Showing papers on "Brightest cluster galaxy published in 2004"

Nearly 5000 Distant Early-Type Galaxies in COMBO-17: A Red Sequence and Its Evolution since z ~ 1

Abstract: We present the rest-frame colors and luminosities of ~25,000 mR 24 galaxies in the redshift range 0.2 < z ≤ 1.1 drawn from 0.78 deg2 of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters). We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z ~ 1. This bimodality permits a model-independent definition of red early-type galaxies and blue late-type galaxies at any given redshift. The colors of the blue peak become redder toward the present day, and the number density of blue luminous galaxies has dropped strongly since z ~ 1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Λ-dominated cold dark matter universe. When we account for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of 2 since z ~ 1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue star-forming population are required to fully explain the properties of these galaxies.

A new nonparametric approach to galaxy morphological classification

Abstract: We present two new nonparametric methods for quantifying galaxy morphology: the relative distribution of the galaxy pixel flux values (the Gini coefficient or G) and the second-order moment of the brightest 20% of the galaxy's flux (M20). We test the robustness of G and M20 to decreasing signal-to-noise ratio (S/N) and spatial resolution and find that both measures are reliable to within 10% for images with average S/N per pixel greater than 2 and resolutions better than 1000 and 500 pc, respectively. We have measured G and M20, as well as concentration (C), asymmetry (A), and clumpiness (S) in the rest-frame near-ultraviolet/optical wavelengths for 148 bright local normal Hubble-type galaxies (E–Sd) galaxies, 22 dwarf irregulars, and 73 0.05 < z < 0.25 ultraluminous infrared galaxies (ULIRGs). We find that most local galaxies follow a tight sequence in G-M20-C, where early types have high G and C and low M20 and late-type spirals have lower G and C and higher M20. The majority of ULIRGs lie above the normal galaxy G-M20 sequence because of their high G and M20 values. Their high Gini coefficients arise from very bright nuclei, while the high second-order moments are produced by multiple nuclei and bright tidal tails. All of these features are signatures of recent and on-going mergers and interactions. We also find that in combination with A and S, G is more effective than C at distinguishing ULIRGs from the normal Hubble types. Finally, we measure the morphologies of 491.7 < z < 3.8 galaxies from HST NICMOS observations of the Hubble Deep Field North. We find that many of the z ~ 2 galaxies possess G and A higher than expected from degraded images of local elliptical and spiral galaxies and have morphologies more like low-redshift ULIRGs.

Weak-lensing mass reconstruction of the interacting cluster 1E 0657-558: Direct evidence for the existence of dark matter

Abstract: We present a weak-lensing mass reconstruction of the interacting cluster 1E 0657-558, in which we detect both the main cluster and a subcluster. The subcluster is identified as a smaller cluster that has just undergone initial infall and pass-through of the primary cluster and has been previously identified in both optical surveys and X-ray studies. The X-ray gas has been separated from the galaxies by ram pressure-stripping during the pass-through. The detected mass peak is located between the X-ray peak and galaxy concentration, although the position is consistent with the galaxy centroid within the errors of the mass reconstruction. We find that the mass peak for the main cluster is in good spatial agreement with the cluster galaxies and is offset from the X-ray halo at 3.4 σ significance, and we determine that the mass-to-light ratios of the two components are consistent with those of relaxed clusters. The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster. This proof of dark matter existence holds true even under the assumption of modified Newtonian dynamics (MOND); based on the observed gravitational shear-optical light ratios and the mass peak-X-ray gas offsets, the dark matter component in a MOND regime would have a total mass that is at least equal to the baryonic mass of the system.

Galaxy ecology: groups and low-density environments in the SDSS and 2dFGRS

Abstract: We analyse the observed correlation between galaxy environment and Halpha emission-line strength, using volume-limited samples and group catalogues of 24 968 galaxies at 0.05 < z < 0.095, drawn from the 2dF Galaxy Redshift Survey (M-bJ < -19.5) and the Sloan Digital Sky Survey (M-r < -20.6). We characterize the environment by: (1) Sigma(5), the surface number density of galaxies determined by the projected distance to the fifth nearest neighbour; and (2) rho(1.1) and rho(5.5), three-dimensional density estimates obtained by convolving the galaxy distribution with Gaussian kernels of dispersion 1.1 and 5.5 Mpc, respectively. We find that star-forming and quiescent galaxies form two distinct populations, as characterized by their H equivalent width, W-0(Halpha). The relative numbers of star-forming and quiescent galaxies vary strongly and continuously with local density. However, the distribution of W-0(Halpha) amongst the star-forming population is independent of environment. The fraction of star-forming galaxies shows strong sensitivity to the density on large scales, rho(5.5), which is likely independent of the trend with local density, rho(1.1). We use two differently selected group catalogues to demonstrate that the correlation with galaxy density is approximately independent of group velocity dispersion, for sigma = 200-1000 km s(-1). Even in the lowest-density environments, no more than similar to70 per cent of galaxies show significant Halpha emission. Based on these results, we conclude that the present-day correlation between star formation rate and environment is a result of short-time-scale mechanisms that take place preferentially at high redshift, such as starbursts induced by galaxy-galaxy interactions.

