Podcasts about sloan digital sky survey sdss

In this episode I talk to KeShawn Ivory, a 2nd year PhD student in astrophysics at Vanderbilt University and a Fisk-Vanderbilt Master's-to-PhD Bridge scholar originally from Garland, TX. KeShawn's personal and professional choices are guided by his firm belief that research is meaningless if it can't be accessed by all who wish to participate, and shared with all who wish to learn. When he isn't working on galaxy mergers, KeShawn loves to write (mostly prose, sometimes music) and sing.Find KeShawn on Twitter @keshawnrantsReach out to KeShawn at keshawn.ivory@vanderbilt.eduReach out to AstronomerAnd: Twitter: @astronomerand Instagram: @astronomerandpodEmail: astronomerand@gmail.comShow Notes:One of KeShawn's Medium pieces: https://kekeivory.medium.com/post-grad-mythos-and-inevitable-disenchantment-e8330e99d5e2Rice Acapella Group: https://open.spotify.com/artist/6SWt8NggN8nn2Y2uUYoCQA?si=5rqxMKUBSga_FDSb0VTTnw&utm_source=copy-link Website for other organizations discussed: Vanderbilt Bridge Program: https://www.fisk-vanderbilt-bridge.orgVanderbilt Astronomy: https://as.vanderbilt.edu/physics-astronomy/ Fisk Astronomy: https://www.fisk.edu/academics/nsm/academic-department-of-life-and-physical-sciences/physics/ The Sloan Digital Sky Survey (SDSS): https://www.sdss.orgPython Module -- emcee: https://emcee.readthedocs.io/en/stable/Cover Art: iritmogilevsky.com, Instagram: irit.mog Theme Tune: Omar Chakor - https://www.fiverr.com/ch6k0rLand Acknowledgement: This episode was recorded in Nashville and we recognize that this land belonged to the Indigenous People before it came to be known as the state of Tennessee, or as the United States of America. We recognize that that Indigenous Peoples were the first residents of what we now know as the State of Tennessee and that Nashville is built upon the homelands and villages of the Indigenous Peoples of this region including the Shawnee, Cherokee, Choctaw, Chickasaw, and Yuchi tribal nations. Our ability to live and work in this state is the result of direct coercion, forced dispossession, and deliberate colonization. To ignore that is to perpetuate injustice to populations of people that continue to seek redistribution of land that was once theirs. We provide this land acknowledgement in recognition and respect of the indigenous people to whom this land truly belongs. We recognize that acknowledgment by itself is a small gesture, and that it becomes meaningful when coupled with authentic relationship and informed action. Inclusivity Statement: I wish for this podcast to be a safe space for those of every gender expression, sexual orientation, ability, race, religious group and age group. I am always trying to learn and improve on my unconscious bias. Any suggestions for improvement will be gratefully received at astronomerand@gmail.com.

Chemodynamical Analysis of Metal-rich High-eccentricity Stars in the Milky Way's Disk by Ayeon Lee et al. on Monday 17 October We present a chemodynamical analysis of 11,562 metal-rich, high-eccentricity halo-like main-sequence (MS) stars, which has been referred to as the Splash or Splashed Disk, selected from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). When divided into two groups, a low-[$alpha$/Fe] population (LAP) and a high-[$alpha$/Fe] population (HAP), based on kinematics and chemistry, we find that they exhibit very distinct properties, indicative of different origins. From a detailed analysis of their orbital inclinations, we suggest that the HAP arises from a large fraction (~ 90%) of heated disk stars and a small fraction (~ 10%) of in situ stars from a starburst population, likely induced by interaction of the Milky Way with Gaia Sausage/Enceladus (GSE) or other early merger. The LAP comprises about half accreted stars from the GSE and half formed by the GSE-induced starburst. Our findings further imply that the Splash stars in our sample originated from at least three different mechanisms - accretion, disk heating, and a merger induced starburst. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2210.07905v1

Chemodynamical Analysis of Metal-rich High-eccentricity Stars in the Milky Way's Disk by Ayeon Lee et al. on Monday 17 October We present a chemodynamical analysis of 11,562 metal-rich, high-eccentricity halo-like main-sequence (MS) stars, which has been referred to as the Splash or Splashed Disk, selected from Sloan Digital Sky Survey (SDSS) and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). When divided into two groups, a low-[$alpha$/Fe] population (LAP) and a high-[$alpha$/Fe] population (HAP), based on kinematics and chemistry, we find that they exhibit very distinct properties, indicative of different origins. From a detailed analysis of their orbital inclinations, we suggest that the HAP arises from a large fraction (~ 90%) of heated disk stars and a small fraction (~ 10%) of in situ stars from a starburst population, likely induced by interaction of the Milky Way with Gaia Sausage/Enceladus (GSE) or other early merger. The LAP comprises about half accreted stars from the GSE and half formed by the GSE-induced starburst. Our findings further imply that the Splash stars in our sample originated from at least three different mechanisms - accretion, disk heating, and a merger induced starburst. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2210.07905v1

