Clusters of Galaxies

By exploiting the data of three large surveys (WINGS, NFPS and SDSS), we present here a comparative analysis of the Fundamental Plane Log(R e) = a Log(σ) + b µ e + c (FP hereafter) of the early-type galaxies (ETGs) belonging to 59 galaxy clusters in the redshift range 0.04 < z < 0.07. We show that the variances of the distributions of the FP coefficients derived for the clusters in our sample are just marginally consistent with the hypothesis of universality of the FP relation. By investigating the origin of such remarkable variances we find that, besides a couple of obvious factors, such as the adopted fitting technique and the method used to measure the photometric and kinematic variables, the coefficients of the FP are also influenced by the distribution of photometric/kinematic properties of galaxies in the particular sample under analysis. In particular, some indication is found that the FP coefficients intrinsically depend on the particular luminosity range of the sample, suggesting that bright and faint ETGs could have systematically different FPs. We speculate

Bar-like structures were investigated in a sample of 186 disk galaxies larger than 0.5 arcsec that are in the I-band image of the Tadpole galaxy taken with the Hubble Space Telescope Advanced Camera for Surveys. We found 22 clear cases of barred galaxies, 21 galaxies with small bars that appear primarily as isophotal

We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, F 2−10 keV ≈ 2.4 × 10 −13 erg s −1 cm −2 , X-ray source 1SAXJ0027.2-1930, ∼ 2 arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, z = 0.2409; this is lower than the value derived by Leir and . The source X-ray luminosity, L 2−10 keV = 7.7 × 10 43 erg s −1 , and intracluster gas temperature, T = 2.9 keV, make this cluster interesting for cosmological studies of the cluster L X − T relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at z=0.2274, and an M-type star, whose emission are blended to form an extended X-ray emission ∼ 4 arcmin north of the A33 cluster. A third possibly point-like X-ray source detected ∼ 3 arcmin north-west of A33 lies close to a spiral galaxy at z=0.2863 and to an elliptical galaxy at the same redshift as the cluster.

1 Based on observations obtained with the Hubble Space Telescope of Space Telescope Science Institute, under contract with the National Aeronautics and Space Administration (NASA) 2 Based on observations obtained with the Shane Telescope at Mt. Hamilton, UCO/Lick Observatory

We present a new wide field image of the distant cluster AC114 (z = 0.31) obtained with Wide Field and Planetary Camera II onboard the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond that derived by Smail et al. (1995a) from a single WF/PC-1 pointing. In conjunction with published ground-based spectroscopy, we utilise several newly-discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members.

We present results of ultra-deep ISOCAM observations through a cluster-lens at 7µm and 15µm with the Infrared Space Observatory (ISO) satellite. These observations reveal a large number of luminous Mid-Infrared (MIR) sources. Cross-identification in the optical and Near-Infrared (NIR) wavebands shows that about half of the 7 µm sources are cluster galaxies. The other 7 µm and almost all 15 µm sources are identified as lensed distant galaxies. Thanks to the gravitational amplification they constitute the faintest MIR detected sources, allowing us to extend the number counts in both the 7 and 15 µm bands. In particular, we find that the 15 µm counts have a steep slope α 15µm = −1.5 ± 0.3 and are large, with N 15µm (> 30µJy) = 13 ± 5 arcmin −2 . These numbers rule out non-evolutionary models and favour very strong evolution. Down to our counts limit, we found that the resolved 7 µm and 15 µm background radiation intensity is respectively (2±0.5)×10 −9 and (5±1)×10 −9 W m −2 sr −1 .

