et al. on Thursday 24 November Integral field spectroscopic studies of galaxies in dense environments, such as clusters and groups of galaxies, have provided new insights for understanding how star formation proceeds, and quenches. I present the spatially resolved view of the star formation activity and its link with the multiphase gas in cluster galaxies based on MUSE and multi-wavelength data of the GASP survey. I discuss the link among the different scales (i.e. the link between the spatially resolved and the global star formation rate-stellar mass relation), the spatially resolved signatures and the quenching histories of jellyfish (progenitors) and post-starburst (descendants) galaxies in clusters. Finally, I discuss the multi-wavelength view of star-forming clumps both in galaxy disks and in the tails of stripped gas. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12297v1

Hydrostatic Mass Profiles of Galaxy Clusters in the eROSITA Survey by Dominik Scheck et al. on Wednesday 23 November To assume hydrostatic equilibrium between the intracluster medium and the gravitational potential of galaxy clusters is an extensively used method to investigate their total masses. We want to test hydrostatic masses obtained with an observational code in the context of the SRG/eROSITA survey. We use the hydrostatic modeling code MBProj2 to fit surface-brightness profiles to simulated clusters with idealized properties as well as to a sample of 93 clusters taken from the Magneticum Pathfinder simulations. We investigate the latter under the assumption of idealized observational conditions and also for realistic eROSITA data quality. The comparison of the fitted cumulative total mass profiles and the true mass profiles provided by the simulations allows to gain knowledge about the reliability of our approach. Furthermore, we use the true profiles for gas density and pressure to compute hydrostatic mass profiles based on theory for every cluster. For an idealized cluster that was simulated to fulfill perfect hydrostatic equilibrium, we find that the cumulative total mass at the true $r_{500}$ and $r_{200}$ can be reproduced with deviations of less than 7%. For the clusters from the Magneticum Pathfinder simulations under idealized observational conditions, the median values of the fitted cumulative total masses at the true $r_{500}$ and $r_{200}$ are in agreement with our expectations, taking into account the hydrostatic mass bias. Nevertheless, we find a tendency towards a too high steepness of the cumulative total mass profiles in the outskirts. For realistic eROSITA data quality, this steepness problem intensifies for clusters with high redshifts and thus leads to too high cumulative total masses at $r_{200}$. For the hydrostatic masses based on the true profiles known from the simulations, we find a good agreement with our expectations concerning the hydrostatic mass. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12146v1

The spatially resolved view of star formation in galaxy clusters by Bianca M. Poggianti et al. on Wednesday 23 November Integral field spectroscopic studies of galaxies in dense environments, such as clusters and groups of galaxies, have provided new insights for understanding how star formation proceeds, and quenches. I present the spatially resolved view of the star formation activity and its link with the multiphase gas in cluster galaxies based on MUSE and multi-wavelength data of the GASP survey. I discuss the link among the different scales (i.e. the link between the spatially resolved and the global star formation rate-stellar mass relation), the spatially resolved signatures and the quenching histories of jellyfish (progenitors) and post-starburst (descendants) galaxies in clusters. Finally, I discuss the multi-wavelength view of star-forming clumps both in galaxy disks and in the tails of stripped gas. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12297v1

The spatially resolved view of star formation in galaxy clusters by Bianca M. Poggianti et al. on Wednesday 23 November Integral field spectroscopic studies of galaxies in dense environments, such as clusters and groups of galaxies, have provided new insights for understanding how star formation proceeds, and quenches. I present the spatially resolved view of the star formation activity and its link with the multiphase gas in cluster galaxies based on MUSE and multi-wavelength data of the GASP survey. I discuss the link among the different scales (i.e. the link between the spatially resolved and the global star formation rate-stellar mass relation), the spatially resolved signatures and the quenching histories of jellyfish (progenitors) and post-starburst (descendants) galaxies in clusters. Finally, I discuss the multi-wavelength view of star-forming clumps both in galaxy disks and in the tails of stripped gas. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12297v1

Hydrostatic Mass Profiles of Galaxy Clusters in the eROSITA Survey by Dominik Scheck et al. on Tuesday 22 November To assume hydrostatic equilibrium between the intracluster medium and the gravitational potential of galaxy clusters is an extensively used method to investigate their total masses. We want to test hydrostatic masses obtained with an observational code in the context of the SRG/eROSITA survey. We use the hydrostatic modeling code MBProj2 to fit surface-brightness profiles to simulated clusters with idealized properties as well as to a sample of 93 clusters taken from the Magneticum Pathfinder simulations. We investigate the latter under the assumption of idealized observational conditions and also for realistic eROSITA data quality. The comparison of the fitted cumulative total mass profiles and the true mass profiles provided by the simulations allows to gain knowledge about the reliability of our approach. Furthermore, we use the true profiles for gas density and pressure to compute hydrostatic mass profiles based on theory for every cluster. For an idealized cluster that was simulated to fulfill perfect hydrostatic equilibrium, we find that the cumulative total mass at the true $r_{500}$ and $r_{200}$ can be reproduced with deviations of less than 7%. For the clusters from the Magneticum Pathfinder simulations under idealized observational conditions, the median values of the fitted cumulative total masses at the true $r_{500}$ and $r_{200}$ are in agreement with our expectations, taking into account the hydrostatic mass bias. Nevertheless, we find a tendency towards a too high steepness of the cumulative total mass profiles in the outskirts. For realistic eROSITA data quality, this steepness problem intensifies for clusters with high redshifts and thus leads to too high cumulative total masses at $r_{200}$. For the hydrostatic masses based on the true profiles known from the simulations, we find a good agreement with our expectations concerning the hydrostatic mass. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.12146v1

i cm z , a semi-analytic model for the thermodynamic properties in galaxy clusters: calibrations with mass and redshift, and implication for the hydrostatic bias by S. Ettori et al. on Monday 21 November In the self-similar scenario for galaxy cluster formation and evolution, the thermodynamic properties of the X-ray emitting plasma can be predicted in their dependencies on the halo mass and redshift only. However, several departures from this simple self-similar scenario have been observed. We show how our semi-analytic model $i(cm)z$, which modifies the self-similar predictions through two temperature-dependent quantities, the gas mass fraction $f_g=f_0 T^{f_1} E_z^{f_z}$ and the temperature variation $f_T=t_0 T^{t_1} E_z^{t_z}$, can be calibrated to incorporate the mass and redshift dependencies. We used a published set of 17 scaling relations to constrain the parameters of the model. We were subsequently able to make predictions as to the slope of any observed scaling relation within a few percent of the central value and about one $sigma$ of the nominal error. Contextually, the evolution of these scaling laws was also determined, with predictions within $1.5 sigma$ and within 10 percent of the observational constraints. Relying on this calibration, we have also evaluated the consistency of the predictions on the radial profiles with some observational datasets. For a sample of high-quality data (X-COP), we were able to constrain a further parameter of the model, the hydrostatic bias $b$. By calibrating the model, we have determined that (i) the slopes of the temperature dependence are $f_1=0.403 (pm0.009)$ and $t_1=0.144 (pm0.017)$; and that (ii) the dependence upon $E_z$ are constrained to be $f_z=-0.004 (pm 0.023)$ and $t_z=0.349 (pm 0.059)$. These values permit one to estimate directly how the normalizations of a given quantity $Q_{Delta}$ changes as a function of the mass (or temperature) and redshift halo in the form $Q_{Delta} sim M^{a_M} E_z^{a_z} sim T^{a_T} E_z^{a_{Tz}}$, in very good agreement with the current observational constraints. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.03082v2

Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters by Kuan-Chou Hou et al. on Tuesday 18 October Galaxy clusters accrete mass through large scale, strong, structure-formation shocks. Such a virial shock is thought to deposit fractions $xi_e$ and $xi_B$ of the thermal energy in cosmic-ray electrons (CREs) and magnetic fields, respectively, thus generating a leptonic virial ring. However, the expected synchrotron signal was not convincingly established until now. We stack low-frequency radio data from the OVRO-LWA around the 44 most massive, high latitude, extended MCXC clusters, enhancing the ring sensitivity by rescaling clusters to their characteristic, $R_{500}$ radii. Both high (73 MHz) and co-added low ($36text{--}68text{ MHz}$) frequency channels separately indicate a significant ($4text{--}5sigma$) excess peaked at $(2.4 text{--} 2.6) R_{500}$, coincident with a previously stacked Fermi $gamma$-ray signal interpreted as inverse-Compton emission from virial-shock CREs. The stacked radio signal is well fit (TS-test: $4$--$6sigma$ at high frequency, $4$--$8sigma$ at low frequencies, and $8$--$10sigma$ joint) by virial-shock synchrotron emission from the more massive clusters, with $dot{m}xi_exi_Bsimeq (1text{--}4)times 10^{-4}$, where $dot{m}equiv dot{M}/(MH)$ is the dimensionless accretion rate for a cluster of mass $M$ and a Hubble constant $H$. The inferred CRE spectral index is flat, $p simeq 2.0 pm 0.2$, consistent with acceleration in a strong shock. Assuming equipartition or using $dot{m}xi_esim0.6%$ inferred from the Fermi signal yields $xi_Bsimeq (2text{--}9)%$, corresponding to $B simeq (0.1text{--}0.3)~mutext{G}$ magnetic fields downstream of typical virial shocks. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2210.09317v1