Efficient Analysis Routines for Single and Double Peaked Type 2 AGN Spectra by Matthew Selwood et al. on Tuesday 11 October Driven by the imminent need to rapidly process and classify millions of AGN spectra drawn from next generation astronomical facilities, we present a spectral fitting routine for Type 2 AGN spectra optimised for high volume processing, using the Quasar Spectral Fitting library (QSFit). We analyse an optically selected sample of 813 luminous Type 2 AGN spectra at $z < 0.83$ from the Sloan Digital Sky Survey (SDSS) to qualify its performance. We report a median narrow line H$alpha$/H$beta$ Balmer decrement of 4.5$pm$0.8, alluding to the presence of dust in the narrow line region (NLR). We publish a specialised QSFit fitting routine for high signal to noise ratio spectra and general fitting routine for double peaked Type 2 AGN spectra applied on a sub-sample of 45 spectra from our parent sample. We report a median red and blue peak velocity separation of 390$pm$60kms$^{-1}$. No trend is found for red or blue peaks to exhibit systematically different luminosity or ionization properties. Emission line diagnostics show that the double peaks in all sources are illuminated by an AGN-powered ionizing continuum. Finally, we examine the morphology of host galaxies of our double peaked sample. We find double peaked Type 2 AGN reside in merging systems at a comparable frequency to single peaked AGN. This suggests that the double peaked AGN phenomenon is likely to have a bi-conical outflow origin in the majority of cases. We publicly release the code used for spectral analysis and produced catalogues used in this work. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2210.04827v1

Efficient Analysis Routines for Single and Double Peaked Type 2 AGN Spectra by Matthew Selwood et al. on Tuesday 11 October Driven by the imminent need to rapidly process and classify millions of AGN spectra drawn from next generation astronomical facilities, we present a spectral fitting routine for Type 2 AGN spectra optimised for high volume processing, using the Quasar Spectral Fitting library (QSFit). We analyse an optically selected sample of 813 luminous Type 2 AGN spectra at $z < 0.83$ from the Sloan Digital Sky Survey (SDSS) to qualify its performance. We report a median narrow line H$alpha$/H$beta$ Balmer decrement of 4.5$pm$0.8, alluding to the presence of dust in the narrow line region (NLR). We publish a specialised QSFit fitting routine for high signal to noise ratio spectra and general fitting routine for double peaked Type 2 AGN spectra applied on a sub-sample of 45 spectra from our parent sample. We report a median red and blue peak velocity separation of 390$pm$60kms$^{-1}$. No trend is found for red or blue peaks to exhibit systematically different luminosity or ionization properties. Emission line diagnostics show that the double peaks in all sources are illuminated by an AGN-powered ionizing continuum. Finally, we examine the morphology of host galaxies of our double peaked sample. We find double peaked Type 2 AGN reside in merging systems at a comparable frequency to single peaked AGN. This suggests that the double peaked AGN phenomenon is likely to have a bi-conical outflow origin in the majority of cases. We publicly release the code used for spectral analysis and produced catalogues used in this work. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2210.04827v1

Galaxy Spin Classification I: Z-wise vs S-wise Spirals With Chirality Equivariant Residual Network by He Jia et al. on Monday 10 October The angular momentum of galaxies (galaxy spin) contains rich information about the initial condition of the Universe, yet it is challenging to efficiently measure the spin direction for the tremendous amount of galaxies that are being mapped by the ongoing and forthcoming cosmological surveys. We present a machine learning based classifier for the Z-wise vs S-wise spirals, which can help to break the degeneracy in the galaxy spin direction measurement. The proposed Chirality Equivariant Residual Network (CE-ResNet) is manifestly equivariant under a reflection of the input image, which guarantees that there is no inherent asymmetry between the Z-wise and S-wise probability estimators. We train the model with Sloan Digital Sky Survey (SDSS) images, with the training labels given by the Galaxy Zoo 1 (GZ1) project. A combination of data augmentation tricks are used during the training, making the model more robust to be applied to other surveys. We find a $sim!30%$ increase of both types of spirals when Dark Energy Spectroscopic Instrument (DESI) images are used for classification, due to the better imaging quality of DESI. We verify that the $sim!7sigma$ difference between the numbers of Z-wise and S-wise spirals is due to human bias, since the discrepancy drops to $

