Bianca M. Poggianti

The Star formation histories of galaxies in distant clusters

We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressler et al. We investigate the nature of the different spectral classes defined by Dressler et al., including star-forming, poststarburst, and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous poststarburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [O II] emission, that we believe are the most likely progenitors of the poststarburst population. We present the differences between the field and cluster galaxies at z=0.4-0.5. We then compare the spectral and morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressler et al. We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and morphological transformation.

A Spectroscopic Catalog of 10 Distant Rich Clusters of Galaxies

We present spectroscopic observations of galaxies in the fields of 10 distant clusters for which we have previously presented deep imaging with WFPC2 on board the Hubble Space Telescope. The clusters span the redshift range z=0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry, redshifts, spectral line strengths, and classifications for 657 galaxies in the fields of the 10 clusters. The catalog is composed of 424 cluster members across the 10 clusters and 233 field galaxies, with detailed morphological information from our WFPC2 images for 204 of the cluster galaxies and 71 in the field. We illustrate some basic properties of the catalog, including correlations between the morphological and spectral properties of our large sample of cluster galaxies. A direct comparison of the spectral properties of the high-redshift cluster and field populations suggests that the phenomenon of strong Balmer lines in otherwise passive galaxies (commonly called E + A but renamed here as the k + a class) shows an order-of-magnitude increase in the rich cluster environment compared with a more modest increase in the field population. This suggests that the process or processes involved in producing k + a galaxies are either substantially more effective in the cluster environment or that this environment prolongs the visibility of this phase. A more detailed analysis and modeling of these data is presented in Poggianti et al.

The Evolution of the Galactic Morphological Types in Clusters

The morphological types of galaxies in nine clusters in the redshift range 0.1 < ∼ z < ∼ 0.25 are derived from very good seeing images taken at the NOT and the La Silla–Danish telescopes, with all galaxies at MV < −20 and within the central ∼1 Mpc 2 area being classified. With the purpose of investigating the evolution of the fraction of different morphological types with redshift, we compare our results with the morphological content of nine distant clusters studied by the MORPHS group (Dressler et al. 1997), five clusters observed with HST-WFPC2 at redshift z = 0.2− 0.3 (Couch et al. 1998), and Dressler’s (1980) large sample of nearby clusters. After having checked the reliability of our morphological classification both in an absolute sense and relative to the MORPHS scheme (Smail et al. 1997), we analyze the relative occurrence of elliptical, S0 and spiral galaxies as a function of the cluster properties and redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related to the cluster morphology. In particular, in our cluster sample, clusters with a high concentration of ellipticals display a low S0/E ratio and, vice-versa, low concentration clusters have a high S0/E. At the same time, the trend of the morphological fractions (%Es, %S0s, %Sp) and of the S0/E and S0/Sp ratios with redshift clearly points to a morphological evolution: as the redshift decreases, the S0 population tends to grow at the expense of the spiral population, whereas the frequency of Es remains almost constant. We also analyze the morphology-density (MD) relation in our clusters and find that – similarly to higher redshift clusters – a good MD relation exists in the high-concentration clusters, while it is absent in the less concentrated clusters. Finally, the comparison of the MD relation in our clusters with that of the D97 sample suggests that the transformation of spirals into S0 galaxies becomes more efficient with decreasing local density. Subject headings: galaxies: clusters – galaxies: evolution – galaxies: structureThe morphological types of galaxies in nine clusters in the redshift range 0.1 z 0.25 are derived from very good seeing images taken at the NOT and the La Silla-Danish telescopes, with all galaxies at MV < -20 and within the central ~1 Mpc2 area being classified. With the purpose of investigating the evolution of the fraction of different morphological types with redshift, we compare our results with the morphological content of nine distant clusters studied by the MORPHS group, five clusters observed with HST/WFPC2 at redshift z = 0.2-0.3, and Dresslers large sample of nearby clusters. After having checked the reliability of our morphological classification both in an absolute sense and relative to the MORPHS scheme, we analyze the relative occurrence of elliptical, S0, and spiral galaxies as a function of the cluster properties and redshift. We find a large intrinsic scatter in the S0/E ratio, mostly related to the cluster morphology. In particular, in our cluster sample, clusters with a high concentration of ellipticals display a low S0/E ratio and, vice versa, low concentration clusters have a high S0/E. At the same time, the trend of the morphological fractions (%Es, %S0s, %Sps) and of the S0/E and S0/Sp ratios with redshift clearly points to a morphological evolution: as the redshift decreases, the S0 population tends to grow at the expense of the spiral population, whereas the frequency of Es remains almost constant. We also analyze the morphology-density (MD) relation in our clusters and find that—similarly to higher redshift clusters—a good MD relation exists in the high-concentration clusters, while it is absent in the less concentrated clusters. Finally, the comparison of the MD relation in our clusters with that of the MORPHS sample suggests that the transformation of spirals into S0 galaxies becomes more efficient with decreasing local density.