Clusters of galaxies : probes of cosmological structure and galaxy evolution

Abstract: Introduction List of participants 1. Galaxy clusters as probes of cosmology and astrophysics August E. Evrard 2. Clusters of galaxies in the Sloan Digital Sky Survey Robert C. Nichol 3. Clustering studies with the 2dF Galaxy Redshift Survey Warrick J. Couch, Matthew M. Colless and Roberto de Propris 4. X-ray surveys of low-redshift clusters Alastair C. Edge 5. X-ray clusters at high redshift Piero Rosati 6. The red sequence technique and high-redshift galaxy clusters Michael D. Gladders 7. Probing dark matter in clusters Ian Smail 8. Clusters of galaxies: an x-ray perspective Richard F. Mushotzky 9. Cool gas in clusters of galaxies Megan Donahue and G. Mark Voit 10. Using the Sunyaev-Zel'dovich effect to probe the gas in clusters Mark Birkinshaw 11. The formation of early-type galaxies: observations to z 1 Tommaso Treu 12. Evolution of early-type galaxies in clusters Marijn Franx 13. Star-forming galaxies in clusters Alan Dressler 14. The stellar content of galaxy clusters Roger L. Davies 15. Modeling stellar populations in cluster galaxies Bianca M. Poggianti 16. The chemistry of galaxy clusters Alvio Renzini 17. Interactions and mergers of cluster galaxies J. Christopher Mihos 18. Evolutionary processes in clusters Ben Moore 19. Interaction of galaxies with the intracluster medium Jacqueline H. van Gorkom 20. The difference between clusters and groups: a journey from cluster cores to their outskirts and beyond Richard G. Bower and Michael L. Balogh 21. Galaxy groups at intermediate redshift and the mechanisms of galaxy evolution Ray G. Carlberg 22. The intragroup medium John S. Mulchaey 23. Symposium summary Jeremiah P. Ostriker Credits.

K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light

Abstract: We investigate the near-infrared K-band properties of the brightest cluster galaxies (BCGs) in a sample of 93 X-ray galaxy clusters and groups, using data from the Two Micron All Sky Survey. Our cluster sample spans a factor of 70 in mass, making it sensitive to any cluster mass-related trends. We derive the cumulative radial distribution for the BCGs in the ensemble and find that 70% of the BCGs are centered in the cluster to within 5% of the virial radius r200; this quantifies earlier findings that BCG position coincides with the cluster center as defined by the X-ray emission peak. We study the correlations between the luminosity of the BCGs (Lb) and the mass and the luminosity of the host clusters, finding that BCGs in more massive clusters are more luminous than their counterparts in less massive systems and that the BCGs become less important in the overall cluster light (L200) as cluster mass increases. By examining a large sample of optically selected groups, we find that these correlations hold for galactic systems less massive than our clusters (<3 × 1013 M☉). From the differences between luminosity functions in high- and low-mass clusters, we argue that BCGs grow in luminosity mainly by merging with other luminous galaxies as the host clusters grow hierarchically; the decreasing BCG luminosity fraction (Lb/L200) with cluster mass indicates that the rate of luminosity growth in BCGs is slow compared to the rate at which clusters acquire galaxy light from the field or other merging clusters. Utilizing the observed correlation between the cluster luminosity and mass and a merger tree model for cluster formation, we estimate that the amount of intracluster light (ICL) increases with cluster mass; our calculations suggest that in 1015 M☉ clusters more than 50% of total stellar mass is in ICL, making the role of ICL very important in the evolution and thermodynamic history of clusters. The cluster baryon fraction accounting for the ICL is in good agreement with the value derived from cosmic microwave background observations. The inclusion of ICL reduces the discrepancy between the observed cluster cold baryon fraction and that found in hydrodynamical simulations. Based on the observed iron abundance in the intracluster medium, we find that the ICL predicted by our model, together with the observed galaxy light, match the iron mass-to-light ratio expected from simple stellar population models, provided that the Salpeter initial mass function is adopted. The ICL also makes it easier to produce the "iron excess" found in the central regions of cool-core clusters.

K-Band Properties of Galaxy Clusters and Groups: Luminosity Function, Radial Distribution, and Halo Occupation Number

Abstract: We explore the near-infrared (NIR) K-band properties of galaxies within 93 galaxy clusters and groups using data from the Two Micron All Sky Survey. We use X-ray properties of these clusters to pinpoint cluster centers and estimate cluster masses. By stacking all these systems, we study the shape of the cluster luminosity function and the galaxy distribution within the clusters. We find that the galaxy profile is well described by the Navarro, Frenk, & White (NFW) profile with a concentration parameter c ~ 3, with no evidence for cluster mass dependence of the concentration. Using this sample, whose masses span the range from 3 × 1013 to 2 × 1015 M☉, we confirm the existence of a tight correlation between total galaxy NIR luminosity and cluster binding mass, which indicates that NIR light can serve as a cluster mass indicator. From the observed galaxy profile, together with cluster mass profile measurements from the literature, we find that the mass-to-light ratio is a weakly decreasing function of cluster radius and that it increases with cluster mass. We also derive the mean number of galaxies within halos of a given mass, the halo occupation number. We find that the mean number scales as N M0.84±0.04 for galaxies brighter than MK = -21, indicating that high-mass clusters have fewer galaxies per unit mass than low-mass clusters. Using published observations at high redshift, we show that higher redshift clusters have higher mean occupation numbers than nearby systems of the same mass. By comparing the luminosity function and radial distribution of galaxies in low-mass and high-mass clusters, we show that there is a marked decrease in the number density of galaxies fainter than M* as one moves to higher mass clusters; in addition, extremely luminous galaxies are more probable in high-mass clusters. We explore several processes, including tidal interactions and merging, as a way of explaining the variation in galaxy population with cluster mass.

