Hernan Muriel

Structure and dynamics of the Shapley Supercluster - Velocity catalogue, general morphology and mass

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 10 529 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 ≈ 14 500 km s −1 ) extends further than previous estimates, filling the whole extent of our survey region of 12 degrees by 30 degrees on the sky (30 × 75 h −1 Mpc). There is also a connecting structure associated with the slightly nearer Abell 3571 cluster complex (v ≈ 12 000 km s −1 ). These galaxies seem to link two previously identified sheets of galaxies and establish a connection with a third one at v = 15 000 km s −1 near RA = 13 h . 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 −1 < cz < 18 000 km s −1 ), we found redshift-space overdensities with bj < 17. 5o f� 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.

Galaxy Morphological Segregation in Clusters: Local versus Global Conditions

We study the relative fraction of galaxy morphological types in clusters as a function of the projected local galaxy density and of different global parameters: cluster projected gas density, cluster projected total mass density, and reduced cluster-centric distance. Since local and global densities are correlated, we have considered different tests to search for the parameters on which segregation shows the strongest dependence. We have also explored the results of our analysis as they apply to both the central regions of the clusters and their outskirts. We consider a sample of clusters of galaxies with estimated temperatures and derive the projected mass density profile, using the model of Navarro, Frenk, & White, and the 500 overdensity contrast radius (r500), using the scaling relation. X-ray surface brightness profiles are used to obtain the projected gas density, assuming the hydrostatic equilibrium model. Our results suggest that morphological segregation in clusters is controlled by local galaxy density in the outskirts. On the other hand, global projected mass density shows the strongest correlation with the fractions of morphological types in the central high-density region, with a marginal dependence on local galaxy density.

Groups of galaxies: relationship between environment and galaxy properties

We analyse how the properties of galaxies in groups identified in the Sloan Digital Sky Survey depend on the environment. In particular, we study the relationship between galaxy properties and group mass and group-centric distance. Among the galaxy properties we have considered here, we find that the g - r colour is the most predictive parameter for group mass, while the most predictive pair of properties are g - r colour and r-band absolute magnitude. Regarding the position inside the systems, the g - r colour is the best tracer of group-centric distance and the most predictive pair of properties are g - r colour and spectral type taken together. These results remain unchanged when a subsample of high-mass groups is analysed. The same happens if the brightest group galaxies are excluded.

Galaxy luminosity function in groups

We compute the luminosity function (LF) of galaxies in groups. The data consist of groups of galaxies selected in distance and redshift space in two samples comprising a total number of 66 groups. The assignment of galaxies to the groups was derived from the Edinburgh-Durham Southern Galaxy Catalog, considering a limiting apparent magnitude mlim = 19.4. We find a flat faint end of the mean galaxy LF in groups, which is in contrast to the mean galaxy LF in clusters where a large relative number of faint galaxies is present. We find that a Schechter function with parameters M* -19.6 ± 0.2, α -1.0 ± 0.2 provides a satisfactory fit to the galaxy LF of our total sample of groups in the range of absolute magnitudes -22 < M < -16.

Structure and dynamics of the Shapley Supercluster

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.

Faint Galaxy Population in Clusters: X-Ray Emission, cD Halos, and Projection Effects

We analyze samples of nearby clusters taken from the Abell catalog and the X-Ray Sample of Bright Clusters of De Grandi and coworkers, including a wide range of X-ray luminosities. Using the usually-adopted background subtraction procedures, we find that galaxies in clusters selected by means of their X-ray emission show a flat luminosity function (faint-end slope α -1.1) consistent with that derived for galaxies in the field and groups. By contrast, the sample of Abell clusters that do not have an X-ray counterpart shows a galaxy luminosity function with a steep faint end (α -1.6). We investigate the possibility that cD halos could be formed by the disruption of galaxies in rich relaxed clusters that show an apparently flat faint-end galaxy luminosity function. We find that clusters dominated by a central cD galaxy (Bautz-Morgan classes I and II) show the same systematic trend: X-ray-selected clusters have flatter faint-end slopes than those clusters with no detected X-ray emission. Thus, it is likely the X-ray selection and not the cluster domination by central galaxies is what correlates with background decontamination estimates of the galaxy luminosity function. Moreover, no significant correlation between X-ray luminosity and the galaxy luminosity function faint-end slope is found. These results do not support a scenario in which flat faint-end slopes are a consequence of cD formation via the disruption of faint galaxies. We argue that the clusters without X-ray emission are strongly affected by projection effects which give rise to spurious faint-end slopes estimated using background subtraction procedures.