The luminosity function of ringed galaxies by Daniil V. Smirnov et al. on Thursday 15 September We perform an analysis of the luminosity functions (LFs) of two types of ringed galaxies -- polar-ring galaxies and collisional ring galaxies -- using data from the Sloan Digital Sky Survey (SDSS). Both classes of galaxies were formed as a result of interaction with their environment and they are very rare objects. We constructed LFs of galaxies by different methods and found their approximations by the Schechter function. The luminosity functions of both types of galaxies show a systematic fall-off at low luminosities. The polar structures around bright ($M_r leq -20^m$) and red ($g-r > +0.8$) galaxies are about twice as common as around blue ones. The LF of collisional rings is shifted towards brighter luminosities compared to polar-ring galaxies. We analysed the published data on the ringed galaxies in several deep fields and confirmed the increase in their volume density with redshift: up to z$sim$1 their density grows as $(1+z)^m$, where $m gtrsim 5$. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.06875v1

Fundamentals of Differential and All-Sky Aperture Photometry Analysis for an Open Cluster by Kanwar Preet Kaur et al. on Wednesday 07 September This article provides detailed description on the fundamentals of aperture photometry analysis. The differential and all-sky aperture photometry techniques are described thoroughly to depict the difference between the two techniques and their selection for determining the stars' magnitudes and their respective magnitude errors. The crucial calibration parameters required for the all-sky photometry analysis such as atmospheric extinctioncoefficient, air-mass, zero point, color term and color index are discussed comprehensively with their extraction from the Sloan Digital Sky Survey (SDSS) archive. The all-sky aperture photometry technique is applied on the stars of an open cluster NGC 2420 to determine their calibrated magnitudes and magnitude errors in the g, r, and i bands. The images required for the analysis are extracted from data release DR12 of SDSS III archive. Herein, the photometry analysis is performed by the Makali'i: SUBARU Image Processor, a Windows-based software. This software has a simple yet effective GUI and it provides the starlight minus the background sky light value with a single click. This article would aid in providing the insight into the physics of aperture photometry by manually scanning the astronomical images. In addition, the g, r, and i magnitudes are transformed to B, V, and R band magnitudes of Johnson-Cousins UBVRI photometric system. The color magnitude diagram for both the standard photometry systems are also provided. arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2209.03015v1

durée : 00:55:04 - La Terre au carré - Le 20 juillet dernier, des astrophysiciens ont publié la plus grande carte tridimensionnelle de l’Univers, résultat de 20 ans d’analyse de plus de quatre millions de galaxies et de quasars ultra-lumineux avec le programme de relevés astronomique de la collaboration Sloan Digital Sky Survey (SDSS).

Voilà des milliers d'années que les humains étudient les étoiles, mais l'astrophysicienne Juna Kollmeier a une mission très spéciale : créer les cartes en 3D de l'univers les plus détaillées qui aient jamais été faites. Voyagez à travers le cosmos grâce au récit de ses travaux d'équipe au service du projet Sloan Digital Sky Survey (SDSS), qui cartographie des millions d'étoiles, de trous noirs et de galaxies à un niveau de détail sans précédent. Si nous continuons à ce rythme, dit-elle, nous pouvons cartographier toutes les grandes galaxies de l'univers observable d'ici 2060 : « Nous sommes passés des représentations du ciel à l'aide de coquilles de palourdes à la relativité générale en quelques milliers d'années. Il nous suffit de quarante ans de plus pour pouvoir cartographier toutes les galaxies. »

Une équipe d'astronomes de la collaboration Sloan Digital Sky Survey (SDSS) vient de rendre publiques de nouvelles mesures de masse de 62 trous noirs supermassifs peuplant des quasars lointains. C'est la première fois que la masse de ce type de trous noirs si éloignés est mesurée directement.