We consider the Universe deep inside the cell of uniformity. At these scales, the Universe is filled with inhomogeneously distributed discrete structures (galaxies, groups and clusters of galaxies), which perturb the background Friedmann model. Here, the mechanical approach (Eingorn & Zhuk, 2012) is the most appropriate to describe the dynamics of the inhomogeneities which is defined, on the one hand, by gravitational potentials of inhomogeneities and, on the other hand, by the cosmological expansion of the Universe. In this paper, we present additional arguments in favor of this approach. First, we estimate the size of the cell of uniformity. With the help of the standard methods of statistical physics and for the galaxies of the type of the Milky Way and Andromeda, we get that it is of the order of 190 Mpc which is rather close to observations. Then, we show that the nonrelativistic approximation (with respect to the peculiar velocities) is valid for $z \lesssim 10$, i.e. approximately for 13 billion years from the present moment. We consider scalar perturbations and, within the $\Lambda$CDM model, justify the main equations. Moreover, we demonstrate that radiation can be naturally incorporated into our scheme. This emphasizes the viability of our approach. This approach gives a possibility to analyze different cosmological models and compare them with the observable Universe. For example, we indicate some problematic aspects of the spatially flat models. Such models require a rather specific distribution of the inhomogeneities to get a finite potential at any points outside gravitating masses. We also criticize the application of the Schwarzschild-de Sitter solution to the description of the motion of test bodies on the cosmological background.

We report the first detailed X-ray and optical observations of the medium-distant cluster A33 obtained with the Beppo-SAX satellite and with the UH 2.2m and Keck II telescopes at Mauna Kea. The information deduced from X-ray and optical imaging and spectroscopic data allowed us to identify the X-ray source 1SAXJ0027.2-1930 as the X-ray counterpart of the A33 cluster. The faint, F 2−10 keV ≈ 2.4 × 10 −13 erg s −1 cm −2 , X-ray source 1SAXJ0027.2-1930, ∼ 2 arcmin away from the optical position of the cluster as given in the Abell catalogue, is identified with the central region of A33. Based on six cluster galaxy redshifts, we determine the redshift of A33, z = 0.2409; this is lower than the value derived by Leir and . The source X-ray luminosity, L 2−10 keV = 7.7 × 10 43 erg s −1 , and intracluster gas temperature, T = 2.9 keV, make this cluster interesting for cosmological studies of the cluster L X − T relation at intermediate redshifts. Two other X-ray sources in the A33 field are identified. An AGN at z=0.2274, and an M-type star, whose emission are blended to form an extended X-ray emission ∼ 4 arcmin north of the A33 cluster. A third possibly point-like X-ray source detected ∼ 3 arcmin north-west of A33 lies close to a spiral galaxy at z=0.2863 and to an elliptical galaxy at the same redshift as the cluster.

We describe our ongoing program designed to measure the SN-Ia rate in a sample of massive z=0.5-0.9 galaxy clusters. The SN-Ia rate is a poorly known observable, especially at high z, and in cluster environments. The SN rate and its redshift dependence can serve as powerful discrimiminants for a number of key issues in astrophysics and cosmology. Our observations will put clear constraints on the characteristic SN-Ia "delay time", the typical time between the formation of a stellar population and the explosion of some of its members as SNe-Ia. Such constraints can exclude entire categories of SN-Ia progenitor models, since different models predict different delays. These data will also help to resolve the question of the dominant source of the high metallicity in the intracluster medium (ICM) -SNe-Ia, or core-collapse SNe from an early stellar population with a top-heavy IMF, perhaps those population III stars responsible for the early re-ionization of the Universe. Since clusters are excellent laboratories for studying enrichment (they generally have a simple star-formation history, and matter cannot leave their deep potentials), the results will be relevant for understanding metal enrichment in general, and the possible role of first generation stars in early Universal enrichment. Observations obtained so far during cycles 14 and 15 yield many SNe in our cluster fields, but our follow-up campaign reveals most are not in cluster galaxies.

A Chandra image of the central 100 kpc of the Abell 2597 cluster of galaxies shows bright, irregular, X-ray emission within the central dominant cluster galaxy (CDG), and two low surface brightness cavities located 30 kpc from the CDG's nucleus. Unlike the cavities commonly seen in other clusters, Abell 2597's "ghost" cavities are not coincident with the bright central radio source. Instead, they appear to be associated with faint, extended radio emission seen in a deep VLA radio map. We interpret the ghost cavities as buoyantly-rising relics of a radio outburst that occurred between 50-100 Myr ago. The demography of cavities in the few clusters studied thus far shows that galactic radio sources experience recurrent outbursts on a ∼ 100 Myr timescale. Over the lifetime of a cluster, ghost cavities emerging from CDGs deposit > ∼ 10 59−61 erg of energy into the intracluster medium. If a significant fraction of this energy is deposited as magnetic field, it would account for the high field strengths in the cooling flow regions of clusters. The similarity between the central cooling time of the keV gas and the radio cycling timescale suggests that feedback between cooling gas and the radio source may be retarding or quenching the cooling flow.