Low redshift calibration of the Amati relation using galaxy clusters by Gowri Govindaraj et al. on Sunday 16 October In this work, we use angular diameter distances of 38 galaxy clusters with joint X-ray/SZE observation to circumvent the circularity problem in the Amati relation for Gamma-ray Bursts (GRBs). Assuming the validity of cosmic-distance duality relation, we obtain the luminosity distance from the cluster angular diameter distance and use that to calculate the isotropic equivalent energy of two different GRB datasets, after restricting the GRB redshift range to $z

Improved strong lensing modelling of galaxy clusters using the Fundamental Plane: detailed mapping of the baryonic and dark matter mass distribution of Abell S1063 by Giovanni Granata. on Monday 26 September Strong gravitational lensing (SL) has emerged as a very accurate probe of the mass distribution of cluster- and galaxy-scale dark matter (DM) haloes in the inner regions of galaxy clusters. The derived properties of DM haloes can be compared to the predictions of high-resolution cosmological simulations, providing us with a test of the Standard Cosmological Model. The usual choice of simple power-law scaling relations to link the total mass of members with their luminosity is one of the possible inherent systematics within SL models of galaxy clusters, and thus on the derived cluster masses. Using new information on their structural parameters (from HST imaging) and kinematics (from MUSE data), we build the Fundamental Plane (FP) for the early-type galaxies of the cluster Abell S1063. We take advantage of the calibrated FP to develop an improved SL model of the total mass of the cluster core. The new method allows us to obtain more accurate and complex relations between the observables describing cluster members, and to completely fix their mass from their observed magnitudes and effective radii. Compared to the power-law approach, we find a different relation between the mass and the velocity dispersion of members, which shows a significant scatter. Thanks to a new estimate of the stellar mass of the cluster members from HST data, we measure the cumulative projected mass profiles out to a radius of 350 kpc, for all baryonic and DM components of the cluster. Finally, we compare the physical properties of the sub-haloes in our model and those predicted by high-resolution hydrodynamical simulations. We obtain compatible results in terms of the stellar-over-total mass fraction of the members. On the other hand, we confirm the recently reported discrepancy in terms of sub-halo compactness: at a fixed total mass value, simulated sub-haloes are less compact than what our SL model predicts. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.11776v1

A Semi-Analytical Model for the Formation and Evolution of Radio Relics in Galaxy Clusters by Yihao Zhou et al. on Wednesday 21 September Radio relics are Mpc-sized synchrotron sources located in the peripheral regions of galaxy clusters. Models based on the diffuse shock acceleration (DSA) scenario have been widely accepted to explain the formation of radio relics. However, a critical challenge to these models is that most observed shocks seem too weak to generate detectable emission, unless fossil electrons, a population of mildly energetic electrons that have been accelerated previously, are included in the models. To address this issue, we present a new semi-analytical model to describe the formation and evolution of radio relics by incorporating fossil relativistic electrons into DSA theory, which is constrained by a sample of 14 observed relics, and employ the Press-Schechter formalism to simulate the relics in a $20^{circ} times 20^{circ}$ sky field at 50, 158, and 1400 MHz, respectively. Results show that fossil electrons contribute significantly to the radio emission, which can generate radiation four orders of magnitude brighter than that solely produced by thermal electrons at 158 MHz, and the power distribution of our simulated radio relic catalog can reconcile the observed $P_{1400}-M_{mathrm{vir}}$ relation. We predict that $7.1%$ clusters with $M_{mathrm{vir}} > 1.2times 10^{14},mathrm{M}_{odot}$ would host relics at 158 MHz, which is consistent with the result of $10 pm 6%$ given by the LoTSS DR2. It is also found that radio relics are expected to cause severe foreground contamination in future EoR experiments, similar to that of radio halos. The possibility of AGN providing seed fossil relativistic electrons is evaluated by calculating the number of radio-loud AGNs that a shock is expected to encounter during its propagation. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.09951v1

The Hateful Eight: Connecting Massive Substructures in Galaxy Clusters like Abell 2744 to their Dynamical Assembly State using the Magneticum Simulations by Lucas C. Kimmig et al. on Wednesday 21 September Substructures are known to be good tracers for the dynamical states and recent accretion histories of the most massive collapsed structures in the Universe, galaxy clusters. Observations find extremely massive substructures in some clusters, especially Abell 2744, which are potentially in tension with the $Lambda$CDM paradigm since they are not found in simulations directly. However, the methods to measure substructure masses strongly differ between observations and simulations. Using the fully hydrodynamical cosmological simulation suite Magneticum Pathfinder we develop a method to measure substructure masses in projection from simulations, similar to the observational approach. We identify a simulated Abell 2744 counterpart that not only has eight substructures of similar mass fractions but also exhibits similar features in the hot gas component. This cluster formed only recently through a major merger together with at least 6 massive minor merger events since z=1, where prior the most massive component had a mass of less than $1times10^{14}M_odot$. We show that the mass fraction of all substructures and of the eighth substructure separately are excellent tracers for the dynamical state and assembly history for all galaxy cluster mass ranges, with high fractions indicating merger events within the last 2Gyr. Finally, we demonstrate that the differences between subhalo masses measured directly from simulations as bound and those measured in projection are due to methodology, with the latter generally 2-3 times larger than the former. We provide a predictor function to estimate projected substructure masses from SubFind masses for future comparison studies between simulations and observations. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.09916v1

Galaxy clusters at z~1 imaged by ALMA with the Sunyaev-Zel'dovich effect by T. Kitayama et al. on Tuesday 20 September We present high angular-resolution measurements of the thermal Sunyaev-Zel'dovich effect (SZE) toward two galaxy clusters, RCS J2319+0038 at z=0.9 and HSC J0947-0119 at z=1.1, by the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 3. They are supplemented with available Chandra X-ray data, optical data taken by Hyper Suprime-Cam on Subaru, and millimeter-wave SZE data from the Atacama Cosmology Telescope. Taking into account departures from spherical symmetry, we have reconstructed non-parametrically the inner pressure profile of two clusters as well as electron temperature and density profiles for RCS J2319+0038. This is one of the first such measurements for an individual cluster at $z gtrsim 0.9$. We find that the inner pressure profile of both clusters is much shallower than that of local cool-core clusters. Our results consistently suggest that RCS J2319+0038 hosts a weak cool core, where radiative cooling is less significant than in local cool cores. On the other hand, HSC J0947-0119 exhibits an even shallower pressure profile than RCS J2319+0038 and is more likely a non-cool-core cluster. The SZE centroid position is offset by more than 140 $h_{70}^{-1}$kpc from the peaks of galaxy distribution in HSC J0947-0119, suggesting a stronger influence of mergers in this cluster. We conclude that these distant clusters are at a very early stage of developing the cool cores typically found in clusters at lower redshifts. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.09503v1

Quasi-spiral solution to the mixed intracluster medium and the universal entropy profile of galaxy clusters by Uri Keshet et al. on Tuesday 20 September Well-resolved galaxy clusters often show a large-scale quasi-spiral structure in deprojected density $rho$ and temperature $T$ fields, delineated by a tangential discontinuity known as a cold front, superimposed on a universal radial entropy profile with a linear $K(r)propto Trho^{-2/3}propto r$ adiabat. We show that a spiral structure provides a natural quasi-stationary solution for the mixed intracluster medium (ICM), introducing a modest pressure spiral that confines the locally buoyant or heavy plasma phases. The solution persists in the presence of uniform or differential rotation, and can accommodate both an inflow and an outflow. Hydrodynamic adiabatic simulations with perturbations that deposit angular momentum and mix the plasma thus asymptote to a self-similar spiral structure. We find similar spirals in Eulerian and Lagrangian simulations of 2D and 3D, merger and offset, clusters. The discontinuity surface is given in spherical coordinates ${r,theta,phi}$ by $phipropto Phi(r)$, where $Phi$ is the gravitational potential, combining a trailing spiral in the equatorial ($theta=pi/2$) plane and semicircles perpendicular to the plane, in resemblance of a snail shell. A local convective instability can develop between spiral windings, driving a modified global instability in sublinear $K(r)$ regions; evolved spirals thus imprint the observed $Kpropto r$ onto the ICM even after they dissipate. The spiral structure brings hot and cold phases to close proximity, suggesting that the observed fast outflows could sustain the structure even in the presence of radiative cooling. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.09259v1