Old galaxies in the young Universe

Abstract: More than half of all stars in the local Universe are found in massive spheroidal galaxies1, which are characterized by old stellar populations2,3 with little or no current star formation. In present models, such galaxies appear rather late in the history of the Universe as the culmination of a hierarchical merging process, in which larger galaxies are assembled through mergers of smaller precursor galaxies. But observations have not yet established how, or even when, the massive spheroidals formed2,3, nor if their seemingly sudden appearance when the Universe was about half its present age (at redshift z ≈ 1) results from a real evolutionary effect (such as a peak of mergers) or from the observational difficulty of identifying them at earlier epochs. Here we report the spectroscopic and morphological identification of four old, fully assembled, massive (1011 solar masses) spheroidal galaxies at l.6 < z < 1.9, the most distant such objects currently known. The existence of such systems when the Universe was only about one-quarter of its present age shows that the build-up of massive early-type galaxies was much faster in the early Universe than has been expected from theoretical simulations4.

A high abundance of massive galaxies 3–6 billion years after the Big Bang

Abstract: Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units1. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies2, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies3) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr (refs 4–7), but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.

The ACS Virgo Cluster Survey. I. Introduction to the Survey

Abstract: The Virgo Cluster is the dominant mass concentration in the Local Supercluster and the largest collection of elliptical and lenticular galaxies in the nearby universe. In this paper, we present an introduction to the ACS Virgo Cluster Survey: a program to image, in the F475W and F850LP bandpasses (≈Sloan g and z), 100 early-type galaxies in the Virgo Cluster using the Advanced Camera for Surveys on the Hubble Space Telescope. We describe the selection of the program galaxies and their ensemble properties, the choice of filters, the field placement and orientation, the limiting magnitudes of the survey, coordinated parallel observations of 100 "intergalactic" fields with WFPC2, and supporting ground-based spectroscopic observations of the program galaxies. In terms of depth, spatial resolution, sample size, and homogeneity, this represents the most comprehensive imaging survey to date of early-type galaxies in a cluster environment. We briefly describe the main scientific goals of the survey, which include the measurement of luminosities, metallicities, ages, and structural parameters for the many thousands of globular clusters associated with these galaxies, a high-resolution isophotal analysis of galaxies spanning a factor of ~450 in luminosity and sharing a common environment, the measurement of accurate distances for the full sample of galaxies using the method of surface brightness fluctuations, and a determination of the three-dimensional structure of Virgo itself.

Velocity and spatial biases in cold dark matter subhalo distributions

Abstract: We present a statistical study of substructure within a sample ofCDM clusters and galaxies simulated with up to 25 million particles With thousands of subhalos per object we can accurately measure their spatial clustering and velocity distribution functions and compare these with observational data The substructure properties of galactic halos closely resembles those of galaxy clusters with a small scatter in the mass and circular velocity functions The velocity distribution function is non-Maxwellian and flat topped with a negative kurtosis of about -07 Within the virial radius the velocity bias b = σsub/σDM � 112±004, increasing to b > 13 within the halo centers Slow subhalos are much less common, due to physical disruption by gravitational tides early in the merging history This leads to a spatially anti-biased subhalo distribution that is well fitted by a cored isothermal Observations of cluster galaxies do not show such biases which we interpret as a limitation of pure dark matter simulations - we estimate that we are missing half of the halo population which has been destroyed by physical overmerging High resolution hydrodynamical simulations are required to study these issues further If CDM is correct then the cluster galaxies must survive the tidal field, perhaps due to baryonic inflow during elliptical galaxy formation Spirals can never exist near the cluster centers and the elliptical galaxies there will have little remaining dark matter This implies that the morphology-density relation is set before the cluster forms, rather than a subsequent transformation of disks to S0's by virtue of the cluster environment

On Departures from a Power Law in the Galaxy Correlation Function

Abstract: We measure the projected correlation function wp from the Sloan Digital Sky Survey for a flux-limited sample of 118,000 galaxies and a volume-limited subset of 22,000 galaxies with absolute magnitude Mr M1 = 4.74 × 1013 h-1 M☉ is M = 0.89, with 75% of the galaxies residing in less massive, single-galaxy halos and simple auxiliary assumptions about the spatial distribution of galaxies within halos and the fluctuations about the mean occupation. This physically motivated model has the same number of free parameters as a power law, and it fits the wp data better, with a χ2/dof = 0.93, compared to 6.12 (for 10 degrees of freedom, incorporating the covariance of the correlation function errors). Departures from a power-law correlation function encode information about the relation between galaxies and dark matter halos. Higher precision measurements of these departures for multiple classes of galaxies will constrain galaxy bias and provide new tests of the theory of galaxy formation.