We investigate the origin of the color-magnitude relation (CMR) observed in cluster galaxies by using a combination of cosmological N-body/SPH simulations of galaxy clusters, and a semi-analaytic model of galaxy formation (Lagos, Cora & Padilla 2008). Simulated results are compared with the photometric properties of early-type galaxies in the Antlia cluster (Smith Castelli et al. 2008). The good agreement obtained between observations and simulations allows us to use the information provided by the model for unveiling the physical processes that yield the tigh observed CMR.

Over the past years observations of young and populous star clusters have shown that the stellar IMF appears to be an invariant featureless Salpeter power-law with an exponent α = 2.35 for stars more massive than a few M ⊙ . A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters, and that the mass function of young star clusters in the solar-neighborhood and in interacting galaxies can be described, over the mass range of a few 10 M ⊙ to 10 7 M ⊙ , as a power-law with an exponent β ≈ 2. These two results imply that galactic-field IMFs for early-type stars cannot, under any circumstances, be a Salpeter power-law, but that they must have a steeper exponent α field > ∼ 2.8. This has important consequences for the distribution of stellar remnants and for the chemo-dynamical and photometric evolution of galaxies.

Using photometric redshifts we determine the galaxy population of the clusters of galaxies Cl0016+16 at z = 0.55, Cl1600+41 at z = 0.54, Cl1601+42 at z = 0.54 and MS1008-1224 at z = 0.31. Comparing the clusters, we find no evidence for a universal shape of the total luminosity function (LF) at these redshifts. When dividing the LFs into spectral types, we find that the LF of the early-type galaxies alone can be described by a Gaussian, while the LF of the late-type galaxies is well fitted by a Schechter function, suggesting that the separate LFs for different populations may be universal. The difference in the total LFs can mainly be attributed to the varying relative normalisation of these populations, implying that clusters with an abundant population of late-type galaxies also have steeper faint-end slopes. In MS1008-1224 we detect a faint blue population that dominates over a population with colours consistent with dwarf ellipticals, opposite to clusters at lower redshift.

In previous work on galaxy clusters, several authors reported the discovery of an unusual population of galaxies, which have spiral morphologies, but do not show any star-formation activity. These galaxies are called "passive spirals", and have been interesting since it has been difficult to understand the existence of such galaxies. Using a volume-limited sample (0.05< z <0.1 and Mr * < −20.5; 25813 galaxies) of the Sloan Digital Sky Survey data, we found 73 (0.28±0.03%) passive spiral galaxies and studied their environments. It is found that passive spiral galaxies exist in a local galaxy density of 1-2 Mpc −2 and have a 1-10 cluster-centric virial radius. Thus, the origins of passive spiral galaxies are likely to be cluster-related. These characteristic environments coincide with a previously reported environment where the galaxy star-formation rate suddenly declines and the so-called morphology-density relation turns. It is likely that the same physical mechanism is responsible for all of these observational results. The existence of passive spiral galaxies suggests that a phys-1 ical mechanism that works calmly is preferred to dynamical origins such as major merger/interaction since such a mechanism would destroy the spiral-arm structures. Compared with the observed cluster galaxy evolution such as the Butcher-Oemler effect and the morphological Butcher-Oemler effect, passive spiral galaxies are likely to be a key galaxy population in transition between red, elliptical/S0 galaxies in lowredshift clusters and blue, spiral galaxies more numerous in higher-redshift clusters.

We present optical VLT spectroscopy of 16 dwarf elliptical galaxies (or dEs) comparable in mass to NGC 205, and belonging to the Fornax cluster and to nearby groups of galaxies. Using full-spectrum fitting, we derive radial profiles of the SSP-equivalent ages and metallicities. We make a detailed analysis with ulyss and steckmap of the star-formation history in the core of the galaxies and in an aperture of one effective radius. We resolved the history into 1 to 4 epochs. The statistical significance of these reconstructions were carefuly tested; the two programs give remarkably consistent results.

