Podcasts about millennium simulation

Ruth Grützbauch ist Astronomin, betreibt in Wien ein Popup-Planetarium, und ich lasse mir von ihr erzählen, was es am Himmel nicht zu sehen gibt, obwohl es dort ist. Darin: Dwarf galaxy problem – Milchstraße – Magellansche Wolken – Sternstrom – Millennium-Simulation – Hintergrundstrahlung – Illustris-Projekt – Tidal stripping – Kalte Dunkle Materie

The first episode of 2018 tackles the biggest question of our time: does anyone actually care whether or not we’re living in a computer simulation?As our understanding of the universe expands, the question of whether we’re living in a computer simulation has shifted from the domain of philosophers into a problem for astrophysics. Because at their astonishingly fast rate of discovery, astrophysicists will be the first ones to know. We chat to astrophysicists Martin Bell (University of Technology Sydney) and Geraint Lewis (Sydney University) about just how close we are to simulating the world.You can find more information about the Millennium Simulation here.

One of the most fundamental correlations between the properties of galaxies in the local Universe is the so-called morphology-density relation (Dressler 1980). A plethora of studies utilizing multi-wavelength tracers of activity have shown that late type star forming galaxies favour low density regions in the local Universe (e.g. G´omez et al. 2003). In particular, the cores of massive galaxy clusters are galaxy graveyards full of massive spheroids that are dominated by old stellar populations. A variety of physical processes might be effective in suppressing star formation and affecting the morphology of cluster and group galaxies. Broadly speaking, these can be grouped in two big families: (i) interactions with other cluster members and/or with the cluster gravitational potential and (ii) interactions with the hot gas that permeates massive galaxy systems. Galaxy groups are the most common galaxy environment in our Universe, bridging the gap between the low density field and the crowded galaxy clusters. Indeed, as many as 50%-70% of galaxies reside in galaxy groups in the nearby Universe (Huchra & Geller 1982; Eke et al. 2004), while only a few percent are contained in the denser cluster cores. In addition, in the current bottom-up paradigm of structure formation, galaxy groups are the building blocks of more massive systems: they merge to form clusters. As structures grow, galaxies join more and more massive systems, spending most of their life in galaxy groups before entering the cluster environment. Thus, it is plausible to ask if group-related processes may drive the observed relations between galaxy properties and their environment. To shed light on this topic we have built the largest X-ray selected samples of galaxy groups with secure spectroscopic identification on the major blank field surveys. For this purpose, we combine deep X-ray Chandra and XMM data of the four major blank fields (All-wavelength Extended Groth Strip International Survey (AEGIS), the COSMOS field, the Extended Chandra Deep Field South (ECDFS), and the Chandra Deep Field North (CDFN) ). The group catalog in each field is created by associating any X-ray extended emission to a galaxy overdensity in the 3D space. This is feasible given the extremely rich spectroscopic coverage of these fields. Our identification method and the dynamical analysis used to identify the galaxy group members and to estimate the group velocity dispersion is extensively tested on the AEGIS field and with mock catalogs extracted from the Millennium Simulation (Springel et al. 2005). The effect of dynamical complexity, substructure, shape of X-ray emission, different radial and redshift cuts have been explored on the LX −sigma relation. We also discover a high redshift group at z~1.54 in the AEGIS field. This detection illustrates that mega-second Chandra exposures are required for detecting such objects in the volume of deep fields. We provide an accurate measure of the Star Formation Rate (SFR) of galaxies by using the deepest available Herschel PACS and Spitzer MIPS data available for the considered fields. We also provide a well-calibrated estimate of the SFR derived by using the SED fitting technique for undetected sources in mid- and far-infrared observations. Using this unique sample, we conduct a comprehensive analysis of the dependence of the total SFR , total stellar masses and halo occupation distribution (HOD) of massive galaxies (M*>10^10 M_sun) on the halo mass of the groups with rigorous consideration of uncertainties. We observe a clear evolution in the level of star formation (SF) activity in galaxy groups. Indeed, the total star formation activity in high redshift (0.5

MinutePhysic's take on the Millennium Simulation

This episode is a conversation with Volker Springel about the Millenium Simulation, which at the time was the largest simulation of the growth of cosmic structure, including a detailed model for the formation of galaxies and supermassive black holes. In the episode we talk about the physical/cosmological background, the simulation process and approach as well as some details about the hard- and software.

