Ancient Light from Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-Clusters

For a complete picture of galaxy cluster formation, it is important that we start probing the early epoch of z~2-7 during which clusters and their galaxies first began to form. Because the study of these so-called "proto-clusters" is currently limited by small number statistics, widely varying selection techniques and assumptions, we have performed a systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in ~3,000 clusters from the earliest times to z=0. We define an effective radius R_e for proto-clusters and characterize their growth in size and mass. We show that the progenitor regions of galaxy clusters (M>10^14 M_sun/h) can already be identified at least up to z~5, provided that the galaxy overdensities, delta_gal, are measured on a sufficiently large scale (R_e~5-10 cMpc). We present the overdensities in matter, DM halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the structures found in real surveys. We derive the probability that a structure having a delta_gal, defined by a set of observational selection criteria, is indeed a proto-cluster, and show how their z=0 masses can already be estimated long before virialization. Galaxy overdensity profiles as a function of radius are presented. We further show how their projected surface overdensities decrease as the uncertainties in redshift measurements increase. We provide a table of proto-cluster candidates selected from the literature, and discuss their properties in the light of our simulations predictions. This work provides the general framework that will allow us to extend the study of cluster formation out to much higher redshifts using the large number of proto-clusters that are expected to be discovered in, e.g., the upcoming HETDEX and HSC surveys.Comment: 16 pages, 13 figures, 5 tables; Published in Ap

Discovery of a large number of candidate proto-clusters traced by ~15 Mpc-scale galaxy overdensities in COSMOS

To demonstrate the feasibility of studying the epoch of massive galaxy cluster formation in a more systematic manner using current and future galaxy surveys, we report the discovery of a large sample of proto-cluster candidates in the 1.62 deg^2 COSMOS/UltraVISTA field traced by optical/IR selected galaxies using photometric redshifts. By comparing properly smoothed 3D galaxy density maps of the observations and a set of matched simulations incorporating the dominant observational effects (galaxy selection and photometric redshift uncertainties), we first confirm that the observed ~15 comoving Mpc scale galaxy clustering is consistent with LCDM models. Using further the relation between high-z overdensity and the present day cluster mass calibrated in these matched simulations, we found 36 candidate structures at 1.6<z<3.1, showing overdensities consistent with the progenitors of M_z=0 ~10^15 M_sun clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M_z=0 ~10^14 M_sun are ~70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true proto-cluster members that will merge into a cluster-scale halo by z=0. With solely photometric redshifts, we successfully rediscover two spectroscopically confirmed structures in this field, suggesting that our algorithm is robust. This work generates a large sample of uniformly-selected proto-cluster candidates, providing rich targets for spectroscopic follow-up and subsequent studies of cluster formation. Meanwhile, it demonstrates the potential for probing early cluster formation with upcoming redshift surveys such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime Focus Spectrograph survey.Comment: 6 pages, 3 figures, 1 table; Accepted for publication in ApJ

Emergence of cosmic structures around distant radio galaxies and quasars

This thesis presents observational evidence for the formation of galaxy clusters associated with distant radio galaxies and quasars.Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UBL - phd migration 201

Lyman Break Analogs: Constraints on the Formation of Extreme Starbursts at Low and High Redshift

Lyman Break Analogs (LBAs), characterized by high far-UV luminosities and surface brightnesses as detected by GALEX, are intensely star-forming galaxies in the low-redshift universe ($z\sim 0.2$), with star formation rates reaching up to 50 times that of the Milky Way. These objects present metallicities, morphologies and other physical properties similar to higher redshift Lyman Break Galaxies (LBGs), motivating the detailed study of LBAs as local laboratories of this high-redshift galaxy population. We present results from our recent integral-field spectroscopy survey of LBAs with Keck/OSIRIS, which shows that these galaxies have the same nebular gas kinematic properties as high-redshift LBGs. We argue that such kinematic studies alone are not an appropriate diagnostic to rule out merger events as the trigger for the observed starburst. Comparison between the kinematic analysis and morphological indices from HST imaging illustrates the difficulties of properly identifying (minor or major) merger events, with no clear correlation between the results using either of the two methods. Artificial redshifting of our data indicates that this problem becomes even worse at high redshift due to surface brightness dimming and resolution loss. Whether mergers could generate the observed kinematic properties is strongly dependent on gas fractions in these galaxies. We present preliminary results of a CARMA survey for LBAs and discuss the implications of the inferred molecular gas masses for formation models.Comment: To appear in the proceedings of IAU Symposium 277, "Tracing the Ancestry of Galaxies on the Land of our Ancestors", eds. C. Carignan, K.C. Freeman, and F. Combe

The UV-optical colours of brightest cluster galaxies in optically and X-ray selected clusters

Many brightest cluster galaxies (BCGs) at the centers of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically-selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here we analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the SDSS images of the BCGs to match the resolution of the GALEX data and we measure UV-optical colours in their inner and outer regions. We find that optically-selected BCGs exhibit smaller scatter in optical colours and redder inner NUV-r colours than the control galaxies, indicating that they are a homogenous population with very little ongoing star formation. The BCGs in the X-ray selected cluster sample span a similar range in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs, those located in clusters with central cooling times of less than 1 Gyr are significantly bluer than those located in clusters where the central gas cooling times are long. Our main conclusion is that the location of a galaxy at the centre of its halo is not sufficient to determine whether or not it is currently forming stars. One must also have information about the thermodynamic state of the gas in the core of the halo.Comment: 14 pages, 13 figures, Accepted by MNRA

The galaxy environment of a QSO at z ~ 5.7

High-redshift quasars are believed to reside in massive halos in the early universe and should therefore be located in fields with overdensities of galaxies, which are thought to evolve into galaxy clusters seen in the local universe. However, despite many efforts, the relationship between galaxy overdensities and z~6 quasars is ambiguous. This can possibly be attributed to the difficulty of finding galaxies with accurate redshifts in the vicinity of z~6 quasars. So far, overdensity searches around z~6 quasars have been based on studies of Lyman break galaxies (LBGs), which probe a redshift range of Delta z ~ 1. This range is large enough to select galaxies that may not be physically related to the quasar. We use deep narrow- and broadband imaging to study the environment of the z=5.72 quasar ULAS J0203+0012. The redshift range probed by our narrow-band selection of Lyman alpha emitters (LAEs) is Delta z ~ 0.1, which is significantly narrower than the LBG searches. This is the first time that LAEs were searched for near a z~6 quasar, in an effort to provide clues about the environments of quasars at the end of the epoch of reionization. We find no enhancement of LAEs in the surroundings of ULAS J0203+0012 in comparison with blank fields. We explore different explanations and interpretations for this non-detection of a galaxy overdensity, including that (1) the strong ionization from the quasar may prevent galaxy formation in its immediate vicinity and (2) high-redshift quasars may not reside in the center of the most massive dark matter halos.Comment: text updated to match published versio