We revisit the XMM-Newton observation of M87 to make a new and more detailed measurement of the metal abundance profile. After having verified that spectral fits with single temperature models show a dramatic abundance drop within 1 arcmin of the cluster core, we show that more appropriate models, which include a multi-temperature component to account for the strong temperature gradient observed in M87, and a power-law component to account for the emission of the nucleus and knot A, give a substantially flat abundance profile. The drastic abundance decrement found by fitting a single temperature component to the data is an artifact known as "Fe-bias" (Buote 2000a,b) following from the application of an oversimplified spectral model to the data.

Using extensive kinematical and morphological data on two Virgo cluster galaxies undergoing strong nuclear star formation, we show that ram pressure stripping and gravitational interactions can act together on galaxies that have recently fallen into clusters.

Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z ∼ 0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high redshift EDisCS clusters exhibit a significant deficit of faint red galaxies. Combining clusters in three different redshift bins, and defining as 'faint' all galaxies in the range 0.4 L/L * 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z ∼ 0.8 to z ∼ 0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved onto the red-sequence below z ∼ 0.8.

The relevance of the Tremaine-Weinberg (TW) method is tested to measure the bar, spiral and inner structure pattern speeds using a gaseous velocity field. The TW method is applied to various simulated barred galaxies in order to demonstrate its validity in seven different configurations, including star formation or/and dark matter halo. The reliability of the different physical processes involved and of the various observational parameters are also tested. The simulations show that the TW method could be applied to the gaseous velocity fields to get a good estimate of the bar pattern speed, under the condition that regions of shocks are avoided and measurements are confined to regions where the gaseous bar is well formed. We successfully apply the TW method to the \ha velocity field of the Virgo Cluster Galaxy M100 (NGC 4321) and derive pattern speeds of 55+/-5 km/s/kpc for the nuclear structure, 30+/-2 km/s/kpc for the bar and 20+/-1 km/s/kpc for the spiral pattern, in full agreement with published determinations using the same method or alternative ones.

Abstract: We develop a toy-model for the chemical evolution of the intracluster medium, polluted by the galactic winds from elliptical galaxies. The model follows the&quot; galaxy formation history&quot; of cluster galaxies, constrained by the observed luminosity function.

Fundamental Plane studies provide an excellent means of understanding the evolutionary history of early-type galaxies. Using the Low Resolution Imaging Spectrograph on the Keck telescope, we obtained internal stellar kinematic information for 36 field galaxies in the Groth Strip-21 early-type and 15 disk galaxies. Their redshifts range from 0.3-1.0, with a median redshift 0.8. The slope of the relation shows no difference compared with the local slope. However, there is significant evolution in the zero-point offset; an offset due to evolution in magnitude requires a 2.4 magnitude luminosity brightening at z=1. We see little differences of the offset with bulge fraction, which is a good surrogate for galaxy type. Correcting for the luminosity evolution reduces the orthogonal scatter in the Fundamental Plane to 8%, consistent with the local scatter. This scatter is measured for our sample, and does not include results from other studies which may have different selection effects. The difference in the degree of evolution between our field sample and published cluster galaxies suggests a more recent formation epocharound z = 1.5 for field galaxies compared to z > 2.0 for cluster galaxies. The magnitude difference implies that the field early-type galaxies are about 2 Gyr younger than the cluster ellipticals using standard single-burst models. However, the same models imply a significant change in the rest-frame U − B color from then to present, which is not seen in our sample. Continuous low-level star formation, however, would serve to explain the constant colors over this large magnitude change. A consistent model has 7% of the stellar mass created after the initial burst, using an exponentially decaying star formation rate with an e-folding time of 5 Gyr.