CHEX-MATE: pressure profiles of 6 galaxy clusters as seen by SPT and Planck by Filippo Oppizzi et al. on Tuesday 20 September Pressure profiles are sensitive probes of the thermodynamic conditions and the internal structure of galaxy clusters. The intra-cluster gas resides in hydrostatic equilibrium within the Dark Matter gravitational potential. However, this equilibrium may be perturbed, e.g. as a consequence of thermal energy losses, feedback and non-thermal pressure supports. Accurate measures of the gas pressure over the cosmic times are crucial to constrain the cluster evolution as well as the contribution of astrophysical processes. In this work we presented a novel algorithm to derive the pressure profiles of galaxy clusters from the Sunyaev-Zeldovich (SZ) signal measured on a combination of Planck and South Pole Telescope (SPT) observations. The synergy of the two instruments made it possible to track the profiles on a wide range of spatial scales. We exploited the sensitivity to the larger scales of the Planck High-Frequency Instrument to observe the faint peripheries, and the higher spatial resolution of SPT to solve the innermost regions. We developed a two-step pipeline to take advantage of the specifications of each instrument. We first performed a component separation on the two data-sets separately to remove the background (CMB) and foreground (galactic emission) contaminants. Then we jointly fitted a parametric pressure profile model on a combination of Planck and SPT data. We validated our technique on a sample of 6 CHEX-MATE clusters detected by SPT. We compare the results of the SZ analysis with profiles derived from X-ray observations with XMM-Newton. We find an excellent agreement between these two independent probes of the gas pressure structure. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.09601v1

Searching for High-Energy Neutrino Emission from Galaxy Clusters with IceCube by R. Abbasi et al. on Monday 19 September Galaxy clusters have the potential to accelerate cosmic rays (CRs) to ultra-high energies via accretion shocks or embedded CR acceleration sites. CRs with energies below the Hillas condition will be confined within the cluster and will eventually interact with the intracluster medium (ICM) gas to produce secondary neutrinos and $gamma$ rays. Using 9.5 years of muon-neutrino track events from the IceCube Neutrino Observatory, we report the results of a stacking analysis of 1094 galaxy clusters, with masses $gtrsim 10^{14}$ (textup{M}_odot) and redshifts between 0.01 and $sim$1, detected by the {it Planck} mission via the Sunyaev-Zeldovich (SZ) effect. We find no evidence for significant neutrino emission and report upper limits on the cumulative unresolved neutrino flux from massive galaxy clusters after accounting for the completeness of the catalog up to a redshift of 2, assuming three different weighting scenarios for the stacking and three different power-law spectra. Weighting the sources according to mass and distance, we set upper limits at $90%$ confidence level that constrain the flux of neutrinos from massive galaxy clusters ($gtrsim 10^{14}$ (textup{M}_odot)) to be no more than $4.6%$ of the diffuse IceCube observations at 100~TeV, assuming an unbroken $E^{-2.5}$ power-law spectrum. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2206.02054v2

Simulation view of galaxy clusters with low X-ray surface brightness by Antonio Ragagnin et al. on Sunday 18 September X-ray selected samples are known to miss galaxy clusters that are gas poor and have a low surface brightness. This is different for the optically selected samples such as the X-ray Unbiased Selected Sample (XUCS). We characterise the origin of galaxy clusters that are gas poor and have a low surface-brightness by studying covariances between various cluster properties at fixed mass using hydrodynamic cosmological simulations. We extracted approx. 1800 galaxy clusters from a high-resolution Magneticum hydrodynamic cosmological simulation and computed covariances at fixed mass of the following properties: core-excised X-ray luminosity, gas fraction, hot gas temperature, formation redshift, concentration, galaxy richness, fossilness parameter, and stellar mass of the bright central galaxy. We also compared the correlation between concentration and gas fractions in non-radiative simulations, and we followed the trajectories of particles inside galaxy clusters to assess the role of AGN depletion on the gas fraction. In simulations and in observational data, differences in surface brightness are related to differences in gas fraction. Simulations show that the gas fraction strongly correlates with assembly time, in the sense that older clusters are gas poor. Clusters that formed earlier have lower gas fractions because the feedback of the active galactic nucleus ejected a significant amount of gas from the halo. When the X-ray luminosity is corrected for the gas fraction, it shows little or no covariance with other quantities. Older galaxy clusters tend to be gas poor and possess a low X-ray surface brightness because the feedback mechanism removes a significant fraction of gas from these objects. Moreover, we found that most of the $L_X$ covariance with the other quantities is explained by differences in the gas fraction. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2208.02827v2

New metrics to probe the dynamical state of galaxy clusters by Giulia Cerini et al. on Thursday 15 September We present new diagnostic metrics to probe the dynamical state of galaxy clusters. These novel metrics rely on the computation of the power spectra of the matter and gas distributions and their cross-correlation derived from cluster observations. This analysis permits us to cross-correlate the fluctuations in the matter distribution, inferred from high-resolution lensing mass maps derived from Hubble Space Telescope (HST) data, with those derived from the emitted X-ray surface brightness distribution of the hot Intra-Cluster medium (ICM) from the Chandra X-ray Observatory (CXO). These methodological tools allow us to quantify with unprecedented resolution the coherence with which the gas traces the mass and interrogate the assumption that the gas is in hydro-static equilibrium with the underlying gravitational potential. We characterize departures from equilibrium as a function of scale with a new gas-mass coherence parameter. The efficacy of these metrics is demonstrated by applying them to the analysis of two representative clusters known to be in different dynamical states: the massive merging cluster Abell 2744, from the HST Frontier Fields (HSTFF) sample, and the dynamically relaxed cluster Abell 383, from the Cluster Lensing And Supernova Survey with Hubble (CLASH) sample. Using lensing mass maps in combination with archival Chandra data, and simulated cluster analogs available from the OMEGA500 suite, we quantify the fluctuations in the mass and X-ray surface brightness and show that new insights into the dynamical state of the clusters can be obtained from our gas-mass coherence analysis. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.06831v1

X-IFU Athena view of the most distant galaxy clusters in the Universe by Florent Castellani et al. on Wednesday 07 September The X-ray Integral Field Unit (X-IFU) on-board the second large ESA mission ''Athena'' will be a high spatial (5'') and spectral (2.5eV) resolution X-ray imaging spectrometer, operating in the 0.2-12 keV energy band. It will address the science question of the assembly and evolution through cosmic time of the largest halos of matter in the Universe, groups and clusters of galaxies. To this end, we present an on-going feasibility study to demonstrate the X-IFU capabilities to unveil the physics of massive halos at their epoch of formation. Starting from a distant (z=2) group of galaxies ($M_{500} = 7cdot 10^{13} M_odot/h$) extracted from the HYDRANGEA cosmological and hydrodynamical numerical simulations, we perform an end-to-end simulation of X-IFU observations. From the reconstruction of the global, 1D and 2D quantities, we plan to investigate the various X-IFU science cases for clusters of galaxies, such as the chemical enrichment of the intra-cluster medium (ICM), the dynamical assembly of groups and clusters and the impact of feedback from galaxy and super-massive black hole evolution. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.02539v1

X-IFU Athena view of the most distant galaxy clusters in the Universe by Florent Castellani et al. on Wednesday 07 September The X-ray Integral Field Unit (X-IFU) on-board the second large ESA mission ''Athena'' will be a high spatial (5'') and spectral (2.5eV) resolution X-ray imaging spectrometer, operating in the 0.2-12 keV energy band. It will address the science question of the assembly and evolution through cosmic time of the largest halos of matter in the Universe, groups and clusters of galaxies. To this end, we present an on-going feasibility study to demonstrate the X-IFU capabilities to unveil the physics of massive halos at their epoch of formation. Starting from a distant (z=2) group of galaxies ($M_{500} = 7cdot 10^{13} M_odot/h$) extracted from the HYDRANGEA cosmological and hydrodynamical numerical simulations, we perform an end-to-end simulation of X-IFU observations. From the reconstruction of the global, 1D and 2D quantities, we plan to investigate the various X-IFU science cases for clusters of galaxies, such as the chemical enrichment of the intra-cluster medium (ICM), the dynamical assembly of groups and clusters and the impact of feedback from galaxy and super-massive black hole evolution. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.02539v1

Even the emptiness between galaxies is filled with enough energy over those vast expanses to generate shock waves and giant structures when clusters of galaxies collide. We explore this, the lives of galaxies, and some intriguing results about activity on the asteroid Ceres.