In this show Tim returns to answer your questions about astronomy [09:04-37:16] and we talk to Andreas Faltenbacher about dark matter halos in the Millennium Simulation [37:38-76:56].

In this show Tim returns to answer your questions about astronomy [09:04-37:16] and we talk to Andreas Faltenbacher about dark matter halos in the Millennium Simulation [37:38-76:56].

This Thesis addresses the topic of galaxy formation and evolution in the universe. In collaboration with D. Croton, G. de Lucia, V. Springel, and S.D.M. White, I made use of the Millennium simulation, a very large N-body simulation of dark-matter evolution in a cosmological volume carried out at the MPA in 2005 by Springel 2005, to explore the predictions made by the most recent generation of semi-analytic models for galaxy formation. These models are incorporating a new mode of feedback from active galactic nuclei (AGN), which have their origins in super-massive black holes accreting mass and turning it into energy. Because of its observational signature in the radio regime this feedback is called "radio mode" and it counteracts the cooling flows of cold gas in undisturbed dark-matter haloes hosting galaxy clusters, which would otherwise show much higher star-formation of their central object than is observed. Previous work by Croton 2006 and De Lucia 2006 has shown that with the new semi-analytic model the population of local galaxies can be reproduced quite accurately. In order to study the evolution of the population out to higher redshifts, the semi-analytic predictions have been compared to a number of observations in various filter bands, in particular to two recent efforts to get a comprehensive multi-wavelength dataset of high redshift galaxies carried out by the DEEP2 (Davis 2001) and COSMOS (Scoville 2006) collaborations. The approach taken was to perform as broad a comparison as possible to gain firm constraints on the assumed physics in our model. Therefore a multitude of observational properties was contrasted with the model predictions such as clustering, luminosity functions, stellar mass functions, number counts per area and redshift to a certain magnitude limit. In order to facilitate the comparison between simulations and recent intermediate and high-redshift surveys, it is very useful to have a number of independent mock observations of the simulated galaxies, which provide good enough statistics to get a handle on cosmic variance. To this end I have devised a computer program that calculates the simulated galaxies lying on the backward light cone of a hypothetical observer out to arbitrarily high redshifts, taking advantage of the periodicity of the simulation box but avoiding replications. The output provides accurately interpolated redshifts, positions, observer frame and rest-frame magnitudes, dust extinction, as well as all the intrinsic galaxy properties like stellar mass and star formation rate. Utilising this tool it is also possible to make predictions for future galaxy surveys, deeper in magnitude and redshift than current ones. Presently the mock catalogues are used by the DEEP2 and COSMOS teams as a comparison sample in general and as a means to assess their selection effects and improve their data reduction in particular. First comparisons of counts in apparent magnitude and redshift gave promising results, showing good agreement in the low and intermediate range. The same holds for the angular clustering analysis except for the faintest magnitudes. Thus we conclude that our current understanding of the processes governing galaxy formation and evolution from the very first objects to the present day population is realistic but still incomplete. In particular the treatment of the interplay between star formation and negative feedback and the various processes influencing satellite galaxies in big galaxy clusters have potential for improvement. In the following I will give a brief outline of the thesis. After setting the stage for any kind of model in Chapter 1 by defining the geometry of the universe and the cosmological parameters that determine it, I will describe our semi-analytical model of galaxy formation in Chapter 2, where it will be also explained how to construct realistic mock observations of the simulated galaxies. First in Chapter 3 it will be verified that a simple model which assumes that galaxies are conserved but evolve in luminosity due to their star formation histories cannot account for the observed evolution of the galaxy population in the universe. This fact can be understood in the context of hierarchical models where massive and luminous galaxies assembled from smaller objects. Chapter 4 proceeds with exploring the predictions from the considerably more sophisticated semi-analytic model based on an N-body simulation of the hierarchical growth of dark matter structures. For this analysis a set of mock light-cones was constructed for direct comparison with the data which shows reasonably good agreement between model and observations at low redshift and for bright apparent magnitudes. These light-cones represent one of the largest samples of realistic mock observations currently available. They can be used for testing data analysis techniques usually applied to real observations on a well defined sample of artificial galaxies to verify how well the derivation of galaxy properties from the data works. In Chapter 5 we will demonstrate how one can measure the evolution of the galaxy merger rate from observing close projected galaxy pairs. Interestingly we find that the calibration needed for the conversion is significantly different from what has typically been assumed in previous studies. Additionally we will demonstrate that galaxy merger rates and dark-matter merger rates show considerably different evolution with redshift. Consequently we conclude that merger rate studies are less suitable as a probe of cosmic structure formation than initially assumed, but nonetheless they can be of great help to understand the formation and evolution of galaxies in a hierarchical universe. Finally these results will be summarised and discussed in Chapter 6 where I will also give a brief outlook on the future of this work, a short glimpse of which is already presented in the Appendix.