We study the evolution of spectral early-type galaxies in clusters, groups and the field up to redshift 0.9 using the ESO Distant Cluster SUrvey (EDisCS) dataset. We measure structural parameters (circularized half-luminosity radii R e , surface brightness I e , and velocity dispersions σ) for 154 cluster and 68 field galaxies. On average, we achieve precisions of 10% in R e , 0.1 dex in log I e and 10% in σ. We sample ≈ 20% of cluster and ≈ 10% of field spectral early-type galaxies down to an I band magnitude in a 1 arcsec radius aperture of I 1 = 22. We study the evolution of the zero point of the fundamental plane (FP) and confirm results in the literature, but now also for the low cluster velocity dispersion regime. Taken at face value, the mass-to-light ratio varies as ∆ log M/L B = (−0.54 ± 0.01)z = (−1.61 ± 0.01) log(1 + z) in clusters, independent of their velocity dispersion. The evolution is stronger (∆ log M/L B = (−0.76 ± 0.01)z = (−2.27 ± 0.03) log(1 + z)) for field galaxies. A somewhat milder evolution is derived if a correction for incompleteness is applied. A rotation in the FP with redshift is detected with low statistical significance. The α and β FP coefficients decrease with redshift, or, equivalently, the FP residuals correlate with galaxy mass and become progressively negative at low masses. The effect is visible at z ≥ 0.7 for cluster galaxies and at lower redshifts z ≥ 0.5 for field galaxies. We investigate the size evolution of our galaxy sample. In agreement with previous results, we find that the half-luminosity radius for a galaxy with a dynamical or stellar mass of 2 × 10 11 M ⊙ varies as (1 + z) −1.0±0.3 for both cluster and field galaxies. At the same time, stellar velocity dispersions grow with redshift, as (1 + z) 0.59±0.10 at constant dynamical mass, and as (1 + z) 0.34±0.14 at constant stellar mass. The measured size evolution reduces to R e ∝ (1 + z) −0.5±0.2 and σ ∝ (1 + z) 0.41±0.08 , at fixed dynamical masses, and R e ∝ (1 + z) −0.68±0.4 and σ ∝ (1 + z) 0.19±0.10 , at fixed stellar masses, when the progenitor bias (PB, galaxies that locally are of spectroscopic early-type, but are not very old, disappear progressively from the EDisCS high-redshift sample; often these galaxies happen to be large in size) is taken into account. Taken together, the variations in size and velocity dispersion imply that the luminosity evolution with redshift derived from the zero point of the FP is somewhat milder than that derived without taking these variations into account. When considering dynamical masses, the effects of size and velocity dispersion variations almost cancel out. For stellar masses, the luminosity evolution is reduced to L B ∝ (1 + z) 1.0 for cluster galaxies and L B ∝ (1 + z) 1.67 for field galaxies. Using simple stellar population models to translate the observed luminosity evolution into a formation age, we find that massive (> 10 11 M ⊙ ) cluster galaxies are old (with a formation redshift z f > 1.5) and lower mass galaxies are 3-4 Gyr younger, in agreement with previous EDisCS results from color and line index analyses. This confirms the picture of a progressive build-up of the red sequence in clusters with time. Field galaxies follow the same trend, but are ≈ 1Gyr younger at a given redshift and mass. Taking into account the size and velocity dispersion evolution quoted above pushes all formation ages upwards by 1 to 4 Gyr.

We present results of our wide-field redshift survey of galaxies in a 285 square degree region of the Shapley Supercluster (SSC), based on a set of 10529 velocity measurements (including 1201 new ones) on 8632 galaxies obtained from various telescopes and from the literature. Our data reveal that the main plane of the SSC (v~ 14500 km/s) extends further than previous estimates, filling the whole extent of our survey region of 12~degrees by 30~degrees on the sky (30 x 75~h-1 Mpc). There is also a connecting structure associated with the slightly nearer Abell~3571 cluster complex (v~ 12000km/s. These galaxies seem to link two previously identified sheets of galaxies and establish a connection with a third one at V= 15000 km/s near R.A.= 13h. They also tend to fill the gap of galaxies between the foreground Hydra-Centaurus region and the more distant SSC. In the velocity range of the Shapley Supercluster (9000 km/s < cz < 18000 km/s), we found redshift-space overdensities with b\_j < 17.5 of ~5.4 over the 225 square degree central region and ~3.8 in a 192 square degree region excluding rich clusters. Over the large region of our survey, we find that the intercluster galaxies make up 48 per cent of the observed galaxies in the SSC region and, accounting for the different completeness, may contribute nearly twice as much mass as the cluster galaxies. In this paper, we discuss the completeness of the velocity catalogue, the morphology of the supercluster, the global overdensity, and some properties of the individual galaxy clusters in the Supercluster.

We present the data and methods that we have used to perform a detailed UVoptical study of the nearby dwarf starburst galaxy NGC 4214 using multifilter HST/WFPC2+STIS photometry. We explain the process followed to obtain high-quality photometry and astrometry of the stellar and cluster populations of this galaxy. We describe the procedure used to transform magnitudes and colors into physical parameters using spectral energy distributions. The data show the existence of both young and old stellar populations that can be resolved at the distance of NGC 4214 (2.94 Mpc) and we perform a general description of those populations.