Alignment effects of clusters of galaxies

The measurement os 204 position angles of the brightest cluster galaxies in a sample of southern clusters is presented. Cluster-cluster alignment effects in this sample are analyzed and this is extended with a previous sample of Abell clusters in the northern hemisphere. Results suggest a weak but statically significant signal of alignment of the brightest galaxy in a cluster with the projected vector toward the nearest-neighbor cluster. The anistropy detected amounts approximately to a 30 percent excess of brightest cluster galaxies pointing to the nearest cluster of galaxies. The mean acute angle between the galaxy position angle and the direction toward the nearest neighbor is 30 + or -2 deg, or less. The scale of the detected anisotropy is D approximately equal to 15 Mpc/h. 17 refs.

TIDAL STRIPPING OF GLOBULAR CLUSTERS IN THE VIRGO CLUSTER

With the aim of finding evidence of tidal stripping of globular clusters (GCs) we analyzed a sample of 13 elliptical galaxies taken from the ACS Virgo Cluster Survey (VCS). These galaxies belong to the main concentration of the Virgo Cluster (VC) and present absolute magnitudes –18.5 < Mz < –22.5. We used the public GC catalog of Jordan et al. and separated the GC population into metal poor (blue) and metal rich (red) according to their integrated colors. The galaxy properties were taken from Peng et al. We found the following. (1) The specific frequencies (SN ) of total and blue GC populations increase as a function of the projected galaxy distances rp to M87. A similar result is observed when three-dimensional distances r 3D are used. The same behaviors are found if the analysis is made using the number of GCs per (T). No correlations between SN or T and rp or r 3D is observed for the red GC population. The correlations for the blue GCs (typically more extended) and the lack of correlations for the red GCs (more concentrated) with the clustocentric distance of the host galaxy are interpreted as evidence of GCs stripping due to tidal forces. (2) No correlation is found between the slope of GC density profiles of host galaxies and the galaxy distance to M87 (Virgo central galaxy). The lack of such a correlation is interpreted in terms of a shrinkage of the GC distribution after the stripping of GCs in the outermost region of galaxies. (3) We also computed the local density of GCs (ρout) located further than 6.2 kpc from the galaxy center for nine galaxies of our sample. We find that the GC population around most of these galaxies is mainly composed of blue GCs. The two highest values of ρout are found in the core of the VC (up to 100 kpc) and correspond to the two lowest values of SN . Our results suggest that the number and the fraction of blue and red GCs observed in elliptical galaxies located near the centers of massive clusters could be significantly different from the underlying GC population. These differences could be explained by tidal stripping effects that occur as galaxies approach the centers of clusters.

Systematics in the orientation of galaxies

Possible systematic effects in the orientation of galaxies ar examined with respect to their neighbors on different scales. For accomplishing this program we study different samples of the ESO catalog with redshifts taken from a compilation by A. P. Fairall . The samples are defined according to the morphology and the apparent lattening of the galaxies. In samples of spiral galaxies we find statistical evidence (2.8σ level) for a preferred orientation of the major axis in the direction toward the nearest neighbors on scales of 3 Mpc/h. The redsift information is crucial in the detection of this effect, and when the nearest neighbors are considered in projection no systematics are detected. Elliptical galaxies are found preferentially oriented not only with the nearest but also with all neighbors on scales of 3 Mpc/h

Alignments and filaments on large scales

By means of statistical analyses on the Lick maps, the reported alignment effects of brightest cluster galaxies and ellipticals with the neighboring galaxy distribution, are confirmed. No peculiarities in the distribution around spirals and S0 were found, suggesting a lack of correlation of angular momentum with the neighboring galaxy distribution. It is found that the alignment signals are consistent with the presence of filaments or sheets of galaxies running across clusters and ellipticals rather than with a single overdensity on one side of these structures. 19 refs.