Browsing by Subject "galaxies: clusters: general"

Now showing 1 - 11 of 11

Ancient Light From Young Cosmic Cities: Physical And Observational Signatures Of Galaxy Proto-Clusters
(2013-12) Chiang, Yi-Kuan; Overzier, Roderik; Gebhardt, Karl; Chiang, Yi-Kuan; Overzier, Roderik; Gebhardt, Karl
A growing number of galaxy clusters at z = 1-2 is being discovered as part of deep optical, IR, X-ray, and Sunyaev-Zel'dovich effect surveys. For a complete picture of cluster formation, however, it is important that we also start probing the much earlier epoch, between redshifts of about 2 and 7, during which these clusters and their galaxies first began to form. Because the study of these so-called proto-clusters is currently quite limited by small number statistics, widely varying selection techniques, and many assumptions, we have performed a large systematic study of cluster formation utilizing cosmological simulations. We use the Millennium Simulations to track the evolution of dark matter and galaxies in about 3000 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 with cosmic time. We show that the progenitor regions of galaxy clusters (ranging in mass from similar to 10(14) to a few times 10(15) M-circle dot) can already be identified in galaxy surveys at very early times (at least up to z similar to 5), provided that the galaxy overdensities are measured on a sufficiently large scale (R-e similar to 5-10 Mpc comoving) and with sufficient statistics. We present the overdensities in matter, dark matter halos, and galaxies as functions of present-day cluster mass, redshift, bias, and window size that can be used to interpret the wide range of structures found in real surveys. We also derive the probability that a structure having a galaxy overdensity delta(gal), defined by a set of observational selection criteria, is indeed a proto-cluster, and we show how their z = 0 masses can already be estimated long before virialization. We present overdensity profiles as a function of radius, and we further show how the projected surface overdensities of proto-clusters 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 light of our simulation predictions. This paper 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 Hyper Suprime-Cam surveys.
Angular Momenta, Dynamical Masses, And Mergers Of Brightest Cluster Galaxies
(2013-12) Tran, Kim-Vy; Brough, Sarah; Gebhardt, Karl; von der Linden, Anja; Couch, Warrick J.; Sharp, Rob; Gebhardt, Karl
Using the VIMOS integral field unit (IFU) spectrograph on the Very Large Telescope, we have spatially mapped the kinematic properties of 10 nearby brightest cluster galaxies (BCGs) and 4 BCG companion galaxies located within a redshift of z = 0.1. In the hierarchical formation model, these massive galaxies (10(10.5) M-circle dot < M-dyn < 10(11.9) M-circle dot) are expected to undergo more mergers than lower mass galaxies, and simulations show that dry minor mergers can remove angular momentum. We test whether BCGs have low angular momenta by using the lambda(Re) parameter developed by the SAURON and ATLAS(3D) teams and combine our kinematics with Sloan Digital Sky Survey photometry to analyze the BCGs' merger status. We find that 30% (3/10) of the BCGs and 100% of the companion galaxies (4/4) are fast rotators as defined by the ATLAS(3D) criteria. Our fastest rotating BCG has a lambda(Re) = 0.35 +/- 0.05. We increase the number of BCGs analyzed from 1 in the combined SAURON and ATLAS(3D) surveys to 11 BCGs total and find that above M-dyn similar to 11.5 M-circle dot, virtually all galaxies, regardless of environment, are slow rotators. To search for signs of recent merging, we analyze the photometry of each system and use the G - M-20 selection criteria to identify mergers. We find that 40% +/- 20% of our BCGs are currently undergoing or have recently undergone a merger (within 0.2 Gyr). Surprisingly, we find no correlation between galaxies with high angular momentum and morphological signatures of merging.
