The H I deficiency pattern of the spiral population in the Virgo cluster region reveals a significant number of galaxies at very large clustercentric distances with gaseous deficiencies comparable to those measured in the cluster centers. We have used the output of cosmological $N$-body simulations to investigate whether the gas-deficient galaxies on the outskirts of the Virgo cluster may have previously passed through its core. We find that the maximum radius reached by infalling galaxies as they bounce out of a Virgo-like cluster must be less than 2.5 virial radii, which results in a predicted velocity-distance diagram noticeably different from the one drawn by the data. The latter is fairly well reproduced, however, after including in the simulations distance errors at the 20% relative rms level. Yet, for several objects apparently over 5 Mpc in front or behind the Virgo center the assumption that they are instead close enough to the cluster to have passed already through its core strains the bounds of plausibility. Hence, unless these outlying H I-deficient Virgo's spirals have grossly incorrect distances, they cannot have lost their gas by interactions with the intracluster medium. This suggests that the evolution of spirals might begin already on the suburbs of clusters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss evidence in local, present-day clusters of galaxies (from the ENACS survey) about the way in which those clusters have evolved and about the evolutionary relationships between the galaxies of different morphological types in them. This evidence is complementary to that obtained from the study of clusters at intermediate and high redshifts. We argue that the spatial distribution and the kinematics of the various types of galaxies in and outside substructures support the following picture.

The elliptical and S0 galaxies have been around for a long time and have obtained an isotropic velocity distribution. The spatial distribution and kinematics of the early spirals are consistent with the idea that many of their kind have transformed into an S0, but that they have survived, most likely because of their velocities. The distribution and kinematics of the late spirals are consistent with a picture in which they have been accreted fairly recently. They have mildly radial orbits and hardly populate the central regions, most likely because they suffer tidal disruption. Finally, the distribution and kinematics of the galaxies in substructures, when taken at face value, imply tangential velocity anisotropy for these galaxies, but this result may be (partly) due to the procedure by which these galaxies are selected. A first attempt to take the effects of selection into account shows that isotropic (or even mildly radial) orbits of subcluster galaxies cannot be excluded.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We provide the first direct lifting of the mass/anisotropy degeneracy for a cluster of galaxies, by jointly fitting the line of sight velocity dispersion and kurtosis profiles of the Coma cluster, assuming an NFW tracer density profile, a generalized-NFW dark matter profile and a constant anisotropy profile. We find that the orbits in Coma must be quasi-isotropic, and find a mass consistent with previous analyses, but a concentration parameter 50% higher than expected in cosmological $N$-body simulations. We then test the accuracy of our method on realistic non-spherical systems with substructure and streaming motions, by applying it to the ten most massive structures in a cosmological $N$-body simulation. We find that our method yields fairly accurate results on average (within 20%), although with a wide variation (factor 1.7 at 1$\sigma$) for the concentration parameter, with decreased accuracy and efficiency when the projected mean velocity is not constant with radius.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the first determination of the pairwise velocity dispersion of galaxies at different luminosity with the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). Our result surprisingly shows that the random velocities of the faint galaxies are very high, around $700 {\rm km\,s}^{-1}$, reaching similar values as the brightest galaxies. At intermediate luminosities slightly brighter than the characteristic luminosity $M_*$, the velocities exhibit a well defined steep minimum near $400 {\rm km\,s}^{-1}$. The result challenges the current halo model of galaxies of Yang et al. that was obtained by matching the clustering and luminosity function of 2dFGRS, and can be an important constraint in general on theories of galaxy formation, e.g., the semi-analytical model. Combining the observed luminosity dependence of clustering, our result implies that quite a fraction of faint galaxies are in massive halos of galaxy clusters as the brightest ones, but most of the $M_*$ galaxies are in galactic halos.