The determination of photometric redshifts is essential for many subjects in cosmology and extragalactic astronomy, like the large scale structure of the Universe, gravitational lensing, or galaxy evolution. If the spectral energy distribution (SED) of a galaxy is measured with high enough spectral resolution, the redshift can be easily derived through the absorption and emission lines which are created by the elements in the galaxy. However, currently more telescopes are equipped with large cameras with charged coupled devices (CCDs) that observe the sky through optical filters. With these photometric observations it is possible to detect much fainter astronomical objects than with spectroscopy. Furthermore, photometric observations are less time consuming and cheaper in comparison, wherefore they are preferentially used for observations of statistical meaningful cosmological volumes. Nonetheless, photometric data, which are often gained by observations through broadband filters, are not as precisely resolved as spectra. Therefore one does not have information about the accurate position in wavelength of spectral lines, but only about the overall shape of the SED. This is the reason why so-called photometric redshifts have to be derived by statistical means. One approach to estimate the redshift through photometry alone are template fitting methods which compare the fluxes predicted by model spectra with the observations. After that, a likelihood analysis is performed with which a probability density function P(z) and the most probable value of z can be derived. To achieve high accuracies with photometric redshift template fitting techniques, the model spectra as well as their corresponding prior probabilities have to be chosen carefully. In this work I use photometric and spectroscopic data of luminous red galaxies from the Sloan Digital Sky Survey (SDSS). I analyze the precision of photometric redshifts estimated with model SEDs specifically designed to match the set of luminous red galaxies of SDSS-II at redshifts z ≤ 0.5 in color and I compare them with published results. These models were created without information on their properties at wavelengths shorter than the SDSS u band. However, the galaxy UV characteristics derived from the model SEDs match those of other observations. Furthermore, I investigate the SED properties derived from the best fitting models with respect to spectroscopic data as functions of redshift and luminosity. At lower redshifts less luminous galaxies from our sample on average show increased signs of star formation in comparison to galaxies with higher luminosities. This is supported by analyses of the line strengths in the spectra. Moreover, star formation activity increases with increasing redshift which is caused by the aging of the galaxy population from higher to lower redshifts. I also generate model spectra for red galaxies from the SDSS-III located at even higher redshifts 0.45 ≤ z ≤ 0.9. For this I modify the shape of theoretical spectra to match the data of the analyzed galaxies to a better extent. The multidimensional space defined by the colors and the absolute magnitude of the galaxies is reduced to two dimensions through a self-organizing map. The map is then partitioned by a k-means algorithm which identifies clusters in the data. From the cluster cells I select model spectra which represent the galaxies from within the same cell. A selection of the models is then used as a template set for photometric redshift estimation. I find that our models improve the redshift accuracy in comparison to the results published by SDSS.

The chemical compositions of the stars and gas in galaxies play a significant role in all their key evolutionary processes, from gas cooling, through star formation, to the production of new heavy elements that are released back into the gas as stars die in supernova explosions. A theoretical explanation of the production of elements heavier than helium (known simply as `metals' in astrophysics) in stars and its distribution throughout galaxies has been developing since the first postulation of stellar nucleosynthesis in the 1920s. However, there are still a number of unanswered questions in the field of galactic chemical evolution (GCE). For example, what is the most accurate way to measure the metallicities in galaxies? What are the relative contributions to GCE from different types of stars? How is this metal-rich material circulated throughout the various components of a galaxy? And how can we explain the seemingly incompatible chemical properties observed in different galaxies in the local Universe? This thesis provides an investigation into the chemical enrichment of galaxies, by utilising both observations of nearby galaxies and sophisticated GCE models within a semi-analytic model of galaxy evolution. Its core aims are a) to better quantify the chemical properties seen in low-redshift galaxies and explain there likely causes, and b) to develop an improved GCE model that can simultaneously reproduce the diverse chemical properties seen in different types of galaxies in the local Universe. With these aims in mind, Chapter 1 outlines the key background knowledge required for such an investigation. It discusses the different methods used for measuring the metallicity of real galaxies, and their various shortcomings. It also describes simple, analytic GCE models, and the sophisticated semi-analytic model, L-Galaxies, that is used to simulate galaxy evolution in detail. In Chapters 2 and 3, I provide an investigation into the relation between stellar mass (M*), star formation rate (SFR), and gas-phase metallicity (Zg) in galaxies. It is shown that the L-Galaxies model reproduces the positive correlation between SFR and Zg in massive galaxies that is seen when using sophisticated, theoretical metallicity diagnostics. This lends support to the use of such diagnostics over simpler, emission-line ratios. It is further shown that, in the semi-analytic model, this SFR-Zg correlation is due to the gradual dilution of the gas in low-SFR, elliptical galaxies, after a gas-rich merger event. A number of signatures of this particular evolution can be seen in these model galaxies at redshift zero, including low gas fractions and low values of (Zg-Z*). Crucially, all of these properties are also seen in nearby elliptical galaxies in the Sloan Digital Sky Survey (SDSS), providing indirect evidence that such an evolutionary process is also occurring in the elliptical galaxy population in the real Universe. In Chapter 4, I present a new, sophisticated GCE model implemented into L-Galaxies, that significantly improves on the previous scheme. It does this by accounting for the delayed enrichment of many chemical elements from stars, of various initial masses and metallicities, via stellar winds and supernovae. This new scheme enables a much more detailed study of the chemical evolution of galaxies, and enables a comparison with a larger range of observational data. In Chapter 5, I demonstrate that this new model is able to simultaneously reproduce the chemical properties observed in a) the gas of local, star-forming galaxies, b) the photospheres of G dwarfs in the Milky Way disc, and c) the integrated stellar populations of nearby elliptical galaxies. Furthermore, the model is able to do this without any significant deviation from the standard framework of galaxy formation in the canonical paradigm of hierarchical structure formation. This can be seen as a significant achievement, which has allowed us to form a much more comprehensive view of GCE than was possible before.