We present a wide field multi-color weak lensing survey of 11 X-ray luminous galaxy clusters all selected in a narrow redshift range near z ! 0.2. The weak lensing methodology used is based on our recent analysis used on the massive cluster Abell 1689, with some improved feature. We compare the total mass estimates to other mass indicators such as

We investigate the evolution of the fractions of late type cluster galaxies as a function of redshift, using one of the largest, most uniform cluster samples available. The sample consists of 514 clusters of galaxies in the range 0.02≤z≤0.3 from the Sloan Digital Sky Survey Cut & Enhance galaxy cluster catalog. This catalog was created using a single automated cluster finding algorithm applied to uniform data from a single telescope, with accurate CCD photometry, thus, minimizing selection biases. We use four independent methods to analyze the evolution of the late type galaxy fraction. Specifically, we select late type galaxies based on: restframe g −r color, u−r color, galaxy profile fitting and concentration index. The first criterion corresponds to 1 the one used in the classical Butcher-Oemler analyses. The last two criteria are more sensitive to the morphological type of the galaxies. In all the four cases, we find an increase in the fraction of late type galaxies with increasing redshift, significant at the 99.9% level. The results confirm that cluster galaxies do change colors with redshift (the Butcher-Oemler effect) and, in addition, they change their morphology to latertype toward higher redshift -indicating a morphological equivalent of the Butcher-Oemler effect. We also find a tendency of richer clusters to have lower fractions of late type galaxies. The trend is consistent with a ram pressure stripping model, where galaxies in richer clusters are affected by stronger ram pressure due to higher temperature of clusters.

Using deep two-band imaging from the Hubble Space Telescope, we measure the color-magnitude relations (CMR) of E/S0 galaxies in a set of 9 optically-selected clusters principally from the Red-Sequence Cluster Survey (RCS) at 0.9 < z < 1.23. We find that the mean scatter in the CMR in the observed frame of this set of clusters is 0.049±0.008, as compared to 0.031±0.007 in a similarly imaged and identically analyzed X-ray sample at similar redshifts. Single-burst stellar population models of the CMR scatter suggest that the E/S0 population in these RCS clusters truncated their star-formation at z l ≃ 1.6, some 0.9 Gyrs later than their X-ray E/S0 counterparts which were truncated at z l ≃ 2.1. The notion that this is a manifestation of the differing evolutionary states of the two populations of cluster galaxies is supported by comparison of the fraction of bulge-dominated galaxies found in the two samples which shows that optically-selected clusters contain a smaller fraction of E/S0 galaxies at the their cores.

Massive quiescent galaxies at z > 1 have been found to have small physical sizes, hence to be superdense. Several mechanisms, including minor mergers, have been proposed for increasing galaxy sizes from high-to low-z. We search for superdense massive galaxies in the WIde-field Nearby Galaxy-cluster Survey (WINGS) of X-ray selected galaxy clusters at 0.04<z<0.07. We discover a significant population of superdense massive galaxies with masses and sizes comparable to those observed at high redshift. They approximately represent 22% of all cluster galaxies more massive than 3×10 10 M ⊙ , are mostly S0 galaxies, have a median effective radius R e = 1.61 ± 0.29kpc, a median Sersic index n = 3.0 ± 0.6, and very old stellar populations with a median mass-weighted age of 12.1±1.3 Gyr. We calculate a number density of 2.9×10 −2 Mpc −3 for superdense galaxies in local clusters, and a hard lower limit of 1.3×10 −5 Mpc −3 in the whole comoving volume between z = 0.04 and z = 0.07. We find a relation between mass, effective radius and luminosity-weighted age in our cluster galaxies, which can mimic the claimed evolution of the radius with redshift, if not properly taken into account. We compare our data with spectroscopic high-z surveys and find that -when stellar masses are considered -there is consistency with the local WINGS galaxy sizes out to z ∼ 2, while a discrepancy of a factor of 3 exists with the only spectroscopic z > 2 study. In contrast, there is strong evidence for a large evolution in radius for the most massive galaxies with M * > 4×10 11 M ⊙ compared to similarly massive galaxies in WINGS, i.e. the BCGs.