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a multi-wavelength study of a pair of head-tailed radio galaxies in an in-falling filament of the A3125/A3128 cluster complex which is located at the centre of the massive Horologium Reticulum Supercluster (HRS). We compare the large-scale structure evident from extensive optical observations of the region with the dynamical information gleaned from the morphology of the radio jets. We argue that in addition to the acceleration produced by in-fall toward the centre of A3125, the closest galaxy cluster, the radio jets show some evidence of bulk motion along the major filament of the HRS.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the preliminary results of BVRI imaging of six clusters chosen from a follow-up project of the CNOC cluster survey. In an attempt to ascertain the evolution of galaxies in the cluster environment, we focus on the infalling galaxy groups, especially on their characteristics reflected by their color properties. To investigate the role of cluster environment in the evolution of galaxy groups, the color properties of galaxy groups are explored as a function of cluster radius and the local density of cluster galaxies. Both the radius and the local density affect the properties of groups, but evidence suggests that density may be more important in the relationship between cluster environment and galaxy characteristics.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Photometric properties (effective surface brightness, effective radius, radial profile index, axis ratio, color, color gradient) of 328 galaxies in the Coma cluster fainter than $R=15$ mag are examined as a function of the distance from the cluster center. No significant gradient is found for the effective surface brightness, effective radius and radial profile index. The distribution of axis ratios shows a concentration of round galaxies at the cluster center in the magnitude range $16.5 \lt R \lt 18$; most of these are found to be old and to have intermediate metal abundance, suggesting that they are nucleated dwarf ellipticals. On the other hand, we find a significant gradient in color, in the sense that galaxy colours become bluer with increasing distance from the cluster center. We conclude that this color gradient represents a metallicity gradient.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have studied the quantitative morphology of the galaxies of the Coma cluster brighter than $m_{r}=17$. The surface brightness profiles of all galaxies were decomposed into the structural components: bulge and disc. We found a correlation between the scale length of the discs and the location in the cluster. Galaxies located at small projected distances show smaller discs than those located in the outermost regions of the cluster. We have also investigated the correlation betwenn the B$-$r color of the galaxies and their environment. For late-type galaxies there is a correlation between the color and the position in the cluster. The bluest galaxies are located in the outermost regions of the cluster and show the largest discs. These results can be explained in terms of galaxy harassment.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

$K$ band luminosity functions (LFs) of three, massive, high redshift clusters of galaxies are presented. The evolution of $K^{*}$, the characteristic magnitude of the LF, is consistent with purely passive evolution, and a redshift of formation $z_{{\rm f}} \approx 1.5-2$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have imaged $\sim 1$ deg$^{2}$ in the V-band in the direction of the Hercules cluster (Abell 2151). The data are used to compute, for the first time, the luminosity function (LF) of galaxies in the cluster down to the dwarf regime (M$_{lim}$$\sim -13.85$ ). The global LF is well described by a Schechter function (Schechter 1976) with best-fit parameters $\alpha = -1.30 \pm 0.06$ and M$_V^* = -21.25 \pm 0.25$. The radial dependence of the LF has also been studied, finding that it turns out to be almost constant within the errors even further away than the virial radius. Given the presence of significant substructure within the cluster, we have analized the LFs in different regions. While the LFs of the two subclusters present are consistent with each other and with the global one, the southernmost one exhibits a somewhat steeper faint-end slope.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have undertaken an imaging survey of low surface brightness dwarf galaxies in nearby groups with the main goal of studying the faint-end of the galaxy luminosity function at z $\sim 0$. Here we describe the results on the search for LSB dwarf galaxies in two compact groups: HCG44 and HCG68. Our study explores the surface brightness, sizes, magnitudes and colours of these candidate dwarf galaxies. The selection criteria were their central surface brightness, scale factors and diameters at the limiting isophote of 26 R mag$/"^{2}$. We estimate the faint end of the luminosity function of galaxies through Monte Carlo simulations. The observed magnitude distribution of the candidate dwarf galaxies in the groups (down to $M_{R} \sim - 13$) is compatible with a Schechter function with a slope $\alpha \sim -1.2$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We investigate the environmental dependence of galaxy properties in the local universe based on the data of the Sloan