On the Iron content in rich nearby Clusters of Galaxies

Abstract: In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX .W e fi nd that the global iron mass of clusters is tightly related to the cluster luminosity and that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron mass vs. luminosity and luminosity vs. temperature correlations. The iron mass is found to scale linearly with the intracluster gas mass, implying that the global iron abundance in clusters is roughly constant. This result suggests that enrichment mechanisms operate at a similar rate in all clusters. By employing population synthesis and chemical enrichment models, we show that the iron mass associated with the abundance excess which is always found in the centre of cool core clusters can be entirely produced by the brightest cluster galaxy (BCG), which is always found at the centre of cool core clusters. The iron mass associated with the excess, the optical magnitude of the BCG and the temperature of the cluster are found to correlate with one another suggesting a link between the properties of the BCG and the hosting cluster. These observational facts lends strength to current formation theories which envisage a strong connection between the formation of the giant BCG and its hosting cluster.

A Comparison of the Galaxy Populations in the Coma and Distant Clusters: The Evolution of k+a Galaxies and the Role of the Intracluster Medium

Abstract: The spectroscopic properties of galaxies in the Coma Cluster are compared with those of galaxies in rich clusters at z ~ 0.5, to investigate the evolution of the star formation history in clusters. Luminous galaxies with MV ≤ -20 and poststarburst/post-star-forming (k+a) spectra that constitute a significant fraction of galaxies in distant cluster samples are absent in Coma, where spectacular cases of k+a spectra are found instead at MV > -18.5 and represent a significant proportion of the cluster dwarf galaxy population. A simple inspection of their positions on the sky indicates that this type of galaxy does not show a preferential location within the cluster, but the bluest and strongest lined group of k+a galaxies lie in projection toward the central 1.4 Mpc of Coma and have radial velocities significantly higher than the cluster mean. We find a striking correlation between the positions of these young and strong poststarburst galaxies and substructure in the hot intracluster medium (ICM) identified from XMM-Newton data, with these galaxies lying close to the edges of two infalling substructures. This result strongly suggests that the interaction with the dense ICM could be responsible for the quenching of the star formation (thus creating the k+a spectrum) and, possibly, for any previous starburst. The evolution with redshift of the luminosity distribution of k+a galaxies can be explained by a downsizing effect, with the maximum luminosity/mass of actively star-forming galaxies infalling onto clusters decreasing at lower redshift. We discuss the possible physical origin of this downsizing effect and the implications of our results for current scenarios of environmental effects on the star formation in galaxies.

The Dark Matter Distribution in the Central Regions of Galaxy Clusters: Implications for Cold Dark Matter

Abstract: We have undertaken a spectroscopic survey of gravitational arcs in a carefully chosen sample of six clusters, each containing a dominant brightest cluster galaxy. We use these systems to study the relative distributions of dark and baryonic material in the central regions. Three clusters present both radial and tangential arcs and provide particularly strong constraints on the mass profiles, whereas the other three display only tangential arcs and act as a control set. Following Sand, Treu, & Ellis, we analyze stellar velocity dispersion data for the brightest cluster galaxies in conjunction with the arc redshifts and lens models to constrain the dark and baryonic mass profiles jointly. For the systems containing radial arcs we find that the inner dark matter density profile is consistent with a three-dimensional distribution, ρDM ∝ r-β, with logarithmic slope β = 0.52 (68% CL). Similarly, we find that the tangential arc sample gives an upper limit, β < 0.57 (99% CL). Taking the six clusters together, the mean dark matter distribution is inconsistent with the standard NFW value, β = 1.0, at greater than 99% confidence. In addition, we find considerable cosmic scatter in the β (Δβ ~ 0.3) values of the radial arc sample. We find no evidence that systems with radial arcs preferentially yield flatter dark matter profiles as might be expected if they were a biased subset. We discuss the validity of our one-dimensional mass reconstruction method and verify its conclusions by comparing with results of a more rigorous ray-tracing code that does not assume axial symmetry. Our results extend and considerably strengthen the earlier conclusions presented by Sand and coworkers and suggest that the relationship between dark and visible matter in the cores of clusters is much more complex than anticipated from recent simulations.