In the science of astronomy, it's important to see both the forest and the trees. Galaxy clusters, in many ways, serve as both. They're rich environments with stars, gas, dust, dark matter, black holes and more. The diversity of stars and stellar populations found within them, as well as found within galaxies of different shapes, sizes, and properties within those clusters, are part of a remarkable and coherent cosmic story. But sometimes the cosmic story can help us understand what's going on in these environments, the converse of the way we normally think about it: where we use the environment to learn about the universe. Come take a fascinating journey into these cosmic behemoths that are the gathering grounds for the greatest collections of large galaxies in the universe, and enjoy a delightful conversation with Gourav Khullar as we go along on this wild ride! (Image credit: ESA/Hubble and NASA, H. Ebling)

Galaxies, those enormous accumulations of stars, dust, gas, and other stuff, are a bit like people – they tend to congregate in big groups.

Several hundred million years ago, two galaxy clusters collided and then passed through each other. This mighty event released a flood of hot gas from each galaxy cluster that formed an unusual bridge between the two objects.

In a study published in Physical Review Letters, Fermilab and University of Chicago scientist Brad Benson and colleagues use the polarization, or orientation, of the cosmic microwave background to calculate the masses of enormous galaxy clusters using a new mathematical estimator. This is the first time that scientists have measured these masses using the polarization of the CMB[...]

Today we cover the large scale structures of the universe. Everything from Stars, Star Clusters, Galaxies, Galaxy Clusters, Filaments, Walls, Great Walls, Voids, Super Voids, Super Clusters, Dark Flow, The Great Attractor and questioning the expansion of our universe!

Today we cover the large scale structures of the universe. Everything from Stars, Star Clusters, Galaxies, Galaxy Clusters, Filaments, Walls, Great Walls, Voids, Super Voids, Super Clusters, Dark Flow, The Great Attractor and questioning the expansion of our universe!

Today we cover the large scale structures of the universe. Everything from Stars, Star Clusters, Galaxies, Galaxy Clusters, Filaments, Walls, Great Walls, Voids, Super Voids, Super Clusters, Dark Flow, The Great Attractor and questioning the expansion of our universe!

Galaxy clusters probed with Stampede2, Comet Supercomputers

Imagine the millions of stars in our galaxy, the Milky Way, and the millions more planets orbiting those stars, Now imagine a cluster of galaxies, perhaps thousands of them. Toby Brown (PhD) is a post-doctoral fellow at McMaster University, Hamilton,… »

When two pairs of galaxy clusters collide, the result is not four separate objects, but one giant galaxy cluster.

Galaxy Clusters happen when gravity pulls galaxies together into groups. They are helping scientists get a look at some of the oldest places in the universe. Idaho Matters talks to one astrophysicist about how these clusters are formed and why they are important to Earthlings. Find out more here.

Fellow ComSciCon- Chicago attendee Gourav Khullar joined me to chat about galaxy clusters. He explained how he models galaxies, infrared telescopes, and gravitational lensing. Suggested Reading: What is a galaxy cluster? http://chandra.harvard.edu/learn_galaxyCluster.html How galaxy clusters tell us a story about the universe: https://astrobites.org/2016/03/17/galaxy-clusters-cosmology-and-beethovens-no-6/ Spectroscopy of Galaxy Clusters: https://www.sdss.org/dr14/algorithms/ancillary/boss/massiveclusters/ Gravitational Lensing employed in Galaxy Clusters (super specific): https://arxiv.org/abs/1303.3274 Follow me: PhDrinking@gmail.com, @PhDrinking, @SadieWit, www.facebook.com/PhDrinking/ Follow Gourav Khullar: @isskywalker, gkhullar@uchicago.edu, http://astro.uchicago.edu/people/gourav-khullar.php Thanks to www.bensound.com/ for the intro/outro Thanks to @TylerDamme for audio editing

A new study tries to tackle the questions surrounding dark energy by examining properties of X-ray emission from galaxy clusters.

Feature Guest: Gil HolderHave you heard of the Great Attractor or the Great Wall? The universe evolved from a hot dense not quite perfectly uniform state to now contain galaxies in sheet-like structures separated by huge voids. These clusters and superclusters of galaxies make up the largest scale structure in the observable universe. How exactly did they emerge, what role does dark matter and dark energy play in the evolution of structure and just where is our universe headed? To help us answer those questions today we’re joined at The Star Spot by cosmologist Gil Holder. Current in SpaceWith the building of the James Webb Space Telescope coming along quickly, Anuj explains how the successor to Hubble will open a new window on the universe? And following the recent 40th anniversary of the Apollo 1 disaster, Tony reflect on three major tragedies in the history of space exploration and reflects on why it’s still worth the risk. Finally Dave reports on the groundbreaking discovery of gravitational waves via one of the most powerful phenomena in the universe: binary black hole mergers.  About Our GuestGil Holder is Canada Research Chair in Cosmological Astrophysics at McGill University and a Scholar at the Canadian Institute for Advanced Research. Dr. Holder received his PhD from the University of Chicago and was a Keck Fellow at the Institute for Advanced Study from 2001 to 2004. His research focuses on unique methods of studying structure formation in the universe.

Galaxienhaufen sind die größten und massivstem gravitativ gebundenen Objekte im Universum, die Zeit hatten, zu kollabieren und virialisieren. Das Intracluster-Medium (ICM) innerhalb Galaxienhaufen ist ein Plasma, das durch Röntgenstrahlung sichtbar ist. Galaxien in Galaxienhaufen sind durch optische Strahlung zu sehen, sie sind hauptsächlich rot und haben eine niedrige Sternbildungsrate. Neu akkretierte Galaxien können blauere Farben und eine höhere Sternbildungsrate aufweisen und werden durch Interaktion mit dem ICM röter. Wachstum von Galaxienhaufen findet durch sporadisches Verschmelzen mit anderen Galaxienhaufen und Gruppen statt, oder durch gleichmäßige Akkretion von Galaxien aus dem Milieu. Um die Hauptfrage »ergänzen sich Röntgen- und optische Messungen von Galaxienhaufen, oder zeigen sie dasselbe?« zu beantworten, haben wir eine Studie durchgeführt, bei der die Verteilung von Galaxien und ICM in Galaxienhaufen verglichen wurden. Im Besonderen, haben wir untersucht, ob optische Daten zusätzliche Information wegen der dynamische Befindlichkeit von einzelnen Galaxienhaufen liefern, die nicht aus Röntgendaten allein hervorgehen können. Surveys in Röntgen und optischer Strahlung sind in den nächsten Jahrzehnten zu erwarten, die Daten von viel weiteren Gebieten des Universums liefern werden. Diese Daten können, mit den Methoden, die wir hier vorlegen, untersucht werden. Wir benutzten Weitwinkelbeobachtungen des MPG/ESO 2.2 m Telescopes und Röntgenbeobachtungen von XMM-Newton, um die Distribution von Galaxien innerhalb Galaxienhaufen mit der Distribution des ICM zu vergleichen. Wir haben gefunden, dass die 1D Radialdistribution der roten Galaxien zu der des ICM zusammenpasst, aber die blauen Galaxien folgen einem flacheren Profil. Mit 2D Abbildungen der Galaxienhaufen, haben wir gefunden dass die roten Galaxien sehr ähnlich verteilt sind, wie das ICM, aber fast jeder Galaxienhaufen hat unvirialisierte rote Subklumpen. Blaue Galaxien anderseits haben zu wenig Zeit zum virialisiern bevor sie rot werden, weil sie ihre sternbildendes Gas innerhalb einer Übergangszeit durch ICM-Staudruckstripping verliern. Röntgenbeobachtungen sind besser für die Bestimmung des Verschmelzungsverlaufes von Galaxienhaufen, weil sie die Kennzeichen von Verschmelzung für eine kürzere Zeit zeigen. Wir haben mehrere Subklumpen von roten Galaxien entdeckt, die scheinen auf einfallenden Trajektorien in Galaxienhaufen zu sein und noch merkliche Mengen von Röntgenemittierendem Gas zu haben.

