119 research outputs found
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 (), 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
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