Barred Galaxies In The Abell 901/2 Supercluster With Stages
(2009-06) Marinova, Irina; Jogee, Shardha; Heiderman, Amanda; Barazza, Fabio D.; Gray, M. E.; Barden, Marco; Wolf, Christian; Peng, Chen Y.; Bacon, David; Balogh, Michael; Bell, Eric F.; Bohm, Asmus; Caldwell, John A. R.; Haussler, Boris; Heymans, Catherine; Jahnke, Knud; van Kampen, Eelco; Lane, Kyle; McIntosh, Daniel H.; Meisenheimer, Klaus; Sanchez, Sebastian F.; Somerville, Rachel; Taylor, Andy; Wisotzki, Lutz; Zheng, Xianzhong; Marinova, Irina; Jogee, Shardha; Heiderman, Amanda
We present a study of bar and host disk evolution in a dense cluster environment, based on a sample of similar to 800 bright (M-V <= -18) galaxies in the Abell 901/2 supercluster at z similar to 0.165. We use Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) F606W imaging from the STAGES survey, and data from Spitzer, XMM-Newton, and COMBO-17. We identify and characterize bars through ellipse-fitting, and other morphological features through visual classification. We find the following results. (1) To define the optical fraction of barred disk galaxies, we explore three commonly used methods for selecting disk galaxies. We find 625, 485, and 353 disk galaxies, respectively, via visual classification, a single component Sersic cut (n <= 2.5), and a blue-cloud cut. In cluster environments, the latter two methods suffer from serious limitations, and miss 31% and 51%, respectively, of visually identified disks, particularly the many red, bulge-dominated disk galaxies in clusters. (2) For moderately inclined disks, the three methods of disk selection, however, yield a similar global optical bar fraction (f(bar-opt)) of 34%(+10%)(-3%) (115/340), 31%(+10%)(-3%) (58/189), and 30%(+10%)(-3%) (72/241), respectively. (3) We explore f(bar-opt) as a function of host galaxy properties and find that it rises in brighter galaxies and those which appear to have no significant bulge component. Within a given absolute magnitude bin, f(bar-opt) is higher in visually selected disk galaxies that have no bulge as opposed to those with bulges. Conversely, for a given visual morphological class, f(bar-opt) rises at higher luminosities. Both results are similar to trends found in the field. (4) For bright early-types, as well as faint late-type systems with no evident bulge, the optical bar fraction in the Abell 901/2 clusters is comparable within a factor of 1.1-1.4 to that of field galaxies at lower redshifts (z < 0.04). (5) Between the core and the virial radius of the cluster (R similar to 0.25-1.2 Mpc) at intermediate environmental densities (log(Sigma(10)) similar to 1.7-2.3), the optical bar fraction does not appear to depend strongly on the local environment density tracers (kappa, Sigma(10), and intracluster medium (ICM) density), and varies at most by a factor of similar to 1.3. Inside the cluster core, we are limited by number statistics, projection effects, and different trends from different indicators, but overall f(bar-opt) does not show evidence for a variation larger than a factor of 1.5. We discuss the implications of our results for the evolution of bars and disks in dense environments.
CANDELS Observations Of The Environmental Dependence Of The Color-Mass-Morphology Relation At Z=1.6
(2013-06) Bassett, Robert; Papovich, Casey; Lotz, Jennifer M.; Bell, Eric F.; Finkelstein, Steven L.; Newman, Jeffrey A.; Tran, Kim-Vy; Almaini, Omar; Lani, Caterina; Cooper, Michael; Croton, Darren; Dekel, Avishai; Ferguson, Henry C.; Kocevski, Dale D.; Koekemoer, Anton M.; Koo, David C.; McGrath, Elizabeth J.; McIntosh, Daniel H.; Wechsler, Risa; Finkelstein, Steven L.
We study the environmental dependence of color, stellar mass, and morphology by comparing galaxies in a forming cluster to those in the field at z = 1.6 with Hubble Space Telescope near-infrared imaging in the CANDELS/UDS field. We quantify the morphology of the galaxies using the effective radius, r(eff), and Sersic index, n. In both the cluster and field, approximately half of the bulge-dominated galaxies (n > 2) reside on the red sequence of the color-magnitude diagram, and most disk-dominated galaxies (n < 2) have colors expected for star-forming galaxies. There is weak evidence that cluster galaxies have redder rest-frame U - B colors and higher stellar masses compared to the field. Star-forming galaxies in both the cluster and field show no significant differences in their morphologies. In contrast, there is evidence that quiescent galaxies in the cluster have larger median effective radii and smaller Sersic indices compared to the field with a significance of 2 sigma. These differences are most pronounced for galaxies at clustercentric distances 1 Mpc < R-proj < 1.5 Mpc, which have low Sersic indices and possibly larger effective radii, more consistent with star-forming galaxies at this epoch and in contrast to other quiescent galaxies. We argue that star-forming galaxies are processed under the influence of the cluster environment at distances greater than the cluster-halo virial radius. Our results are consistent with models where gas accretion onto these galaxies is suppressed from processes associated with the cluster environment.