Der Ursprung und die Entwicklung unseres Universums zeigt sich gleichermaßen in der Raumzeit selbst wie in den Strukturen, die in ihr entstehen. Galaxienhaufen sind das Ergebnis hierarchischer Strukturbildung. Sie sind die massivsten Objekte, die sich im heutigen Universum bilden konnten. Aufgrund dieser Eigenschaft ist ihre Anzahl und Struktur hochgradig abhängig von der Zusammensetzung und Evolution des Universums. Die Messung der Anzahldichte von Galaxienhaufen beruht auf Katalogen, die nach einer beobachtbaren Größe ausgewählt werden. Die Anwendung einer Massen-Observablen-Relation (MOR) erlaubt es, die beobachtete Anzahl als Funktion der Observablen und der Rotverschiebung mit Vorhersagen zu vergleichen und so kosmologische Parameter zu bestimmen. Man kann jedoch zu Recht behaupten, dass diese Messungen noch nicht präzise im Prozentbereich sind. Hauptgrund hierfür ist das unvollständige Verständnis der MOR. Ihre Normalisierung, die Skalierung der Observablen mit Masse und Rotverschiebung und die Größe und Korrelation von intrinsischen Streuungen muss bekannt sein, um Anzahldichten korrekt interpretieren zu können. Die Massenbestimmung von Galaxienhaufen durch die differenzielle Lichtablenkung in ihrem Gravitationsfeld, i.e. durch den so genannten schwachen Gravitationslinseneffekt (weak lensing), kann erheblich hierzu beitragen. In dieser Arbeit werden neue Methoden und Ergebnisse solcher Untersuchungen vorgestellt. Zu ersteren gehören, als Teil der Datenaufbereitung, (i) die Korrektur von CCD-Bildern für nichtlineare Effekte durch die elektrischen Felder der angesammelten Ladungen (Kapitel 2) und (ii) eine Methode zur Maskierung von Artefakten in überlappenden Aufnahmen eines Himmelsbereichs durch Vergleich mit dem Median-Bild (Kapitel 3). Schließlich ist (iii) eine Methode zur Selektion von Hintergrundgalaxien, basierend auf deren Farbe und scheinbarer Magnitude, die eine neue Korrektur für die Kontamination durch Mitglieder des Galaxienhaufens einschließt, im Abschnitt 5.3.1 beschrieben. Die wissenschaftlichen Hauptergebnisse sind die folgenden. (i) Für den Hubble Frontier Field-Haufen RXC J2248.7-4431 bestimmen wir Masse und Konzentration mittels weak lensing und bestätigen die durch Röntgen- und Sunyaev-Zel'dovich-Beobachtungen (SZ) vorhergesagte große Masse. Die Untersuchung von Haufengalaxien zeigt die Abhängigkeit von Morphologie und Leuchtkraft sowie Umgebung (Kapitel 4). (ii) Unsere Massenbestimmung für 12 Galaxienhaufen ist konsistent mit Röntgenmassen, die unter Annahme hydrostatischen Gleichgewichts des heißen Gases gemacht wurden. Wir bestätigen die MOR, die für die Signifikanz der Detektion mit dem South Pole Telescope bestimmt wurde. Wir finden jedoch Diskrepanzen zur Planck-SZ MOR. Unsere Vermutung ist, dass diese mit einer flacheren Steigung der MOR oder einem größen-, rotverschiebungs- oder rauschabhängigen Problem in der Signalextraktion zusammenhängt (Kapitel 5). (iii) Schließlich zeigen wir, durch die Verbindung von Simulationen und theoretischer Modellierung, dass die Variation von Dichteprofilen bei fester Masse signifikant zur Ungenauigkeit von Massenbestimmungen von Galaxienhaufen mittels weak lensing beiträgt. Ein Modell für diese Variationen, wie das hier entwickelte, ist daher wichtig für die genaue Bestimmung der MOR, wie sie für kommende Untersuchungen nötig sein wird (Kapitel 6).

Gravitational Lensing is a unique technique to investigate the dark matter distribution of structures in the Universe, from galaxies, through galaxy groups, clusters, up to the large-scale structure. It allows us to map the total projected mass density of structures acting as lenses, and thus to shed light on the distribution and properties of the otherwise-invisible dark matter. Clusters of galaxies are the largest virialized structures in the universe. Gravitational lensing analysis allows us to study their mass distribution in great detail. Weak lensing probes the mass distribution in the outskirts of clusters based on a statistical analysis of the shape distortion observed in hundreds of galaxies behind the cluster. Strong lensing, instead, allows us to reconstruct high resolution mass and magnification maps of the central region of the cluster. In addition, thanks to the lensing magnification of background sources, galaxy clusters act as "Gravitational Telescopes" and can be used to investigate the galaxy population of the early Universe at z>5. In the first part of my Thesis I use the CLASH and Frontier Fields cluster RXC J2248 to investigate sources at z~6. At such and higher redshift galaxies appear as optical dropouts, since the light they emit is redshifted to NIR wavelengths and no flux is observed in the UV and optical filters. I discovered a z~6 lensed galaxy in the core of RXC J2248 which appears as a quintuple lensed optical dropout in the 16 HST filters of the CLASH survey. I perform a detailed photometric analysis of these dropouts to verify that they present the same photometric properties and are actually multiple images of the same source. In addition, by performing the strong lensing analysis of the cluster core I verify that the lensing model supports the quintuple and z~6 nature of this system. In the second part of my Thesis I use strong gravitational analysis of the CLASH cluster A383 to probe the details of the mass distribution of galaxies in the cluster core. Well known luminosity scaling relations allow us to relate the physical properties as stellar velocity dispersion and size of the elliptical galaxies to their observed luminosity. However in clusters, galaxies suffer tidal stripping due to the interaction with other cluster members and the cluster dark matter halo. The goal of this work is to measure the galaxy halo sizes in a cluster core to quantify how much mass was stripped relative to field galaxies. Here I present a new approach to strong lensing analysis of clusters, in which I use measurements of cluster members' velocity dispersions as additional constraints in the lens modeling. I apply this analysis to Abell 383 to separate the galaxy mass content from the smooth dark matter mass component and investigate how the dark matter halo size scales with the galaxy luminosity in the cluster core. In addition I perform the surface brightness reconstruction of the southern giant arcs to improve constraints on close by individual galaxies and study possible deviations from the global scaling law measured for the cluster.