Lawrence Berkeley National Laboratory physicist and dark energy hunter David Schlegel chats with Sabin Russell, former San Francisco Chronicle reporter turned Berkeley Lab science writer. David Schlegel is an astrophysicist at the Lawrence Berkeley National Laboratory and the principal investigator of Baryon Oscillation Spectroscopic Survey (BOSS), the largest of four night-sky surveys being conducted as part of the Sloan Digital Sky Survey (SDSS). Series: "Lawrence Berkeley National Laboratory " [Science] [Show ID: 22550]

Lawrence Berkeley National Laboratory physicist and dark energy hunter David Schlegel chats with Sabin Russell, former San Francisco Chronicle reporter turned Berkeley Lab science writer. David Schlegel is an astrophysicist at the Lawrence Berkeley National Laboratory and the principal investigator of Baryon Oscillation Spectroscopic Survey (BOSS), the largest of four night-sky surveys being conducted as part of the Sloan Digital Sky Survey (SDSS). Series: "Lawrence Berkeley National Laboratory " [Science] [Show ID: 22550]

Lawrence Berkeley National Laboratory physicist and dark energy hunter David Schlegel chats with Sabin Russell, former San Francisco Chronicle reporter turned Berkeley Lab science writer. David Schlegel is an astrophysicist at the Lawrence Berkeley National Laboratory and the principal investigator of Baryon Oscillation Spectroscopic Survey (BOSS), the largest of four night-sky surveys being conducted as part of the Sloan Digital Sky Survey (SDSS). Series: "Lawrence Berkeley National Laboratory " [Science] [Show ID: 22550]

Lawrence Berkeley National Laboratory physicist and dark energy hunter David Schlegel chats with Sabin Russell, former San Francisco Chronicle reporter turned Berkeley Lab science writer. David Schlegel is an astrophysicist at the Lawrence Berkeley National Laboratory and the principal investigator of Baryon Oscillation Spectroscopic Survey (BOSS), the largest of four night-sky surveys being conducted as part of the Sloan Digital Sky Survey (SDSS). Series: "Lawrence Berkeley National Laboratory " [Science] [Show ID: 22550]

Despite being a minority among the total population of galaxies, radio galaxies have gained increasing attention, because of the energetic feedback they can provide to the surrounding environment. These systems host an active nucleus that produces large amounts of energy in the form of radio/X-ray emitting jets, injecting energy that in some cases can balance the radiative loses of the gas that cools and condenses within massive halos of dark matter. Therefore, if we want to understand galaxy formation and evolution, we have to understand the role of active galactic nuclei (AGN) in the galaxy assembly process. My work concentrated, first, on constructing a complete sample of radio galaxies. This is not an easy task as they have very different morphologies in different surveys, and ofter break up into separate components. These components must be correctly associated with a unique optical galaxy. We cross-correlated two different radio surveys (NVSS and FIRST) with a sample of luminous red galaxies derived from the Sloan Digital Sky Survey (SDSS). The NVSS provides accurate flux measurements for extended sources, while the angular resolution of FIRST allows the host galaxy to be identified accurately. We also improved the matching of sources below the nominal 1 mJy detection limit of FIRST, to increase the reliability and completeness of the final catalogue. These techniques allowed us to assemble the largest radio galaxy catalogue to date, consisting of around 14,000 radio–loud AGN at intermediate redshifts (0.4 < z < 0.8), with 1.4 GHz fluxes above 3.5 mJy. The matching criteria were tested and refined using Monte–Carlo simulations, leading to an estimated reliability of ∼98.3% and completeness level of about 95% for our catalogue. With this catalog in hand, we were able to compare radio galaxies at z∼0.55 with similar samples in the local universe. We studied the evolutionary properties of radio galaxies, how their spatial density changes as function of time and how the fraction of radio emitting sources varies with galaxies properties such as stellar mass, radio luminosity and redshift. We present a new determination of the luminosity function of radio AGN at z∼0.55 and compare this to the luminosity function of nearby (z∼0.1) radio sources from the SDSS main survey. The comoving number density of radio AGN with luminosities less than 10^25 W/Hz increases by a factor ∼1.5 between z=0.1 and z=0.55. At higher luminosities, this factor increases sharply, reaching values of more than 10 at radio luminosities larger than 10^26 W/Hz. We then study how the relation between radio AGN and their host galaxies evolves with redshift. Our main conclusion is that the fraction of radio–loud AGN increases towards higher redshift in all massive galaxies, but the evolution is particularly strong for the lower mass galaxies in our sample. These trends may be understood if there are two classes of radio galaxies (likely associated with the “radio” and “quasar mode” dichotomy) that have different fuelling/triggering mechanisms and hence evolve in different ways. We conclude that stellar mass seems to be the a very important factor in deciding whether a galaxy develops bright, powerful radio jets. There is also the suggestion that the environment of a galaxy is also crucial in deciding whether it becomes radio-loud. To address this question, we studied how radio galaxies are clustered in the universe and quantified the clustering dependence on stellar mass and radio power. We do this by computing the cross-correlation function between radio galaxies and the parent LRG population. In order to isolate the true clustering of RLAGN, we compare respect to control radio-quiet galaxies selected with the same properties as radio AGN. The main result is that RLAGN are significantly more clustered than radio-quiet objects, particularly below ∼1 Mpc/h, indicating that the gaseous environment of a radio sources at the scale of its dark matter halo is important in modulating the observed output power and determining its radio loudness. Unification models predict that the environments of radio-loud galaxies and radio-loud quasars should be equivalent, as they represent the same object that is observed at different orientations with respect to the line of sight. We have compared our clustering measurements for these two types of objects, setting important restrictions on the conditions that must be met if the models are valid. We find evidence that the idea of unification can hold only for the most luminous radio galaxies in our sample, with radio power above ∼10^26 W/Hz. Finally, we have also modeled the radio-loud population of AGN at z=0.5 by applying a simple prescription for the distribution of radio galaxies in dark matter haloes extracted from N-body simulations. This proves the applicability at high redshift of models that have been shown to work well in the local universe and test directly the radio-mode heating recipe implemented in semianalyitcal models of galaxy formation. By combining an estimate for the mechanical power of radio sources with our determination of the luminosity function we have been able to estimate the heating effect that RLAGN produce in haloes of different mass. The heating power in haloes of ∼10^15 M_sun/h, the most massive in our sample, is a factor of 6-7 larger than in less massive systems of ∼10^13 M_sun/h, and a factor of ∼2.5 larger at z=0.5 than at in the local universe. This result can be used to improve the implementation of radio feedback in models of galaxy formation.