We present the results of a search for Be stars in six fields centered on the young clusters NGC 330 and NGC 346 in the SMC, and NGC 1818, NGC 1948, NGC 2004 and NGC 2100 in the LMC. Be stars were identified by differencing R band and narrow-band Hα CCD images. Our comparatively large images provide substantial Be star populations both within the clusters and in their surrounding fields. Magnitudes, positions and finding charts are given for the 224 Be stars found. The fraction of Be stars to normal B stars within each cluster is found to vary significantly although the average ratio is similar to the average Be to B star ratio found in the Galaxy. In some clusters, the Be star population is weighted to magnitudes near the main sequence turn-off. The Be stars are redder in V −I than normal main-sequence stars of similar magnitude and the redness increases with increasing Hα emission strength.

The distribution of galaxy properties in groups and clusters holds important information on galaxy evolution and growth of structure in the Universe. While clusters have received appreciable attention in this regard, the role of groups as fundamental to formation of the present day galaxy population has remained relatively unaddressed. Here we present stellar ages, metallicities and α-element abundances derived using Lick indices for 67 spectroscopically confirmed members of the NGC 5044 galaxy group with the aim of shedding light on galaxy evolution in the context of the group environment.

We summarize several results based on the velocity data-set for cluster galaxies provided by ENACS (the ESO Nearby Abell Cluster Survey). We describe the distribution of velocity dispersions of a complete sample of rich galaxy clusters, and compare it to the distribution of cluster X-ray temperatures, and with predictions of theoretical models. We then address the issue of the existence of a Fundamental Plane (FP) for rich clusters, first suggested by Schaeffer et al. We confirm the existence of this FP with the ENACS data-set. The cluster FP is different from the FP of elliptical galaxies, and from the virial prediction. Some implications of the cluster FP are discussed. Finally, we describe the phase-space distributions of different populations of cluster galaxies. Different galaxy classes are defined according to their morphological or spectral type, and the presence of emission-lines. Star-forming (late-type) galaxies have a velocity dispersion profile that (in combination with ...

We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV; λ ef f = 2267Å, ∆λ = 732Å) and far-ultraviolet (FUV; λ ef f = 1516 A, ∆λ = 268Å), respectively. Our UV catalog has been complemented with existing optical and near-infrared photometry. The UV properties of GCs have

We present MUSE, a software framework for combining existing computational tools for different astrophysical domains into a single multiphysics, multiscale application. MUSE facilitates the coupling of existing codes written in different languages by providing inter-language tools and by specifying an interface between each module and the framework that represents a balance between generality and computa

The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W (%SDSS g) and F850LP (%SDSS z) filters. We derive distances for 84 of these galaxies using the method of surface brightness fluctuations (SBFs), present the SBF distance catalog, and use this database to examine the three-dimensional distribution of early-type galaxies in the Virgo Cluster. The SBF distance moduli have a mean (random) measurement error of 0.07 mag (0.5 Mpc), or roughly 3 times better than previous SBF measurements for Virgo Cluster galaxies. Five galaxies lie at a distance of d % 23 Mpc and are members of the W 0 cloud. The remaining 79 galaxies have a narrow distribution around our adopted distance of hdi ¼ 16:5 AE 0:1 (random mean error) AE1.1 Mpc (systematic). The rms distance scatter of this sample is (d ) ¼ 0:6 AE 0:1 Mpc, with little or no dependence on morphological type or luminosity class (i.e., 0:7 AE 0:1 and 0:5 AE 0:1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of early-type galaxies is 2:4 AE 0:4 Mpc (i.e., AE 2 of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16:7 AE 0:2 and 16:4 AE 0:2 Mpc, respectively. There may be a third subcluster associated with M86. A weak correlation between velocity and line-of-sight distance may be a faint echo of the cluster velocity distribution not having yet completely virialized. In three dimensions, Virgo's early-type galaxies appear to define a slightly triaxial distribution, with axis ratios of (1: 0.7 : 0.5). The principal axis of the best-fit ellipsoid is inclined $20 Y 40 from the line of sight, while the galaxies belonging to the W 0 cloud lie on an axis inclined by $10 Y15 .