Galaxy clusters, the massive systems host hundreds of galaxies, are invaluable cosmological probes and astrophysical laboratories. Besides these fascinating galaxies, the concentration of dark matter creates a deep gravitational potential well, where even light passing by is bended and the background image is distorted. The baryonic gas falling into the potential well is heated up to more than 10^7 K that free electrons start to emitting in X-ray. Observing those phenomena leads to a throughout understanding of gravity, particle physics and hydrodynamics. In addition, residing on the top of the density perturbations, clusters are sensitive to the initial condition of the Universe, such that they are complimentary tools for cosmology studies. In this thesis we first introduce the basic framework of the Universe and supporting observational evidence. Following that, we sketch the principle to use clusters for cosmology study via their redshift and mass distribution. However cluster mass is not a direct observable, so we need to estimate it by other channels. We briefly exhibit cluster observations in optical, X-ray and microwave bands and discuss the challenges in estimating the underlying cluster mass with them. After this introduction, we present our results on three aspects of the cluster cosmology study. First, we present a study of Planck Sunyaev-Zel’dovich effect (SZE) selected galaxy cluster candidates using Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) imaging data. To fulfil the strength of SZE survey, the redshifts of clusters are required. In this work we examine 237 Planck cluster candidates that have no redshift in the Planck source catalogue. Among them, we confirmed 60 galaxy clusters and measure their redshifts. For the remaining sample, 83 candidates are so heavily contaminated by stars due to their location near the Galactic plane that we do not identify galaxy members and assign reliable redshifts. For the rest 94 candidates we find no optical counterparts. By examining with 150 Planck confirmed clusters with spectroscopy redshifts, our redshift estimations have an accuracy of σ_{z/(1+z)}~0.022. Scaling for the already published Planck sample, we expect the majority of the unconfirmed candidates to be noise fluctuations, except a few at high redshift that the Pan-STARRS1 (PS1) data are not sufficiently deep for confirmation. Thus we use the depth of the optical imaging for each candidate together with a model of the expected galaxy population for a massive cluster to estimate a redshift lower limit, beyond which we would not have expected to detect the optical counterpart. Second, we use 95GHz, 150GHz, and 220GHz observations from South Pole Telescope (SPT) to study the SZE signatures of a sample of 46 X-ray selected groups and clusters drawn from ~6 deg^2 of the XMM-Newton Blanco Cosmology Survey (XMM-BCS). The wide redshift range and low masses make this analysis complementary to previous studies. We develop an analysis tool that using X-ray luminosity as a mass proxy to extract selection-bias corrected constraints on the SZE significance- and Y_{SZ}-mass relations. The SZE significance- mass relation is in good agreement with an extrapolation of the relation obtained from high mass clusters. However, the fit to the Y_{SZ}-mass relation at low masses, while in agreement with the extrapolation from high mass SPT sample, is in tension at 2.8σ with the constraints from the Planck sample. We examine the tension with the Planck relation, discussing sample differences and biases that could contribute. We also analyse the radio galaxy point source population in this ensemble of X-ray selected systems. We find 18 of our systems have 1 GHz Sydney University Molonglo Sky Survey (SUMSS) sources within 2 arcmin of the X-ray centre, and three of these are also detected at significance >4 by SPT. Among these three, two are associated with the brightest cluster galaxies, and the third is a likely unassociated quasar candidate. We examined the impact of these point sources on our SZE scaling relation result and find no evidence of biases. We also examined the impact of dusty galaxies. By stacking the 220 GHz data, we found 2.8σ significant evidence of flux excess, which would correspond to an average underestimate of the SZE signal that is (17±9) % in this sample of low mass systems. Finally we predict a factor of four to five improvements on these SZE mass-observable relation constraints based on future data from SPTpol and XMM-XXL. In the end we present a study using clusters as tools to probe deviations from adiabatic evolution of the Cosmic Microwave Background (CMB) temperature. The expected adiabatic evolution is a key prediction of standard cosmology. We measure the deviation of the form T(z)=T_0(1+z)^{1-α} using measurements of the spectrum of the SZE with SPT. We present a method using the ratio of the SZE signal measured at 95 and 150 GHz in the SPT data to constrain the temperature of the CMB. We validate that this approach provides unbiased results using mock observations of cluster from a new set of hydrodynamical simulations. Applying this method to a sample of 158 SPT-selected clusters, we measure α=0.017^{+0.030}_{−0.028} consistent with the standard model prediction of α=0. Combining with other published results, we find α=0.005±0.012, an improvement of ~ 10% over published constraints. This measurement also provides a strong constraint on the effective equation of state, w_{eff}=−0.994±0.010, which is presented in models of decaying dark energy.

We present a study of the galaxy populations in massive galaxy clusters selected by their Sunyaev–Zel’dovich Effect (SZE) signatures. Selection via the SZE is approximately mass- limited where the mass limit varies only slightly with redshift, making it an ideal selection method for studying the evolution of the galaxy content of clusters. We begin by introducing the SZE, the South Pole Telescope (SPT), and the larger research project in which this Thesis is embedded. We then present the core galaxy population studies of this Thesis. In Chapter 3, we present the first large-scale follow-up of an SZE-selected galaxy cluster sample. Of 224 galaxy cluster candidates in the sample, we optically confirm 158 clusters and measure their photometric redshifts. We find a redshift range of 0.1

The morphological analysis of galaxy clusters in X-rays allows a reliable determination of their dynamical state. Substructures on (sub-)Mpc scale influence the gravitational potential of a cluster and manifest themselves in the X-ray surface brightness distribution as secondary peaks or overall irregular shape. They lead to deviations from the hydrostatic equilibrium and spherical shape, two assumptions which are widely used in galaxy cluster studies to derive global astrophysical properties. Analyzing the X-ray morphology of clusters thus yields valuable information, provided that the employed substructure measures are well-tested and well-calibrated. In this work, the X-ray morphology of galaxy clusters is quantified using three common substructure parameters (power ratios, center shift and the asymmetry parameter), which are subsequently employed to study the disturbed cluster fraction as a function of redshift. To ensure a reliable application of these substructure parameters on a variety of X-ray images, a detailed parameter study is conducted. It focuses on the performance and reliability of the parameters for varying data quality using simulated and observed X-ray images. In particular, when applying them to X-ray images with low photon counts such as observations of distant clusters or survey data, it is important to know the characteristics of the parameters. Comparing the three substructure measures, the center shift parameter is most robust against Poisson noise and allows a reliable determination of the clusters' dynamical state even for low-count observations. Power ratios, especially the hexapole P3/P0, and the asymmetry parameter, on the other hand, are severely affected by noise, which results in spuriously high substructure signals. Furthermore, this work presents methods to minimize the noise bias. The results of the parameter study provide a step forward in the morphological analysis of high-redshift clusters and are employed in the framework of this thesis to quantify the evolution of the disturbed cluster fraction. The sample used for this analysis comprises 78 low-z (z < 0.3) and 51 high-z (0.3 < z < 1.08) galaxy clusters with varying photon statistics. The low-redshift objects were observed with the XMM-Newton observatory, contain a high number of photon counts and are part of several well-known and representative samples. For z > 0.3, the high-redshift subsets of the 400d2 and SPT survey catalog are used. These objects were mainly observed with the Chandra observatory and have low photon counts. To ensure a fair comparison, which is independent of the data quality, the photon statistics of the low- and high-redshift observations are aligned before performing the morphological analysis. In agreement with the hierarchical structure formation model, a mild positive evolution with redshift, i.e. a larger fraction of clusters with disturbed X-ray morphologies at higher redshift, is found. Owing to the low photon counts and small number of high-redshift observations, the statistical significance of this result is low. For two of the three substructure parameters (power ratios and center shift) the findings are also consistent within the significance limits with no evolution, but a negative evolution of the disturbed cluster fraction can be excluded for all parameters.

Multi-wavelength large-scale surveys are currently exploring the Universe and establishing the cosmological scenario with extraordinary accuracy. There has been recently a significant theoretical and observational progress in efforts to use clusters of galaxies as probes of cosmology and to test the physics of structure formation. Galaxy clusters are the most massive gravitationally bound systems in the Universe, which trace the evolution of the large-scale structure. Their number density and distribution are highly sensitive to the underlying cosmological model. The constraints on cosmological parameters which result from observations of galaxy clusters are complementary with those from other probes. This dissertation examines the crucial role of clusters of galaxies in confirming the standard model of cosmology, with a Universe dominated by dark matter and dark energy. In particular, we examine the clustering of optically selected galaxy clusters as a useful addition to the common set of cosmological observables, because it extends galaxy clustering analysis to the high-peak, high-bias regime. The clustering of galaxy clusters complements the traditional cluster number counts and observable-mass relation analyses, significantly improving their constraining power by breaking existing calibration degeneracies. We begin by introducing the fundamental principles at the base of the concordance cosmological model and the main observational evidence that support it. We then describe the main properties of galaxy clusters and their contribution as cosmological probes. We then present the theoretical framework of galaxy clusters number counts and power spectrum. We revise the formulation and calibration of the halo mass function, whose high mass tail is populated by galaxy clusters. In addition to this, we give a prescription for modelling the cluster redshift space power spectrum, including an effective modelling of the weakly non-linear contribution and allowing for an arbitrary photometric redshift smoothing. Some definitions concerning the study of non-Gaussian initial conditions are presented, because clusters can provide constraints on these models. We dedicate a Chapter to the data we use in our analysis, namely the Sloan Digital Sky Survey maxBCG optical catalogue. We describe the data sets we derived from this large sample of clusters and the corresponding error estimates. Specifically, we employ the cluster abundances in richness bins, the weak-lensing mass estimates and the redshift-space power spectrum, with their respective covariance matrices. We also relate the cluster masses to the observable quantity (richness) by means of an empirical scaling relation and quantify its scatter. In the next Chapter we present the results of our Monte Carlo Markov Chain analysis and the cosmological constraints obtained. With the maxBCG sample, we simultaneously constrain cosmological parameters and cross-calibrate the mass-observable relation. We find that the inclusion of the power spectrum typically brings a 50% improvement in the errors on the fluctuation amplitude and the matter density. Constraints on other parameters are also improved, even if less significantly. In addition to the cluster data, we also use the CMB power spectra from WMAP7, which further tighten the confidence regions. We also apply this method to constrain models of the early universe through the amount of primordial non-Gaussianity of the initial density perturbations (local type) obtaining consistent results with the latest constraints. In the last Chapter, we introduce some preliminary calculations on the cross-correlation between clusters and galaxies, which can provide additional constraining power on cosmological models. In conclusion, we summarise our main achievements and suggest possible future developments of research.