In this thesis, gravitational lensing in the concordance LambdaCDM cosmology is investigated by carrying out ray-tracing along past light cones through the Millennium Simulation, a very large N-body simulation of cosmic structure formation. The method used for tracing light rays substantially extends previous ray-tracing methods that are based on the Multiple-Lens-Plane approximation. Strong lensing is investigated by shooting random light rays through the Millennium Simulation. The probability is evaluated that an image of a small distant light source will be highly magnified, will be highly elongated or will be one of a set of multiple images. It is found that these probabilities increase strongly with increasing source redshift. It is shown that strong-lensing events can almost always be traced to a single dominant lensing object, and the mass and redshift distribution of these primary lenses is studied. The observed lens-mass range extends to lower masses than those found in earlier studies using simulations with lower spatial and mass resolution. Furthermore, effects of additional material along the line-of-sight are investigated. Although strong-lensing lines-of-sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few percent of the total surface density. The influence of stellar mass in galaxies on strong lensing is investigated by comparing the results obtained for lensing by dark matter alone to those obtained by also including the luminous matter. The dark-matter component of the lensing matter is constructed directly from the dark-matter particle distribution of the Millennium Simulation, while the luminous component is inferred from semi-analytic galaxy-formation models implemented within the evolving dark-matter distribution of the simulation. It is found that the inclusion of the stellar mass strongly enhances the probability for strong lensing compared to a 'dark-matter only' universe. The identification of the lenses associated with strong-lensing events reveals that the stellar mass of galaxies (i) significantly enhances the strong-lensing cross-sections of group and cluster halos, and (ii) gives rise to strong lensing in smaller halos, which would not produce noticeable effects in the absence of the stars. Even if only image splittings >10 arcsec are considered, the luminous matter can still enhance the strong-lensing optical depths by up to a factor of two. Finally, the potential capabilities of future radio telescopes for imaging the cosmic matter distribution are discussed. The Millennium Simulation is used to simulate large-area maps of the lensing convergence with the noise, resolution and redshift-weighting achievable with a variety of idealised surveys. It is shown that by observing lensing of 21-cm emission during reionization with a sufficiently large radio telescope, an image of the matter distribution could be obtained whose signal-to-noise far exceeds that of any map made using galaxy lensing. These mass images would allow the dark-matter halos of individual galaxies to be viewed directly, giving a wealth of statistical and morphological information about the relative distributions of mass and light. For telescopes like the planned Square Kilometre Array, mass imaging may be possible near the resolution limit of the core array of the telescope.