Digital Sky Survey. We focus on how star formation and morphology of galaxies correlate with luminosity, local environment, and global environment. We find that galaxy properties abruptly change at a critical local density of $\log \Sigma_{\rm crit}\sim 0.4\ {\rm galaxies}\ h_{75}^{2}\ \rm Mpc^{-2}$. The ‘break’ at the critical density is found only for faint galaxies ($M^*_r+1 < M_r < M^*_r+2$). Bright galaxies ($M_r < M^*_r+1$) show no break. That is, the star formation-density and the morphology-density relations depend on galaxy luminosity. Next, we focus on global environment, i.e., richness of galaxy groups and clusters. Most galaxies are not forming stars in groups as poor as $\sigma\sim200\rm\ km\ s^{-1}$. This fact suggests that environmental mechanisms that are effective only in rich clusters, such as ram-pressure stripping of cold gas and harassment, have not played a major role in suppressing galaxy star formation. Our results may suggest that evolution of bright galaxies is not strongly related to galaxy systems such as groups and clusters. On the other hand, evolution of faint galaxies may have a close connection.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present the values of CN and Mg overabundances with respect to Fe, for early-type galaxies in 8 clusters over a range of richness and morphology. Spectra were taken from the Sloan Digital Sky Survey (SDSS) DR1 spectroscopic database, and from WHT and CAHA observations. Abundances were derived from absorption lines and single burst population models, by comparing galaxy spectra with appropriately broadened synthetic model spectra. We find robust correlations between [Mg/CN] and [CN/Fe] abundance ratios and cluster X-ray luminosity. No correlation was observed for [Mg/Fe]. We also see a clear trend with the richness and morphology of the clusters. This behavior is interpreted given varying formation timescales for CN, Mg and Fe, combined with a different star formation history in early-type galaxies as a function of their environment. This result supports the scenario in which early-type galaxies in more massive clusters are assembled on shorter timescales than those galaxies within less massive clusters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have discovered four intergalactic HII regions in the Stephan's quintet, at more than 25 kpc projected distance from the center of the nearest group galaxy, with no apparent optical connection to it. They have ${\rm M}_B$ ranging from −11.9 to −12.5 mag, colors B$-$R=0.7 to 1.1 mag, radial velocities from 6565 to 6651 km/s and they are superposed onto the HI tail east of NGC 7319, with a mean radial velocity of 6610 km/s. In addition, they have metalicities of the order of 12+log(O/H)=8.58$\pm$0.25, which suggests that they were formed from pre-enriched material. We derive a mean age of $4.6 \pm 0.6$ Myr and a mean stellar mass of $(2.9 \pm 1.4)\times10^4~M_\odot$ for the four objects. The masses, ages, colors, velocities, metalicities and location of the objects suggest that they are HII regions that were formed far away from the galaxies through compression of the intergalactic HI gas by galaxy collisions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A new volume-limited sample of 84 isolated pairs of galaxies has been selected from the UZC catalogue. Galaxies in pairs are brighter than $M_{Zw}-5\log h =-18.9$. The projected separation between pair members is $r < 200 h^{-1}$ kpc, the isolation criterion requires the pair to have no further companion within a $R=1 h^{-1}$ Mpc distance. Morphological classification for both members is available for 57 pairs. Out of these, 9 are early-type (E+E) pairs, 29 late type (S+S) pairs, and 19 mixed (E+S) pairs. We find a deficit of luminous dominants among E+E and E+S pairs relative to S+S pairs, while the distance between pair members is statistically undistinguishable. However, among the 12 pairs that are not only close but also luminous, the fraction of E+E and S+S pairs is 8% and 83% respectively. This result is consistent with star-formation being enhanced in S+S pairs (but not in spirals in S+E pairs) in extremely close isolated pairs.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We investigate whether the spectral-type of a locally dominant (most luminous) galaxy can be used to select sets of galaxies that are physically associated (groups). We assume that passive dominants trace a group-like potential, and SF-dominants a field-like environment. The group sample includes 988 groups selected in the 2dFGRS applying a maximum magnitude difference criterion. We find that the average number of passive galaxies associated to a dominant is larger when the dominant is passive, a result supporting our assumption that galaxy associations around a passive dominant are reliable groups. Finally we show that, to reduce the contamination by unbound galaxy associations (SF-dominant), ${\rm a}\geq 3$ passive-members criterion is more efficient than ${\rm a}\geq 6$ all-members criterion.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss the mass assembly history both on cluster and galaxy scales and their impact on galaxy evolution.