CANDELS Observations Of The Structural Properties Of Cluster Galaxies At Z=1.62
(2012-05) Papovich, Casey; Bassett, Robert; Lotz, Jennifer M.; van der Wel, A.; Tran, K. V.; Finkelstein, Steven L.; Bell, Eric F.; Conselice, Christopher J.; Dekel, Avishai; Dunlop, J. S.; Guo, Y. C.; Faber, S. M.; Farrah, D.; Ferguson, Henry C.; Finkelstein, Keely D.; Haussler, Boris; Kocevski, D. D.; Koekemoer, A. M.; Koo, D. C.; McGrath, E. J.; McLure, R. J.; McIntosh, Daniel H.; Momcheva, I.; Newman, Jeffrey A.; Rudnick, Gregory; Weiner, B.; Willmer, Christopher N. A.; Wuyts, S.; Finkelstein, Steven L.; Finkelstein, Keely D
We discuss the structural and morphological properties of galaxies in a z = 1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology relation: galaxies with colors of quiescent stellar populations generally have morphologies consistent with spheroids, and galaxies with colors consistent with ongoing star formation have disk-like and irregular morphologies. The size distribution of the quiescent cluster galaxies shows a deficit of compact (less than or similar to 1 kpc), massive galaxies compared to CANDELS field galaxies at z = 1.6. As a result, the cluster quiescent galaxies have larger average effective sizes compared to field galaxies at fixed mass at greater than 90% significance. Combined with data from the literature, the size evolution of quiescent cluster galaxies is relatively slow from z similar or equal to 1.6 to the present, growing as (1 + z)(-0.6 +/- 0.1). If this result is generalizable, then it implies that physical processes associated with the denser cluster region seem to have caused accelerated size growth in quiescent galaxies prior to z = 1.6 and slower subsequent growth at z < 1.6 compared to galaxies in the lower density field. The quiescent cluster galaxies at z = 1.6 have higher ellipticities compared to lower redshift samples at fixed mass, and their surface-brightness profiles suggest that they contain extended stellar disks. We argue that the cluster galaxies require dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk material and to match the relations for ellipticity, stellar mass, size, and color of early-type galaxies in z < 1 clusters.
Discovery of a Large Number of Candidate Protoclusters Traced By Similar to 15 Mpc-Scale Galaxy Overdensities in COSMOS
(2014-02) Chiang, Yi-Kuan; Overzier, Roderick; Gebhardt, Karl; Chiang, Yi-Kuan; Overzier, Roderick; Gebhardt, Karl
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 protocluster candidates in the 1.62 deg(2) COSMOS/UltraVISTA field traced by optical/infrared selected galaxies using photometric redshifts. By comparing properly smoothed three-dimensional 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 similar to 15 comoving Mpc-scale galaxy clustering is consistent with Lambda CDM 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 similar to 10(15) M-circle dot clusters. Taking into account the significant upward scattering of lower mass structures, the probabilities for the candidates to have at least M-z=0 similar to 10(14) M-circle dot are similar to 70%. For each structure, about 15%-40% of photometric galaxy candidates are expected to be true protocluster 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 protocluster 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.
Frequency And Properties Of Bars In Cluster And Field Galaxies At Intermediate Redshifts
(2009-04) Barazza, F. D.; Jablonka, P.; Desai, V.; Jogee, S.; Aragon-Salamanca, A.; De Lucia, G.; Saglia, R. P.; Halliday, C.; Poggianti, B. M.; Dalcanton, J. J.; Rudnick, G.; Milvang-Jensen, B.; Noll, S.; Simard, L.; Clowe, D. I.; Pello, R.; White, S. D. M.; Zaritsky, D.; Jogee, S.