Galaxy clusters are the largest gravitationally bound systems in the universe. Clusters consist of three components: galaxies, gas, and dark matter. The galaxies themselves contribute the least, at most a few percent, to the total mass. The remainder consists of diffuse, hot gas (the intracluster medium, or ICM) and an unseen component which is needed to explain the gravitational stability of clusters (the dark matter). The two most obvious means of studying clusters of galaxies are by observing the optical light emitted from the constituent galaxies or the X-ray emission from the ICM. Clusters of galaxies, bound ensambles of hundreds of galaxies, are an ideal environment to study galaxy evolution and to learn how this is affected by different physical processes: gravity, starbursts and star formation, interactions with the intergalactic medium and galaxy-galaxy encounters. Since the very early works of Hubble in the thirties, it has been recognized that galaxies in dense environments differ systematically from those in low-density regions in their morphological types, stellar populations and gaseous content. When during the history of the Universe and why such environmental differences were established is currently one of the subjects of most intensive investigation in the international astrophysical community. On the other hand, clusters can teach us a great deal about cosmology. The distributions of galaxies on the sky shows a net-like structure in which thin walls and filaments surround large voids. The galaxy clusters are the nodes of this network. Therefore, they trace out the Large-Scale Structure (LSS) of the universe and can be used to study the LSS formation. Moreover, if clusters provide a 'fair sample' of the universe, then the fraction of their mass in baryons should equal the universal baryon fraction, known as $Omega_b/Omega_m$. Moreover, the evolution of cluster number density with redshift can determine the mass density parameter, known as $Omega_M$, and possibly determine the equation of state (and nature) of the dark energy believed to be causing the expansion of the universe to accelerate. Thus, galaxy clusters have a twofold importance: first as laboratories of galaxy formation and evolution, and second as cosmological tool. The aim of this project is to study galaxy clusters from these two perspectives. For this purpose we use the largest optical and X-ray surveys ever realized, the Sloan Digital Sky Survey (SDSS) and the Rosat All Sky Survey (RASS), respectively, to conduct a multiwavelenght study of the properties of galaxy clusters. The project is called RASS-SDSS Galaxy Cluster Survey reflecting the name of the two big surveys used for this work. All the analyses are performed on two cluster samples specially created for the survey: the X-ray selected RASS-SDSS galaxy cluster catalog and a subsample of optically selected, isolated and spectroscopically confirmed Abell clusters. The project consists of two parts. The aim of the first part is to understand which role play the gravitational processes, galaxy mergers and collisions and the interaction with ICM in the process of galaxy formation and evolution. For this purpose, we study the variations of several properties of the cluster galaxy population such as the luminosity and spatial distribution, the morphological type mix, the Star Formation Rate (SFR) and stellar mass as a function of the environmental conditions and the cluster global properties. Our detailed analysis of the cluster individual and composite luminosity functions reveals that the LF clearly shows a bimodal behavior with an upturn and a evident steepening in the faint magnitude range in any SDSS band. The LF is well fitted by the sum of two Schechter functions. The bright end of the LF is found to be universal in all the clusters. The faint end of the LF is much steeper and varies significantly from system to system, when calculated within a fixed metric aperture. The variations are not ramdom however. The more massive a cluster, the lower its fraction of dwarf galaxies. This effect disappears when the cluster LF is calculated within the physical size of the system, as the virial radius ($r_{200}$). This indicates that the previously observed variations are due to aperture effects caused by the observed increase of the fraction of dwarf galaxies with the clustercentric distance. Our conclusion is that the shape of the cluster LF is universal in all the magnitude ranges when the LF is calculated within the virial region. Moreover, the analysis of the composite cluster LF per morphological type, shows that the upturn and the steepening at the faint end of the LF is caused by dwarf early type galaxies. These systems are quite rare in low density regions and appear to be a typical cluster population. