Stellar populations in superclusters of galaxies

A catalogue of superclusters of galaxies is used to investigate the influence of the supercluster environment on galaxy populations, considering galaxies brighter than M$_r<$-21+5$\log$ h. Empirical spectral synthesis techniques are applied to obtain the stellar population properties of galaxies which belong to superclusters and representative values of stellar population parameters are attributed to each supercluster. We show that richer superclusters present denser environments and older stellar populations. The galaxy populations of superclusters classified as filaments and pancakes are statistically similar, indicating that the morphology of superclusters does not have a significative influence on the stellar populations. Clusters of galaxies within superclusters are also examined in order to evaluate the influence of the supercluster environment on their galaxy properties. Our results suggest that the environment affects galaxy properties but its influence should operate on scales of groups and clusters, more than on the scale of superclusters.Comment: 7 pages, 4 figures; accepted to MNRA

Spitzer Observations of Galaxy Clusters

We present preliminary results of a project to study three rich nearby clusters of galaxies with the Spitzer space telescope. The Spitzer observations in the four IRAC and three MIPS bands cover a region up to three virial radii, approximately, and have been recently completed. On the basis of the first Spitzer images, we followed up spectroscopically the far-infrared sources with the multi-fiber spectrograph HYDRA on the WIYN telescope. 70% of the sources brighter than 0.3 mJy at 24 μm and r’ < 20.5 have been observed for a total of 1078 spectra. For 87% of them we were able to measure redshifts obtaining 50 to 100 members for the different clusters. This first study shows that the far-IR sources in these clusters are predominantly powered by star formation and clustered in regions far from the center. In the case of A1763, they seem to be situated along a filament supporting the idea of infalling galaxies experiencing bursts of star formation during their first contact with the hot intra-cluster medium

Spitzer observations of Abell 1763. III. The infrared luminosity function in different supercluster environments

Context. The study of galaxy luminosity functions (LFs) in different environments provides powerful constraints on the physics of galaxy evolution. The infrared (IR) LF is a particularly useful tool since it is directly related to the distribution of galaxy star-formation rates (SFRs). Aims. We aim to determine the galaxy IR LF as a function of the environment in a supercluster at redshift 0.23 to shed light on the processes driving galaxy evolution in and around clusters. Methods. We base our analysis on multi-wavelength data, which include optical, near-IR, and mid- to far-IR photometry, as well as redshifts from optical spectroscopy. We identify 467 supercluster members in a sample of 24-μm-selected galaxies, on the basis of their spectroscopic (153) and photometric (314) redshifts. IR luminosities and stellar masses are determined for supercluster members via spectral energy distribution fitting. Galaxies with active galactic nuclei are identified by a variety of methods and excluded from the sample. SFRs are obtained for the 432 remaining galaxies from their IR luminosities via the Kennicutt relation. Results. We determine the IR LF of the whole supercluster as well as the IR LFs of three different regions in the supercluster: the cluster core, a large-scale filament, and the cluster outskirts (excluding the filament). A comparison of the IR LFs of the three regions, normalized by the average number densities of r-band selected normal galaxies, shows that the filament (respectively, the core) contains the highest (respectively, the lowest) fraction of IR-emitting galaxies at all levels of IR luminosities, and the highest (respectively, the lowest) total SFR normalized by optical galaxy richness. Luminous IR galaxies (LIRGs) are almost absent in the core region. The relation between galaxy specific SFRs and stellar masses does not depend on the environment, and it indicates that most supercluster LIRGs are rather massive galaxies with relatively low specific SFRs. A comparison with previous IR LF determinations from the literature confirms that the mass-normalized total SFR in clusters increases with redshift, but more rapidly than previously suggested for redshifts ≲ 0.4. Conclusions. The IR LF shows an environmental dependence that is not simply related to the local galaxy density. The filament, an intermediate-density region in the A1763 supercluster, contains the highest fraction of IR-emitting galaxies. We interpret our findings within a possible scenario for the evolution of galaxies in and around clusters

Starburst Galaxies in Cluster-feeding Filaments Unveiled by Spitzer

We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770, which lies at the same redshift as Abell 1763 (z = 0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation. According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region