We present a study of large-scale bars in field and cluster environments out to redshifts of similar to 0.8 using a final sample of 945 moderately inclined disk galaxies drawn from the EDisCS project. We characterize bars and their host galaxies and look for relations between the presence of a bar and the properties of the underlying disk. We investigate whether the fraction and properties of bars in clusters are different from their counterparts in the field. The properties of bars and disks are determined by ellipse fits to the surface brightness distribution of the galaxies using HST/ACS images in the F814W filter. The bar identification is based on quantitative criteria after highly inclined (> 60 degrees) systems have been excluded. The total optical bar fraction in the redshift range z = 0.4-0.8 (median z = 0.60), averaged over the entire sample, is 25% (20% for strong bars). For the cluster and field subsamples, we measure bar fractions of 24% and 29%, respectively. We find that bars in clusters are on average longer than in the field and preferentially found close to the cluster center, where the bar fraction is somewhat higher (similar to 31%) than at larger distances (similar to 18%). These findings however rely on a relatively small subsample and might be affected by small number statistics. In agreement with local studies, we find that disk-dominated galaxies have a higher optical bar fraction (similar to 45%) than bulge-dominated galaxies (similar to 15%). This result is based on Hubble types and effective radii and does not change with redshift. The latter finding implies that bar formation or dissolution is strongly connected to the emergence of the morphological structure of a disk and is typically accompanied by a transition in the Hubble type. The question whether internal or external factors are more important for bar formation and evolution cannot be answered definitely. On the one hand, the bar fraction and properties of cluster and field samples of disk galaxies are quite similar, indicating that internal processes are crucial for bar formation. On the other hand, we find evidence that cluster centers are favorable locations for bars, which suggests that the internal processes responsible for bar growth are supported by the typical interactions taking place in such environments.
Gravitational Fragmentation In Turbulent Primordial Gas And The Initial Mass Function Of Population III Stars
(2011-02) Clark, Paul C.; Glover, Simon C. O.; Klessen, Ralf S.; Bromm, Volker; Bromm, Volker
We report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III. 1 stars-those which form in the pristine halos unaffected by prior star formation-and the formation of Population III. 2 stars-those forming in halos where the gas has an increased ionization fraction. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e., for rms velocity dispersions as low as 20% of the sound speed. Fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. If such levels of turbulent motions were indeed present in star-forming minihalos, Population III. 1 stars would be on average of somewhat lower mass, and form in larger groups, than Population III. 2 stars. We find that fragment masses cover over two orders of magnitude, suggesting that the Population III initial mass function may have been much broader than previously thought. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help to determine the applicability of our results to primordial star formation.
Interacting Galaxies In The A901/902 Supercluster With Stages
(2009-11) Heiderman, Amanda; Jogee, Shardha; Marinova, Irina; van Kampen, Eelco; Barden, Marco; Peng, Chien Y.; Heymans, Catherine; Gray, Meghan E.; Bell, Eric F.; Bacon, David; Balogh, Michael; Barazza, Fabio D.; Boehm, Asmus; Caldwell, John A. R.; Haeussler, Boris; Jahnke, Knud; Lane, Kyle; McIntosh, Daniel H.; Meisenheimer, Klaus; Sanchez, Sebastian F.; Somerville, Rachel S.; Taylor, Andy; Wisotzki, Lutz; Wolf, Christian; Zheng, Xianzhong; Heiderman, Amanda; Jogee, Shardha; Marinova, Irina