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds $sim 500$ times the critical density of the Universe. We interpret our results in the context of the 'harassment' scenario, where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies. In the same context, we investigate whether the cluster total star formation rate ($Sigma SFR$) depends on the cluster global properties for a sample of 90 very nearby clusters. The total cluster SFR is given by the sum of the SFR of all the cluster members within the virial region. It is found to be proportional to the number of cluster galaxies involved ($N_{gal}$). The best relation between the total SFR and the cluster mass reflects the $N_{gal}-M$ relation, which is a power law with exponent smaller than 1. As a consequence, the more massive a cluster, the lower its number of cluster galaxies and total SFR per unit mass. The mean SFR per cluster galaxy ($Sigma SFR/N_{gal}$) is constant troughout our cluster sample and does not depend on the global properties of the system. Moreover, in order to account for projection effects, we study the galaxy surface number density profile in our cluster sample. We find that clusters of different mass exibit different profiles. In the low and intermediate mass systems the best fit is provided by a core King profile, with the core radius decreasing with cluster mass, until, at the highest cluster masses, the profile is better represented by a cuspy Navarro, Frenk & White profile. All these different analysis converge to the conclusion that the global properties of the cluster galaxy population, such as the luminosity distribution, the galaxy type mix, the mean and total cluster SFR are only weakly dependent on the cluster mass and X-ray luminosity. This suggests that the gravitational processes and the interaction galaxy-ICM are not likely to affect those properties of the cluster galaxy population. Only the spatial distribution of the cluster galaxies depends on the cluster mass, probably reflecting the different relaxation status of systems of different masses. Instead, the variations of the LF and the galaxy type mix with the clustercentric distance reflect a link between the galaxy formation process and the galaxy-galaxy encounters, as suggested by the 'harassment' scenario. In the second part of the thesis, galaxy clusters are used as cosmological tool. The aim of this work is to elucidate which component, galaxies or ICM, traces better the cluster mass in order to understand whether different selection methods select the same cluster population. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. For this porpuse, we analyse as a first step the relation between the optical ($L_{op}$) and the X-ray ($L_X$) luminosity, respectively, to the cluster mass in the X-ray selected RASS-SDSS cluster sample. The main motivation in deriving these dependences is to evaluate $L_{op}$ and $L_X$, as predictors of the cluster mass and to compare the quality of the two quantities as predictors. Our analysis reveals that $L_{op}$ is a key measure of the cluster mass. In this respect, the optical luminosity performs even better than the X-ray luminosity, which suggests that the mass distribution of a cluster is better traced by cluster galaxies rather than by intracluster gas. On the other hand, our conclusion is at odds with the generally accepted view that a cluster main physical properties are more easily revealed in the X-ray than in the optical. Such a view was established at an epoch when the lack of optical wide field surveys precluded a reliable determination of the optical luminosities of a large sample of clusters. With the advent of the Sloan Digital Sky survey, this problem is now overcome. The application of the same analysis to an optically selected cluster sample (the Abell subsample) confirms the result. Neverthless, the Abell sample comprises a subpopulation of systems which scatter significantly in the $L_X-M$ relation and appear to be extremely X-ray underluminous (on average one order of magnitude) with regard to their mass. On the other hand, these systems do follow the general scaling relation between optical luminosity and virial mass. Therefore, we call them 'Abell X-ray Underluminous clusters' or AXU clusters for short. To understand the particular nature of these systems, we examine the properties of their galaxy population. The velocity distribution of the AXU clusters is Gaussian within the virial region but is leptokurtic (more centrally concentrated than a Gaussian) in the outskirts, as expected for the systems in accretion. In addition, the AXU clusters have a higher fraction of blue galaxies in the external region and show a marginally significant paucity of galaxies at the center. Our results seem to support the interpretation suggested by Bower et al. (1997) that the AXU clusters are systems in formation undergoing a phase of mass accretion. Their low X-ray luminosity should be due to the still accreting Intracluster gas or to an ongoing merging process. Our results give supports to the conclusion of Donahue et al. (2002) concerning the biases inherent in the selection of galaxy clusters in different wavebands. While the optical selection is prone to substantial projection effects, also the X-ray selection is not perfect or not simple to characterize. The existence of X-ray underluminous clusters, even with large masses, makes it difficult to reach the needed completeness in mass for cosmological studies. Clearly, a multi-waveband approach is needed for optimizing the completeness and reliability of clusters samples. The 'RASS-SDSS Galaxy Clusters Survey' series comprises 7 scientific papers which are inserted as part of the thesis. Four of the papers are accepted for pubblication on a scientific Journal ('Astronomy & Astrophysics') and three are submitted.