A Redshift Survey of Nearby Galaxy Groups: the Shape of the Mass Density Profile

We constrain the mass profile and orbital structure of nearby groups and clusters of galaxies. Our method yields the joint probability distribution of the density slope n, the velocity anisotropy beta, and the turnover radius r0 for these systems. The measurement technique does not use results from N-body simulations as priors. We incorporate 2419 new redshifts in the fields of 41 systems of galaxies with z < 0.04. The new groups have median velocity dispersion sigma=360 km/s. We also use 851 archived redshifts in the fields of 8 nearly relaxed clusters with z < 0.1. Within R < 2 r200, the data are consistent with a single power law matter density distribution with slope n = 1.8-2.2 for systems with sigma < 470 km/s, and n = 1.6-2.0 for those with sigma > 470 km/s (95% confidence). We show that a simple, scale-free phase space distribution function f(E,L^2) ~ (-E)^(alpha-1/2) L^(-2 \beta) is consistent with the data as long as the matter density has a cusp. Using this DF, matter density profiles with constant density cores (n=0) are ruled out with better than 99.7% confidence.Comment: 22 pages; accepted for publication in the Astrophysical Journa

CAIRNS: The Cluster And Infall Region Nearby Survey I. Redshifts and Mass Profiles

The CAIRNS (Cluster And Infall Region Nearby Survey) project is a spectroscopic survey of the infall regions surrounding eight nearby, rich, X-ray luminous clusters of galaxies. We collect 15665 redshifts (3471 new or remeasured) within \sim 5-10 Mpc of the centers of the clusters, making it the largest study of the infall regions of clusters. We determine cluster membership and the mass profiles of the clusters based on the phase space distribution of the galaxies. All of the clusters display decreasing velocity dispersion profiles. The mass profiles are fit well by functional forms based on numerical simulations but exclude an isothermal sphere. Specifically, NFW and Hernquist models provide good descriptions of cluster mass profiles to their turnaround radii. Our sample shows that the predicted infall pattern is ubiquitous in rich, X-ray luminous clusters over a large mass range. The caustic mass estimates are in excellent agreement with independent X-ray estimates at small radii and with virial estimates at intermediate radii. The mean ratio of the caustic mass to the X-ray mass is 1.03\pm0.11 and the mean ratio of the caustic mass to the virial mass (when corrected for the surface pressure term) is 0.93\pm0.07. We further demonstrate that the caustic technique provides reasonable mass estimates even in merging clusters.Comment: 54 pages, 18 figures, to appear in The Astronomical Journa

Physical Conditions in the Seyfert Galaxy NGC 2992

This paper presents long slit spectral maps of the bi-cone shaped extended narrow line region (ENLR) in the Seyfert galaxy NGC 2992. We investigate the physical properties of the ENLR via emission line diagnostics, and compare the observations to shock and photoionization models for the excitation mechanism of the gas. The line ratios vary as a function of position in the ENLR, and the loci of the observed points on line ratio diagrams are shown to be most consistent with shock+precursor model grids. We consider the energetics of a nuclear ionizing source for the ENLR, and perform the q-test in which the rate of ionizing photons from the nucleus is inferred from measurements of the density and ionization parameter. The q-test is shown to be invalid in the case of NGC 2992 because of the limitations of the [S II]6717/6731 density diagnostic. The excitation of the gas is shown to be broadly consistent with the kinematics, with higher [N II]6583/H-alpha present in the more dynamically active region. We also show that the pressure associated with the X-ray emitting plasma may provide a large fraction of the pressure required to power the ENLR via shocks.Comment: 55 pages, 49 figures, ApJ accepted September 9, 1998. Figures 1a-f are provided in jpeg forma

Gravo-thermal properties and formation of elliptical galaxies

We have analyzed a sample of galaxies belonging to three clusters: Coma, Abell 85, and Abell 496 (real galaxies) and a sample of simulated elliptical galaxies formed in a hierarchical merging scheme (virtual galaxies). We use the Sersic law to describe their light profile. The specific entropy (Boltzmann-Gibbs definition) is then calculated supposing that the galaxies behave as spherical, isotropic, one-component systems. We find that, to a good approximation (about 10%), both real and virtual galaxies have an almost unique specific entropy. Within this approximation the galaxies are distributed in a thin plane in the space defined by the three Sersic law parameters, which we call the Entropic Plane. A further analysis shows that both real and virtual galaxies are in fact located on a thin line, therefore indicating the existence of another - and yet unknown - physical property, besides the uniqueness of the specific entropy. A more careful examination of the virtual galaxies sample indicates a very small increase of their specific entropy with merging generation. In a hierarchical scenario, this implies a correlation between the specific entropy and the total mass, which is indeed seen in our data. The scatter and tilt of the Entropic Line, defined by Lima Neto et al. (1999a), are reduced when this correlation is taken into account. Although one cannot distinguish between various generations for real galaxies, the distribution of their specific entropy is similar to that in the virtual sample, suggesting that hierarchical merging processes could be an important mechanism in the building of elliptical galaxies