We present a study of galaxy mergers and the influence of environment in the Abell 901/902 supercluster at z similar to 0.165, based on 893 bright (R(Vega) <= 24) intermediate-mass (M(*) >= 10(9) M(circle dot)) galaxies. We use HST ACS F606W data from the Space Telescope A901/902 Galaxy Evolution Survey, COMBO-17, Spitzer 24 mu m, and XMM-Newton X-ray data. Our analysis utilizes both a physically driven visual classification system and quantitative CAS parameters to identify systems which show evidence of a recent or ongoing merger of mass ratio >1/10 (i.e., major and minor mergers). Our results are (1) after visual classification and minimizing the contamination from false projection pairs, we find that the merger fraction f(merge) is 0.023 +/- 0.007. The estimated fractions of likely major mergers, likely minor mergers, and ambiguous cases are 0.01 +/- 0.004, 0.006 +/- 0.003, and 0.007 +/- 0.003, respectively. (2) All the mergers lie outside the cluster core of radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large galaxy velocity dispersion in the core. The mergers, instead, populate the region (0.25 Mpc < R <= 2 Mpc) between the core and the cluster outskirt. In this region, the estimated frequency of mergers is similar to those seen at typical group overdensities in N-body simulations of accreting groups in the A901/902 clusters. This suggests the ongoing growth of the clusters via accretion of group and field galaxies. (3) We compare our observed merger fraction with those reported in other clusters and groups out to z similar to 0.4. Existing data points on the merger fraction for L <= L* galaxies in clusters allow for a wide spectrum of scenarios, ranging from no evolution to evolution by a factor of similar to 5 over z similar to 0.17-0.4. (4) In A901/902, the fraction of interacting galaxies, which lie on the blue cloud is 80% +/- 18% (16/20) versus 34% +/- 7% or (294/866) for non-interacting galaxies, implying that interacting galaxies are preferentially blue. (5) The average star formation rate (SFR), based on UV or a combination of UV+IR data, is enhanced by a factor of similar to 1.5-2 in mergers compared to non-interacting galaxies. However, mergers in the A901/902 clusters contribute only a small fraction (between 10% and 15%) of the total SFR density, while the rest of the SFR density comes from non-interacting galaxies.
A Large-Scale Galaxy Structure At Z=2.02 Associated With The Radio Galaxy MRC 0156-252
(2013-11) Galametz, Audrey; Stern, Daniel; Pentericcil, Laura; De Breuck, Carlos; Vernet, Joël; Wylezalek, Dominika; Fassbender, Rene; Hatch, Nina; Kurk, Jaron; Overzier, Roderik; Rettura, Alessandro; Seymour, Nick; Overzier, Roderik
We present the spectroscopic confirmation of a structure of galaxies surrounding the radio galaxy MRC 0156-252 at z = 2.02. The structure was initially discovered as an overdensity of both near-infrared selected z > 1.6 and mid-infrared selected z > 1.2 galaxy candidates. We used the VLT/FORS2 multi-object spectrograph to target similar to 80 high-redshift galaxy candidates, and obtain robust spectroscopic redshifts for more than half the targets. The majority of the confirmed sources are star-forming galaxies at z > 1.5. In addition to the radio galaxy, two of its close-by companions (<6 '') also show AGN signatures. Ten sources, including the radio galaxy, lie within vertical bar z - 2.020 vertical bar < 0.015 (i.e., velocity offsets < 1500 km s(-1)) and within projected 2 Mpc comoving of the radio galaxy. Additional evidence suggests not only that the galaxy structure associated with MRC 0156-252 is a forming galaxy cluster but also that this structure is most probably embedded in a larger-scale structure.
A Massive, Distant Proto-Cluster at Z=2.47 Caught in a Phase of Rapid Formation?
(2015-08) Casey, C. M.; Cooray, A.; Capak, P.; Fu, H.; Kovac, K.; Lilly, S.; Sanders, D. B.; Scoville, N. Z.; Treister, E.; Casey, C. M.
Numerical simulations of cosmological structure Formation show that the universe's most massive clusters, and the galaxies living in those clusters, assemble rapidly at early times (2.5 < z < 4). While more than 20 proto-clusters have been observed at z greater than or similar to 2 based on associations of 5-40 galaxies around rare sources, the observational evidence for rapid cluster Formation is weak. Here we report observations of an asymmetric filamentary structure at z = 2.47 containing 7 starbursting, submillimeter-luminous galaxies and 5 additional active galactic nuclei (AGNs) within a comoving volume of 15,000 Mpc(3). As the expected lifetime of both the luminous AGN and starburst phase of a galaxy is similar to 100 Myr, we conclude that these sources were likely triggered in rapid succession by environmental factors or, alternatively, the duration of these cosmologically rare phenomena is much longer than prior direct measurements suggest. The stellar mass already built up in the structure is similar to 10(12) M-circle dot and we estimate that the cluster mass will exceed that of the Coma supercluster at z similar to 0. The filamentary structure is in line with hierarchical growth simulations that predict that the peak of cluster activity occurs rapidly at z > 2.