COLORS OF ACTIVE GALACTIC NUCLEUS HOST GALAXIES AT 0.5 < z < 1.1 FROM THE GEMS SURVEY

Abstract: We present the results from a study of the host galaxies of 15 optically selected active galactic nuclei (AGNs) with 0.5 < z < 1.1 from the Galaxy Evolution from Morphology and SEDs project (GEMS). GEMS is a Hubble Space Telescope imaging survey of a ~28' × 28' contiguous field centered on the Chandra Deep Field-South in the F606W and F850LP filter bands. It incorporates the spectral energy distributions and redshifts of ~10,000 objects, obtained by the COMBO-17 project. We have detected the host galaxies of all 15 AGNs in the F850LP band (and 13 of 15 in the F606W band), recovering their fluxes, morphologies, and structural parameters. We find that 80% of the host galaxies have early-type (bulge-dominated) morphologies, while the rest have structures characteristic of late-type (disk-dominated) galaxies. We find that 25% of the early types and 30% of the late types exhibit disturbances consistent with galaxy interactions. The hosts show a wide range of colors, from those of red-sequence galaxies to blue colors consistent with ongoing star formation. Roughly 70% of the morphologically early-type hosts have rest-frame blue colors, a much larger fraction than those typical of nonactive morphologically early-type galaxies in this redshift and luminosity range. Yet, we find that the early-type hosts are structurally similar to red-sequence elliptical galaxies, inasmuch as they follow an absolute magnitude versus half-light size correlation that is consistent with the mean relation for early-type galaxies at similar redshifts.

Selection and Photometric Properties of K+A Galaxies

Abstract: Two different simple measurements of galaxy star formation rate with different timescales are compared empirically on 156,395 fiber spectra of galaxies with r < 17.77 mag taken from the Sloan Digital Sky Survey in the redshift range 0.05 < z < 0.20: a ratio A/K found by fitting a linear sum of an average old stellar population spectrum (K) and average A star spectrum (A) to the galaxy spectrum, and the equivalent width (EW) of the Hα emission line. The two measures are strongly correlated, but there is a small, clearly separated population of outliers from the median correlation that display excess A/K relative to Hα EW. These "K+A" (or "E+A") galaxies must have dramatically decreased their star formation rates over the last ~1 Gyr. The K+A luminosity distribution is similar to that of the total galaxy population. The K+A population appears to be bulge-dominated, but bluer and with higher surface brightness than normal bulge-dominated galaxies; it appears that K+A galaxies will fade with time into normal bulge-dominated galaxies. The inferred rate density for K+A galaxy formation is ~10-4 h3 Mpc-3 Gyr-1 at redshift z ~ 0.1. These events are taking place in the field; the K+A galaxies found in this study do not primarily lie in the high-density environments or clusters typical of bulge-dominated populations.

Evolution and Color Dependence of the Galaxy Angular Correlation Function: 350,000 Galaxies in 5 Square Degrees

Abstract: When applied to deep photometric catalogs, the two-point angular correlation function, ?(?), is a sensitive probe of the evolution of galaxy clustering properties. Here we present measurements of ?(?) as a function of IAB magnitude and (R - I) color to a depth of IAB = 24 on scales ? = 7''-3', using a sample of ~350,000 galaxies covering 5 deg2 in total over five separate fields. Using redshifts of 2954 galaxies in early DEEP2 Galaxy Redshift Survey data, we construct robust galaxy redshift distributions as a function of IAB magnitude and (R - I) color for galaxies in the range 0 1.4 galaxies, as well as broad-line active galactic nuclei. We find that local blue dwarfs are relatively unclustered, with x0 = 1.6 ? 0.2 h-1 Mpc. The z > 1.4 blue galaxies have a larger clustering scale length, x0 5 h-1 Mpc.

Pre-Processing of Galaxies before Entering a Cluster

Abstract: We consider several mechanisms that possibly affect the evolution of disk galaxies in clusters using analytical models based on a hierarchical clustering scenario. We especially focus on the evolution of disk galaxies in subclusters located around a main cluster. We show that ram-pressure stripping cannot be always ignored in subclusters, although their masses are much smaller than that of the main cluster. The star-formation rate of a galaxy may gradually decrease by the stripping of warm gas (‘strangulation’) in a main cluster. However, we find that ram-pressure stripping could start before the strangulation is completed, if a field galaxy directly falls into the main cluster. Since this conflicts with some recent observations, many galaxies might have been affected by some environmental effects when they were in subclusters before they fell into the main cluster (‘pre-processing’). We show that strangulation and evaporation of the cold gas by the surrounding hot ICM in subclusters are the possible candidates. We also show that the observed morphological transformation of disk galaxies at z < ∼ 1 is not chiefly due to galaxy mergers.

The Diffuse Light in Simulations of Galaxy Clusters

Abstract: We study the properties of the diffuse light in galaxy clusters forming in a large hydrodynamical cosmological simulation of the Λ cold dark matter cosmology. The simulation includes a model for radiative cooling, star formation in dense cold gas, and feedback by Type II supernova explosions. We select clusters having mass M > 1014 h-1 M☉ and study the spatial distribution of their star particles. While most stellar light is concentrated in gravitationally bound galaxies orbiting in the cluster potential, we find evidence for a substantial diffuse component, which may account for the extended halos of light observed around central cD galaxies. We find that more massive simulated clusters have a larger fraction of stars in the diffuse light than the less massive ones. The intracluster light is more centrally concentrated than the galaxy light, and the stars in the diffuse component are on average older than the stars in cluster galaxies, supporting the view that the diffuse light is not a random sampling of the stellar population in the cluster galaxies. We thus expect that at least ~10% of the stars in a cluster may be distributed as intracluster light, largely hidden thus far because of its very low surface brightness.