On cluster scale, we introduce our on-going PISCES project on Subaru, which plans to target $\sim 15$ clusters at $0.4\le z\le 1.3$ using the unique wide-field ($30'$) optical camera Suprime-Cam and the spectrograph both in optical (FOCAS, $6'$) and near-infrared (FMOS, $30'$). The main objectives of this project are twofold: (1) Mapping out the large scale structures in and around the clusters on 10–14~Mpc scale to study the hierarchical growth of clusters through assembly of surrounding groups. (2) Investigating the environmental variation of galaxy properties along the structures to study the origin of the morphology-density and star formation-density relations. Some initial results are presented.

On galactic scale, we first present the stellar mass growth of cluster galaxies out to $z\sim1.5$ based on the near-infrared imaging of distant clusters and show that the mass assembly process of galaxies is largely completed by $z\sim1.5$ and is faster than the current semi-analytic models' predictions. We then focus on the faint end of the luminosity function at $z\sim1$ based on the Subaru/XMM-Newton Deep Survey imaging data. We show the deficit of red galaxies below $M^*+2$ or 10$^{10}\,{\rm M}_{\odot}$, which suggest less massive galaxies are either genuinely young or still vigorously forming stars in sharp contrast to the massive galaxies where mass is assembled and star formation is terminated long time ago.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We here present our result of deep and wide-field narrow-band imaging of the region of the ‘proto-cluster’ at $z=3.1$ around the SSA22 field. We detected 283 highly confident Ly$\alpha$ emitter candidates (LAEs) and discovered that the belt-like high surface density region of these LAEs extends over $\approx 60$ Mpc in comoving scale, which is much larger than it was known previously. We then successfully detected the 35 extended Ly$\alpha$ blobs (LABs) which are larger than 16 arcsec$^2$ in isophotal area and brighter than $0.7 \times 10^{-16}$ ergs s$^{-1}$ cm$^{-2}$. The distributions of average surface brightness and morphology are widespread from relatively compact high surface brightness objects to very diffuse low surface brightness ones. For one third of these 35 LABs, simple photo-ionization by massive stars is not sufficient to explain the Ly$\alpha$ luminosities, and other mechanisms, such as gravitational heating or superwind must be considered. From their large size and strong spatial clustering, we consider that these LABs are objects closely related to the massive galaxies in their forming phase in the environment of the proto-cluster or large-scale structure.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We analyse the rest–frame (U$-$V) colour–magnitude relation for 2 clusters at redshift 0.7 and 0.8, drawn from the ESO Distant Cluster Survey. By comparing them with the population of red galaxies in the Coma cluster, we show that the high redshift clusters exhibit a deficit of passive faint red galaxies. Our results show that the red–sequence population cannot be explained in terms of a monolithic and synchronous formation scenario. A large fraction of faint passive galaxies in clusters today has moved onto the red sequence relatively recently as a consequence of the fact that their star formation activity has come to an end at $z<0.8$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a method for deriving light-weighted stellar metallicities and ages from high-resolution galaxy spectra, based on the new population synthesis code of Bruzual & Charlot (2003). The method relies on the simultaneous fit of several optical spectral absorption features that are sensitive to either age or metallicity, but not to the $\alpha$-elements abundance ratio. We have constructed a library of stochastic star formation histories, which we have used to derive median-likelihood estimates of ages and metallicities for $\sim10^5$ galaxies extracted from the Sloan Digital Sky Survey Data Release One (SDSS DR1), spanning the full range in star formation activities, from dormant early type to actively star forming. Here we discuss the results for early-type galaxies. We show that the $g-r$, $M_r$ color-magnitude relation for these galaxies is driven primarily by changes in metallicity and in heavy-element abundance ratios. Changes in light-weighted age contribute mainly to the scatter about the relation. This is consistent with previous interpretations of this relation based on lower resolution models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html