Tue, 27 Nov 2012 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/15232/ https://edoc.ub.uni-muenchen.de/15232/1/Nastasi_Alessandro.pdf Nastasi, Alessandro ddc:530, ddc:500, Fakultät für Physik

Gravitational lensing shows that two galaxy clusters are connected by a filament of dark matter. John Matson reports

Galaxy clusters are optimal laboratories to test cosmology as well as models for physical processes acting on smaller scales. X–ray observations of the hot gas filling their dark matter potential well, i.e. the intra–cluster medium (ICM), still provides one of the best ways to investigate the intrinsic properties of clusters. Methods based on X–ray observations of the ICM are commonly used to estimate the total mass, assuming that the gas traces the underlying potential well and satisfies spherical symmetry, and thermal motions dominate the total pressure support. However, non–thermal motions are likely to establish in the ICM, hence, contribute to the total pressure and have to be taken into account in the mass estimate. In this thesis I study the ICM thermo–dynamical structure by combining hydrodynamical simulations and synthetic X–ray observations of galaxy clusters. The main goal is to study their gas velocity field and the implications due to non–thermal motions: first, by analysing directly the velocity patterns in simulated clusters and, secondly, by reconstructing the internal ICM structure from mock X–ray spectra. To this aim, I developed and applied an X–ray photon simulator to obtain synthetic X–ray spectra from the gas component in hydrodynamical simulations of galaxy clusters. The main findings of this work are as follows. (i) Ordered, rotational patterns in the gas velocity field in cluster cores can establish during the mass assembly process, but are found to be transient phenomena, easily destroyed by passages of gas–rich subhaloes. This suggests that in smoothly growing haloes the phenomenon is in general of minor effect. Nonetheless, major mergers or highly disturbed systems can indeed develop significant ordered motions and rotation, which contribute up to 20% to the total mass. (ii) It is indeed possible to reconstruct the thermal structure of the ICM in clusters from X–ray spectral analysis, by recovering the emission measure (EM) distribution of the gas as a function of temperature. This is possible with current X–ray telescopes (e.g. Suzaku) via multi–temperature fitting of X– ray spectra. (iii) High–precision X–ray spectrometers, such as ATHENA, will allow us to measure velocity amplitudes of ICM non–thermal motions, from the velocity broadening of heavy–ion (e.g. iron) emission lines. In this work, these achievements are obtained by applying the virtual X–ray simulator to generate ATHENA synthetic spectra of simulated clusters. The non–thermal velocity of the ICM in the central region is used to further characterise the cluster and the level of deviation from the expected self–similarity. By excluding the clusters with the highest non–thermal velocity dispersion, the scatter of the LX −T relation for the sample is significantly reduced, which will allow for a more precise comparison between observations and simulations.

Observational constraints on the average radial distribution profile of AGN in distant galaxy clusters can provide important clues on the triggering mechanisms of AGN activity in dense environments and are essential for a completeness evaluation of cluster selection techniques in the X-ray and mm wavebands. The aim of this work is a statistical study with XMM-Newton of the presence and distribution of X-ray AGN in the large-scale structure environments of 22 X-ray luminous galaxy clusters in the redshift range 0.9 < z less than or similar to 1.6 compiled by the XMM-Newton Distant Cluster Project (XDCP). To this end, the X-ray point source lists from detections in the soft band (0.35-2.4 keV) and full band (0.3-7.5 keV) were stacked in cluster-centric coordinates and compared to average background number counts extracted from three independent control fields in the same observations. A significant full-band (soft-band) excess of similar to 78 (67) X-ray point sources is found in the cluster fields within an angular distance of 8' (4 Mpc) at a statistical confidence level of 4.0 sigma (4.2 sigma), corresponding to an average number of detected excess AGN per cluster environment of 3.5 +/- 0.9 (3.0 +/- 0.7). The data point towards a rising radial profile in the cluster region (r < 1Mpc) of predominantly low-luminosity AGN with an average detected excess of about one point source per system, with a tentative preferred occurrence along the main cluster elongation axis. A second statistically significant overdensity of brighter soft-band-detected AGN is found at cluster-centric distances of 4'-6' (2-3 Mpc), corresponding to about three times the average cluster radius R-200 of the systems. If confirmed, these results would support the idea of two different physical triggering mechanisms of X-ray AGN activity in dependence of the radially changing large-scale structure environment of the distant clusters. For high-z cluster studies at lower spatial resolution with the upcoming eROSITA all-sky X-ray survey, the results suggest that cluster-associated X-ray AGN may impose a bias in the spectral analysis of high-z systems, while their detection and flux measurements in the soft band may not be significantly affected.

Tue, 8 Nov 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13627/ https://edoc.ub.uni-muenchen.de/13627/1/Zhuravleva_Irina.pdf Zhuravleva, Irina ddc:530, ddc:500, Fakultät fü

Thu, 21 Jul 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13348/ https://edoc.ub.uni-muenchen.de/13348/1/Donnert_Julius.pdf Donnert, Julius ddc:530, ddc:500, Fakultät für Physik

Wed, 8 Jun 2011 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/13164/ https://edoc.ub.uni-muenchen.de/13164/1/Misgeld_Ingo.pdf Misgeld, Ingo ddc:530, ddc:500, Fa

In the framework of the current cosmological paradigm, cosmic evolution is mostly driven by gravity through the hierarchical growth of cold dark matter structures. However, the evolution of the directly observed luminous component involves complex non-gravitational processes such as cooling, star formation and feedback mechanisms involving the conventional matter well known to us, termed shortly as baryons. Clusters of galaxies are the largest virialized systems in the Universe, hence are ideal laboratories to study the evolution of baryons. The baryon content of clusters accounts for roughly 15% of their total mass, encompassing a “cold phase” in the form of luminous galaxy masses, and a “hot phase” corresponding to the X-ray emitting intracluster medium (ICM). The thermodynamics of baryons is affected by non-trivial phenomena and the interplay of the intricate processes between these two phases remains, to a large extent, unclear. In this thesis I investigate the properties of both the ICM and the underlying galaxy populations in X-ray selected distant clusters, with the aim of constraining physical processes governing the evolution of clusters and their galaxies. The inner regions of local clusters often exhibit radiative cooling, termed cool cores (CC). I have made an important step in investigating the abundance of cool cores in the distant cluster population, by devising efficient methods to characterize local CCs, that were applied to the highest redshift cluster sample currently available (0.7 < z < 1.4) from the Chandra archive. The fraction of CCs seems to decrease with redshift, since I find that the majority of the distant clusters are in an intermediate state of cooling. High-z (z∼ >1) clusters are hard to find. The XMM-Newton Distant Cluster Project (XDCP) is a survey aimed to construct a complete sample of z∼1 clusters from the XMM-Newton archive. Within this scope a large effort has been done to confirm potential distant cluster candidates by exploring a new optical & near-infrared imaging technique to identify overdensities of galaxies. Twenty-two cluster candidates were imaged during two runs at the ESO/La Silla Observatory, of which around half are potential distant clusters, based on their I-H images. The applied photometric technique has thus proven to be reliable in identifying z≥0.8 overdensities of galaxies. The formation and evolution of massive early-type galaxies (ETGs) is still an open question, since the observational data cannot be easily reconciled with the preferred, hierarchical galaxy formation scenario. Using high resolution Hubble Space Telescope/ACS imaging and VLT/FORS2 spectra, I studied the galaxy population of XMM1229 at z=0.975, discovered in the XDCP. The results show a red-sequence populated by galaxies with stellar masses in the range 5e10 - 2e11 solar masses and old (3-4 Gyr) underlying stellar population formed at zf ∼ 4. The color-magnitude relation at this high redshift is found to be already very tight (with a 0.04 spread similar to the local Coma cluster). This confirms that ETGs in clusters assembled early on and in short timescales, and their star formation processes have already completed the essential part of their chemical enrichment, as elucidated by the high metal abundance (Z ∼ 0.3 solar) of the ICM, measured with XMM spectra.