Distributions of galaxy spectral types in the Sloan Digital Sky Survey

Abstract: We perform an objective classification of 170,000 galaxy spectra in the Sloan Digital Sky Survey (SDSS) using the Karhunen-Loeve (KL) transform. With about one-sixth of the total set of galaxy spectra that will be obtained by the survey, we are able to carry out the most extensive analysis of its kind to date. The formalism proposed by Connolly and Szalay is adopted to correct for gappy regions in the spectra and to derive eigenspectra and eigencoefficients. From this analysis, we show that this gap-correction formalism leads to a converging set of eigenspectra and KL-repaired spectra. Furthermore, KL eigenspectra of galaxies are found to be convergent not only as a function of iteration, but also as a function of the number of randomly selected galaxy spectra used in the analysis. From these data a set of 10 eigenspectra of galaxy spectra are constructed, with rest-wavelength coverage 3450–8350 A. The eigencoefficients describing these galaxies naturally place the spectra into several classes defined by the plane formed by the first three eigencoefficients of each spectrum. Spectral types corresponding to different Hubble types and galaxies with extreme emission lines are identified for the 170,000 spectra and are shown to be complementary to existing spectral classifications. From a nonparametric classification technique, we find that the population of galaxies can be divided into three classes that correspond to early late-type through intermediate late-type galaxies. This finding is believed to be related to the color separation of SDSS galaxies discussed in earlier works. Bias in the spectral classifications due to the aperture spectroscopy in the SDSS is small and within the signal-to-noise limit for a majority of galaxies, except for the reddest nearby galaxies and large galaxies (>30 kpc) with prominent emissions. The mean spectra and eigenspectra derived from this work can be downloaded from the SDSS Web site.

Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey

Abstract: Using a nearest neighbor analysis, we construct a sample of void galaxies from the Sloan Digital Sky Survey (SDSS) and compare the photometric properties of these galaxies to the population of nonvoid (wall) galaxies. We trace the density field of galaxies using a volume-limited sample with zmax ¼ 0:089. Galaxies from the flux-limited SDSS with zzmax and fewer than three volume-limited neighbors within 7 h � 1 Mpc are classif ied as void gal- axies. This criterion implies a density contrast ��=� < � 0:6 around void galaxies. From 155,000 galaxies, we obtain a subsample of 13,742 galaxies with zzmax, from which we identify 1010 galaxies as void galaxies. To identify an additional 194 faint void galaxies from the SDSS in the nearby universe, r P 72 h � 1 Mpc, we employ volume-limited samples extracted from the Updated Zwicky Catalog and the Southern Sky Redshift Survey with zmax ¼ 0:025 to trace the galaxy distribution. Our void galaxies span a range of absolute magnitude from Mr ¼ � 13: 5t o� 22.5. Using SDSS photometry, we compare the colors, concentration indices, and Sersic indices of the void and wall samples. Void galaxies are significantly bluer than galaxies lying at higher density. The population of void galaxies with Mr P M � þ 1 and brighter is on average bluer and more concentrated (later type) than galaxies outside of voids. The latter behavior is only partly explained by the paucity of luminous red galaxies in voids. These results generally agree with the predictions of semianalytic models for galaxy formation in cold dark matter models, which indicate that void galaxies should be relatively bluer, more disklike, and have higher specific star formation rates. Subject heading gs: cosmology: observations — galaxies: photometry — galaxies: structure — large-scale structure of universe — methods: statistical

Linking Gas Fractions to Bimodalities in Galaxy Properties

Abstract: Galaxies over 4 decades in stellar mass are shown to obey a strong correlation between u - K colors and atomic-gas-to-stellar mass ratios (G/S), using stellar mass-to-light ratios derived from optical colors. The correlation holds for G/S ranging from nearly 10 : 1 to 1 : 100 for a sample obtained by merging the SDSS DR2, 2MASS, and HyperLeda H I catalogs. This result implies that u - K colors can be calibrated to provide photometric gas fractions for statistical applications. Here this technique is applied to a sample of ~35,000 SDSS-2MASS galaxies to examine the relationship of gas fractions to observed bimodalities in galaxy properties as a function of color and stellar mass. The recently identified transition in galaxy properties at stellar masses ~(2-3) × 1010 M☉ corresponds to a shift in gas richness, dividing low-mass late-type galaxies with G/S ~ 1 : 1 from high-mass galaxies with intermediate-to-low G/S. Early-type galaxies below the transition mass also show elevated G/S, consistent with formation scenarios involving mergers of low-mass gas-rich systems and/or cold-mode gas accretion.