Die moderne Astrophysik steht vor der Herausforderung, neueste Beobachtungen mit den theoretischen und numerischen Modellen der Galaxienentstehung und -entwicklung zu konfrontieren. So hofft man, die wichtigsten physikalischen Prozesse und ihre Zeitskalen identifizieren zu koennen. In dieser Arbeit nehmen wir eine komplette, helligkeits--limitierte Auswahl von 1862 Galaxien aus der Sloan Digital Sky Survey (SDSS), um eine Anzahl von globalen und strukturellen Parametern zu untersuchen. Diese Auswahl beinhaltet helle Objekte mit einer r--Band Helligkeit von < 15.9 im nahen Universum mit einer Rotverschiebung von z < 0.12. Sie enthaelt elliptische, Spiral- und irregulaere Galaxien. Photometrische Daten sind fuer die u, g, r, i und z--Baender angegeben und von 1588 Galaxien wurden nachtraeglich Spektra genommen. Die `Bulge' Komponente der Galaxien wird mit Sersic und de Vaucouleurs Modellen modelliert, waehrend die Scheibenkomponente mit einer exponentiellen Verteilung modelliert wird. Die Messung des Lichtanteils in `Bulge' und Scheibenkomponente gibt Aufschluss ueber die Effizienz des hierarchischen Strukturbildungsprozesses. In Kapitel 3 zeigen wir, dass der mittlere Anteil des Lichts aus der Scheibe stark mit der totalen absoluten Helligkeit der Galaxie zunimmt. Unabhaengige r und i Band Analysen ergeben einen sehr aehnlichen Trend. Zum ersten Mal schaetzen wir den volumengemittelten Anteil des Lichts aus der Scheibenkomponente von Galaxien ab und stellen fest, dass ungefaehr (55 +- 2) % des gesamten Lichts im lokalen Universum aus Scheiben kommt. Wir ermitteln auch die Leuchtkraftfunktion fuer reine 'Bulges', also fuer Strukturen ohne Scheibenanteil, die nicht einfache Spheroide sind. In Kapitel 4 studieren wir die Abhaengigkeiten von visuellen und quantitativen morphologischen Klassifikationskriterien mit dem Ziel sauberere Galaxienkataloge zu erstellen, besonders bei hohen Rotverschiebungen, wo die Klassifikation schwierig ist. Wir finden, dass Galaxienfarben, effektive Oberflaechenhelligkeit, Masse/Licht Anteil, und Asymmetrie Parameter einen Mehrparameter Raum aufspannen, in der alle Galaxien je nach morphologischem Typ eindeutig positioniert sind. In Kapitel 5 beobachten wir einen klaren Trend, mit dem die Skalenlaenge der Scheiben mit ihrer Helligkeit zunimmt, und dieser Trend ist unabhaengig vom photometrischen Band und der morphologischen Klasse. Es existiert auch eine klare Abhaengigkeit zwischen dem effektiven Radius des `Bulge' und seiner Helligkeit, aber die Steigung dieser Relation aendert sich mit dem morphologischem Typ. Sie ist steiler fuer fruehere Typus, was uns zu der Schlussfolgerung fuehrt, dass die Skalenlaenge weniger von der Morphologie abhaengt als die Skalenlaenge des `Bulges'. Dies legt nahe, dass `Bulges' in fruehen und spaeteren Galaxien in unterschiedlichen Prozessen gebildet werden. Wir finden auch eine Korrelation zwischen den strukturellen Parametern von Scheiben und `Bulges', insbesondere zwischen effektivem Radius der `Bulges' und der Skalenlaenge der Scheiben in Systemen fruehen Typus. Wir interpretieren dies als Beweisstueck zugunsten von saekularen Evolutionsmodellen.