Fri, 17 Oct 2008 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/9179/ https://edoc.ub.uni-muenchen.de/9179/1/Braglia_Filiberto_Giorgio.pdf Braglia, Filiberto Giorgio ddc:500, ddc:530, Fakultät für Physik

The study of the formation and growth of cosmic structures is one of the most fascinating and challenging fields of astrophysics. In the currently favoured cosmological model, the so-called LCDM cosmogony, dark matter structures grow hierarchically, with small clumps forming first at very early epochs. The merging of these dark matter halos in the following evolution leads to the formation of more massive objects with time, ultimately resulting in a complex cosmic web composed of filaments of dark matter and galaxies, rich galaxy clusters, and voids in between. While we have some knowledge how these dark matter structures evolve with cosmic time, the relationship between the "dark" and the "luminous" content of the Universe is still far from being fully understood and it poses many puzzling questions, both for observational and theoretical investigations. Galaxy clusters, the largest virialized objects in the Universe, are especially interesting for cosmological studies because they are ideal laboratories to study the physical processes relevant in structure formation, like those that shape the properties of galaxies, the intergalactic and intracluster media, and the active galactic nuclei (AGN) that originate from super-massive black holes (BHs) in cluster centres. The study of clusters is remarkably promising right now, both because of the wealth of new data from X-ray telescopes such as XMM-Newton and Chandra or from optical surveys such as SDSS, and also due to the increasing power of cosmological simulations as a theoretical tool. The latter can track the growth of cosmological structures far into the highly non-linear regime, and have recently become faithful enough to include for the first time physical processes such as AGN activity and its effect on galaxy evolution. Therefore the aim of this Thesis was to incorporate AGN heating process in fully self-consistent cosmological simulations of structure formation, and to constrain the relevance of this feedback mechanism for galaxy and galaxy cluster formation and evolution.

In the semi-analytical work presented here the feedback from supermassive black holes on galaxy clusters is investigated. In particular we aim at providing simple diagnostics tools to constrain the characteristic velocities and spatial scales of the hot Intra Cluster Medium (ICM) motions. In the so-called "cold core'' clusters these motions are believed to be driven by the activity of a central black hole. The methods developed here, together with present-day and future observations, are designed to help to solve the puzzle of cooling flow clusters (see section $1.3$) and understand better the AGN/gas interaction in smaller systems (down to individual galaxies). Clusters of galaxies are the largest gravitationally bound systems in the Universe: they are composed of hundreds to thousands of galaxies, moving in a deep potential well set by the dominating dark matter. The whole volume of clusters is filled with hot (temperature $sim 10^7-10^8$~K) and rarefied (electron density $10^{-4}-10^{-1} {rm cm^{-3}}$) gas. In such a high-temperature regime even heavy elements (e.g. silicon, sulfur, iron etc.) are highly ionized up to [H]- or [He]-like ions and they emit in bright lines with energies from $sim 0.7$ to $sim 8$ keV. Using X-ray observations one can reliable measure all the major gas properties: the temperature, density and abundance of heavy elements. A significant fraction of clusters (called "cool core'' clusters) show distinct signatures in the central region: the gas temperature drops inward, while the gas density increases. The central gas radiative cooling time in such clusters is much shorter than the age of the cluster and without any external source of energy the gas would cool well below X-ray temperatures. However observations suggest that the gas temperature drops only to 1-2 keV. One plausible explanation of this problem is that the activity of a central supermassive black hole deposits large amounts of mechanical energy into the cluster gas and that this balances the gas radiative losses. A direct implication of this hypothesis is that the hot gas is not at rest, but it is continuously stirred by the AGN activity. The same class of cool core clusters is characterized by a centrally peaked distribution of the heavy elements abundance (usually measured using the He-like iron 6.7 keV line). The peaked abundance profiles are likely associated with the metals ejection by the stars of very massive elliptical galaxies, that are always present at the centers of these clusters. However the observed abundance distributions are significantly broader than the central galaxy light profiles, suggesting that some gas motions are spreading the metals ejected from the galaxy. We treat this process in a diffusion approximation to derive, from the X-ray observations, constraints on the characteristic velocities and spatial scales of the gas motions for a sample of cool core clusters and groups (Chapters $2$ and $3$). The parameters derived from a simple semi-analytic model are then compared with the results of numerical simulations of the AGN/gas interaction in the cluster core (Chapter $4$). In Chapter $5$ we discuss the impact of the gas motions on the width of the strongest X-ray emission lines. Since the characteristic thermal velocities of heavy ions (e.g. iron) are much smaller than the sound speed of the gas, the width of the lines sensitively depends on the presence of gas motions. We show that both the absolute value of the linewidth and its dependence on the projected distance from the cluster center provide valuable diagnostics of the gas motions. Such measurements will soon become possible with the launch of X-ray micro-calorimeters in space. This work has been done in collaboration with E.Churazov, R.Sunyaev, H.B"ohringer, M.Br"uggen, W.Forman and E.Roediger.

Thu, 27 Oct 2005 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/4628/ https://edoc.ub.uni-muenchen.de/4628/1/Zhang_Yu-Ying.pdf Zhang, Yu-Ying ddc:530, ddc:500, Fa

In currently favoured scenarios, the Universe evolves from a highly homogeneous phase, emerging from the hot Big Bang, to the present-day state, which is characterised by a wealth of hierarchically organised structures, spanning several orders of magnitudes in size: galaxies, clusters of galaxies, superclusters, walls and filaments. Structures are formed via gravitational instability and grow hierarchically: the smallest ``haloes'' collapse first and then grow by accreting mass from other haloes or by merging with other similar structures. Gravitational and dynamical interactions, like mergers, accretions, tidal distortions and disruptions thus play a fundamental role in shaping galaxies and galaxy clusters. As a natural by-product of these interactions, stars, originally located within galaxies, are ejected into the space surrounding galaxies and into intracluster space, giving rise to diffuse stellar components. The study of these components can reveal important details of galaxy and cluster formation, and are therefore of great interest. Observations in this field are severely hampered by the extremely low surface brightness that has to be measured, corresponding to less than 1/1,000 of the typical surface brightness of the sky. So far, this has prevented observing large statistical samples of stellar haloes and intracluster stellar populations. The statistical characterisation of stellar haloes and of the intracluster light is the objective of this thesis. In order to do this, I have developed a new method in which a large number (approx. 1,000) of relatively shallow images of homogeneous objects are stacked to produce an extremely deep average image. Systematic effects that arise from contaminating sources and instrumental biases in the observation of individual objects are cancelled out by taking the average of many different observations and by adopting a conservative masking of the polluting sources. The large image database required for this technique has been provided by the Sloan Digital Sky Survey, the largest optical survey of the Universe ever undertaken, which will eventually cover approximately 10,000 square degrees of sky with 5-band photometry and spectroscopy. In the first part of the thesis I have studied the stellar haloes of disc galaxies. From the stacking of 1043 galaxies I have been able to infer the almost ubiquitous presence of this component around discs, thus demonstrating that haloes are essential ingredients of galaxy evolution. On average, stellar haloes have power-law profiles, consistent with those of the Milky Way and M31. Their shape is moderately flattened. The average halo colours hint at old and fairly metal-enriched stars. However, a puzzling emission excess in the redmost bands has been measured that cannot be explained by any stellar emission, but suggests the presence of ionised gas. These results have been confirmed by the analysis of a galaxy, which has been observed by the Hubble Space Telescope in its deepest exposure (the Ultra Deep Field). The second part of the thesis is devoted to a study of the intracluster light (ICL) from the stacking of a sample of 683 clusters of galaxies in the redshift range 0.2--0.3. The average contribution of the ICL to the total light of a cluster is 17.5 +- 2.0% within 700 kpc from the cluster centre. The ICL is significantly more centrally concentrated than the light in galaxies, consistent with the idea that the ICL is formed via tidal stripping and disruption of galaxies that plunge deep into the cluster potential. The colours of the ICL are consistent with those of the cluster galaxies, thus indicating that the intracluster stars stem from the same population as the stars in galaxies. The amount of ICL correlates more strongly with the luminosity of the central galaxy of the cluster than with cluster richness. Furthermore, the ICL aligns more strongly with the central galaxy than with the larger scale galaxy distribution in the cluster. This strongly suggest that the mechanism of formation of the ICL is strongly coupled with the process of growth of the cluster central galaxy.

Mon, 10 Dec 2001 12:00:00 +0100 https://edoc.ub.uni-muenchen.de/423/ https://edoc.ub.uni-muenchen.de/423/1/Reiprich_Thomas.pdf Reiprich, Thomas H. ddc:530, d