Down‐sizing in galaxy formation at z∼ 1 in the Subaru/XMM–Newton Deep Survey (SXDS)

Abstract: We use the deep wide-field optical imaging data of the Subaru/XMM‐Newton Deep Survey to discuss the luminosity- (mass-)dependent galaxy colours down to z � = 25.0 (5 × 10 9 h −2 70 M� ) for z ∼ 1 galaxies in colour-selected high-density regions. We find an apparent absence of galaxies on the red colour‐magnitude sequence below z � ∼ 24.2, corresponding to ∼M ∗ + 2( ∼ 10 10 M� ) with respect to passively evolving galaxies at z ∼ 1. Galaxies brighter than M ∗ − 0.5 (8 × 10 10 M� ), however, are predominantly red passively evolving systems, with few blue star-forming galaxies at these magnitudes. This apparent age gradient, where massive galaxies are dominated by old stellar populations while less massive galaxies have more extended star formation histories, supports the ‘downsizing’ idea where the mass of galaxies hosting star formation decreases as the Universe ages. Combined with the lack of evolution in the shape of the stellar mass function for massive galaxies since at least z ∼ 1, it appears that galaxy formation processes (both star formation and mass assembly) should have occurred in an accelerated way in massive systems in highdensity regions, while these processes should have been slower in smaller systems. This result provides an interesting challenge for modern cold dark matter based galaxy formation theories which predict later formation epochs of massive systems, commonly referred to as ‘bottom-up’.

The Buildup of the red sequence in galaxy clusters since z ∼ 0.8

Abstract: We study the rest-frame ( ) color-magnitude relation in four clusters at redshifts 0.7–0.8, drawn from the U V ESO Distant Cluster Survey (EDisCS). We confirm that the red-sequence galaxies in these clusters can be described as an old, passively evolving population, and we demonstrate that, by comparison with the Coma Cluster, there has been significant evolution in the stellar mass distribution of red-sequence galaxies since . The EDisCS z ∼ 0.75 clusters exhibit a deficiency of low-luminosity passive red galaxies. Defining as “faint” all galaxies in the passive evolution–corrected range , the luminous-to-faint ratio of red-sequence galaxies varies from 0.4 L/L 0.1 ∗ for the Coma Cluster to for the high-redshift clusters. These results exclude a syn0.34 0.06 0.81 0.18 chronous formation of all red-sequence galaxies and suggest that a large fraction of the faint red galaxies in current clusters moved on to the red sequence relatively recently. Their star formation activity presumably came to an end at . z 0.8 Subject headings: galaxies: clusters: general — galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: formation

Rotationally Supported Virgo Cluster Dwarf Elliptical Galaxies: Stripped Dwarf Irregular Galaxies?

Abstract: New observations of 16 dwarf elliptical galaxies (dEs) in the Virgo Cluster indicate that at least seven dEs have significant velocity gradients along their optical major axis, with typical rotation amplitudes of 20–30 km s-1. Of the remaining nine galaxies in this sample, six have velocity gradients of less than 20 km s-1 kpc-1, while the other three observations had too low a signal-to-noise ratio to determine an accurate velocity gradient. Typical velocity dispersions for these galaxies are ~44 ± 5 km s-1, indicating that rotation can be a significant component of the stellar dynamics of Virgo dEs. When corrected for the limited spatial extent of the spectral data, the rotation amplitudes of the rotating dEs are comparable to those of similar-brightness dwarf irregular galaxies (dIs). Evidence of a relationship between the rotation amplitude and galaxy luminosity is found and, in fact, agrees well with the Tully-Fisher relation. The similarity in the scaling relations of dIs and dEs implies that it is unlikely that dEs evolve from significantly more luminous galaxies. These observations reaffirm the possibility that some cluster dEs may be formed when the neutral gaseous medium is stripped from dIs in the cluster environment. We hypothesize that several different mechanisms are involved in the creation of the overall population of dEs and that stripping of infalling dIs may be the dominant process in the creation of dEs in clusters like Virgo.

Discovery of Face-on Counterparts of Chain Galaxies in the Tadpole Advanced Camera for Surveys Field

Abstract: The background field of the Tadpole galaxy image taken with the Hubble Space Telescope Advanced Camera for Surveys contains 87 clump clusters that are probably low-inclination versions of the 69 chain galaxies that we found in a previous paper. This conclusion follows from the similarity in colors and apparent magnitudes of the brightest clumps in the clump clusters and chain galaxies and from the similarity between the distribution of axial ratios for the combined sample and the distribution for normal disk galaxies. These distributions are consistent with chains and clump clusters being disks viewed at random angles with intrinsic axial ratios of 0.1-0.2 or less. The disks appear to be gas-rich galaxies at z = 1-2 that are forming an early generation of stars in a small number of very large star complexes.

The origin and properties of intracluster stars in a rich cluster

Abstract: We use a multimillion particle N-body + smooth particle hydrodynamics (SPH) simulation to follow the formation of a rich galaxy cluster in a � -cold dark matter (� CDM) cosmology, with the goal of understanding the origin and properties of intracluster stars. The simulation includes gas cooling, star formation, the effects of a uniform ultraviolet background and feedback from supernovae. Haloes that host galaxies as faint as MR =− 19.0 are resolved by this simulation, which includes 85 per cent of the total galaxy luminosity in a rich cluster. We find that the accumulation of intracluster light (ICL) is an ongoing process, linked to infall and stripping events. The unbound star fraction varies with time between 10 and 22 per cent of the total amount of cluster stars, with an overall trend to increase with time. The fraction is 20 per cent at z = 0, consistent with observations of galaxy clusters. The surface brightness profile of the cD galaxy shows an excess compared with a de Vaucouleur profile near 200 kpc, which is also consistent with observations. Both massive and small galaxies contribute to the formation of the ICL, with stars stripped preferentially from the outer, lower-metallicity parts of their stellar distributions. Simulated observations of planetary nebulae (PNe) show significant substructure in velocity space, tracing separate streams of stripped intracluster stars. Despite an unrelaxed distribution, individual intracluster PNe might be useful mass tracers if more than five fields at a range of radii have measured line-of-sight velocities, where an accurate mass calculation depends more on the number of fields than the number of PNe measured per field. However, the orbits of intracluster stars are more anisotropic than those of galaxies or dark matter, which leads to a systematic underestimate of cluster mass relative to that calculated with galaxies, if not accounted for in dynamical models. Overall, the properties of ICL formed in a hierarchical scenario are in good agreement with current observations, supporting a model where ICL originates from the dynamical evolution of galaxies in dense environments. ICL should thus be ubiquitous in galaxy clusters.

Galaxy Clustering and Galaxy Bias in a ΛCDM Universe

Abstract: We investigate galaxy clustering and the correlations between galaxies and mass in the CDM cosmological model (inflationary cold dark matter with m ¼ 0:4, ¼ 0:6, h ¼ 0:65, n ¼ 0:95, and � 8 ¼ 0:8), using a large, smoothed particle hydrodynamics simulation (SPH; with 2 � 144 3 particles in a 50 h � 1 Mpc cube). Simulated galaxies can be unambiguously identified as clumps of stars and cold gas a few kpc to a few tens of kpc across, residing in extended halos of hot gas and dark matter; the space density of the resolved galaxy population at z ¼ 0 corresponds to that of observed galaxies with luminosity LkL� =4. We investigate the galaxy correlation function, the pairwise velocity dispersion and mean pairwise velocity, and the second and third moments of counts in cells; we also investigate the galaxy-mass correlation function and the average extended mass distributions around galaxies, both of which can be measured via galaxy-galaxy lensing. For the most part, the predicted biases between galaxies and dark matter lead to good agreement with current observations, including (1) a nearly constant comoving correlation length from z ¼ 3 to 0 for mass-selected galaxy samples of constant comoving space density; (2) an rms bias factor b� � 1a tz ¼ 0; (3) a scale-dependent bias on small scales that transforms the curved dark matter correlation function into a nearly power-law galaxy correlation function; (4) galaxy pairwise dispersion and hierarchical skewness ratio S3 in good agreement with observed values, lower than values for the dark matter by � 20%; (5) a ratio of galaxy-galaxy to galaxy-mass correlation functions consistent with recent measurements from the Red Cluster Sequence survey; and (6) a mean excess mass M ð260 h � 1 kpcÞ approximately proportional to galaxy baryon mass Mb, in agreement with estimates from the Sloan Digital Sky Survey (SDSS). All these clusterin gp roperties vary with galaxy baryon mass and, more strongly, with the age of a galaxy’s stellar population. The predicted dependences are in good qualitative agreement with the observed dependence of galaxy clustering and the galaxy-mass correlation function on galaxy type. The predicted ratio Mð260 h � 1 kpcÞ=Mb is lower than the SDSS estimates by a factor of � 1.5‐3 for galaxies with Mb k2 � 10 11 M� . A test with a higher resolution (smaller volume) simulation suggests that this discrepancy is largely a numerical artifact; if so, then the SDSS weak-lensing comparison leaves limited room for feedback or other astrophysical processes to reduce the stellar masses of luminous galaxies, at least given our adopted cosmological parameters. On the whole, our results show that the CDM model and the galaxy formation physics incorporated in the SPH simulation give a good account of observed galaxy clustering, but anticipated improvements in clustering and weak-lensing measurements will soon test this picture in much greater detail. Subject headings: cosmology: theory — galaxies: formation — large-scale structure of universe — methods: numerical

The environmental dependence of radio-loud AGN activity and star formation in the 2dFGRS

Abstract: By combining the 2-degree Field Galaxy Redshift Survey with the NRAO VLA Sky Survey at 1.4 GHz, the environments of radio-loud active galactic nuclei (AGN) in the nearby Universe are investigated using both local projected galaxy densities and a friends-of-friends group-finding algorithm. Radio-loud AGN are preferentially located in galaxy groups and poor-to-moderate richness galaxy clusters. The AGN fraction appears to depend more strongly on the large-scale environment (group, cluster, etc.) in which a galaxy is located than on its more local environment, except at the lowest galaxy surface densities where practically no radio-loud AGN are found. The ratio of absorption-line to emission-line AGN changes dramatically with environment, with essentially all radio-loud AGN in rich environments showing no emission lines. This result could be connected with the lack of cool gas in cluster galaxies, and may have important consequences for analyses of optically selected AGN, which are invariably selected on emission-line properties. The local galaxy surface density of the absorption-line AGN is strongly correlated with radio luminosity, implying that the radio luminosities may be significantly boosted in dense environments due to confinement by the hot intracluster gas. The environments of a radio-selected sample of star-forming galaxies are also investigated to provide an independent test of optical studies. In line with those studies, the fraction of star-forming galaxies is found to decrease strongly with increasing local galaxy surface density; this correlation extends across the whole range of galaxy surface densities, with no evidence for the density threshold found in some optical studies.