The Evolution of the Star Formation Activity in Galaxies and Its Dependence on Environment

We study how the proportion of star-forming galaxies evolves between z ¼ 0:8 and 0 as a function of galaxy environment,usingtheOiilineinemissionasasignatureofongoingstarformation.Our high-zdatasetcomprises16 clusters, 10 groups, and another 250 galaxies in poorer groups and the field at z ¼ 0:4 0:8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use galaxy systems selected from the Sloan Digital Sky Survey (SDSS) at 0:04 < z < 0:08. At high z most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z ¼ 0:4 0:8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the O ii equivalent width) in star-forming galaxies is also found to vary systematically with environment. SDSS clusters have much lower fractions of starforming galaxies than clusters at z ¼ 0:4 0:8 and, in contrast with the distant clusters, show a plateau for velocity dispersions � 550kms � 1 ,where thefraction ofgalaxieswithOiiemission doesnotvarysystematicallywithvelocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find that it is strongest in intermediate-mass systems (� � 500 600 km s � 1 at z ¼ 0). To understandtheoriginoftheobservedtrends,weusethePress-Schechter formalismandtheMillenniumSimulationandshow thatgalaxystarformationhistoriesmaybecloselyrelatedtothegrowthhistoryofclustersandgroups.Ifthescenariowe propose is roughly correct, the link between galaxy properties and environment is extremely simple to predict purely from a knowledge of the growth of dark matter structures. Subject headings: cosmology: observations — galaxies: clusters: general — galaxies: evolution — galaxies: fundamental parameters — galaxies: stellar content

Optical Spectral Signatures of Dusty Starburst Galaxies

We analyze the optical spectral properties of the complete sample of Very Luminous Infrared Galaxies presented by Wu et al., and we find a high fraction (~50%) of spectra showing both a strong Hδ line in absorption and relatively modest [O II] emission [e(a) spectra]. The e(a) signature has been proposed as an efficient method to identify dusty starburst galaxies, and we study the star formation activity and the nature of these galaxies, as well as the effects of dust on their observed properties. We examine their emission-line characteristics, in particular their [O II]/Hα ratio, and we find this to be greatly affected by reddening. A search for AGN spectral signatures reveals that the e(a) galaxies are typically H II/LINER galaxies. We compare the star formation rates derived from the FIR luminosities with the estimates based on the Hα line and find that the values obtained from the optical emission lines are a factor of 10-70 (Hα) and 20-140 ([O II]) lower than the FIR estimates (50-300 M☉ yr-1). We then study the morphological properties of the e(a) galaxies, looking for a near companion or signs of a merger/interaction. In order to explore the evolution of the e(a) population, we present an overview of the available observations of e(a) galaxies in different environments both at low and high redshift. Finally, we discuss the role of dust in determining the e(a) spectral properties and we propose a scenario of selective obscuration in which the extinction decreases with the stellar age.

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

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.

Superdense Massive Galaxies in Wings Local Clusters

Massive quiescent galaxies at z > 1 have been found to have small physical sizes, and 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 2 study. In contrast, there is strong evidence for a large evolution in radius for the most massive galaxies with M * > 4 × 1011 M ☉ compared to similarly massive galaxies in WINGS, i.e., the brightest cluster galaxies.

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

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

A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. III. Spectral Ages and Metallicities

We present a detailed analysis of the spectroscopic catalog of galaxies in the Coma Cluster from Mobasher et al. (Paper II of the series). This catalog comprises ~300 spectra of cluster members with absolute magnitudes in the range MB = -20.5 to -14 in two areas of ~1 × 1.5 Mpc toward the center and the southwest region of the cluster. In order to study the star formation and metallicity properties of the Coma galaxies as a function of their luminosity and environment, spectral indices of the Lick/IDS system and equivalent widths of the emission lines were measured in the range λ = 3600-6600 A. In this paper, the analysis is restricted to the 257 galaxies with no emission lines in their spectra. The strength of the age-sensitive indices (such as Hβ, HγF, and HδF) is found to correlate with galaxy magnitude over the whole magnitude range explored in this study. Similarly, the metallicity-sensitive indices (such as Mg2, Fe, and C24668) anticorrelate with magnitude. By comparing the observed indices with model grids based on the Padova isochrones, we derive luminosity-weighted ages and metallicities. We present the distributions of ages and metallicities for galaxies in various magnitude bins. The mean metallicity decreases with galaxy magnitude and, at a given luminosity, appears to be generally lower for galaxies in the southwest region of Coma as compared to the center of the cluster. A broad range of ages, from younger than 3 Gyr to older than 9 Gyr, is found in galaxies of any magnitude. However, systematic trends of age with luminosity are present among galaxies in the central field, including a slight decrease of the mean age for fainter galaxies. Furthermore, in the central Mpc of Coma, a large fraction of galaxies at any luminosity (50%-60% of the giants, more than 30% of the dwarfs) show no evidence in their central regions of star formation occurred at redshift z < 2, while the proportion of galaxies with significant star formation occurring at intermediate (0.35 < z < 2) and low (z < 0.35) redshifts is found to depend on galaxy luminosity. An additional surprising result is that the faint galaxies with young luminosity-weighted ages appear to have a bimodal metallicity distribution that, if confirmed, would point to a composite formation scenario involving different physical processes. Coadding the spectra of these metal-rich and metal-poor galaxies separately supports the reality of the metallicity bimodality, although higher signal-to-noise ratio spectra of the individual galaxies will be needed to draw definite conclusions. An anticorrelation between age and metallicity is found to be present in galaxies of any given luminosity bin, and it is especially evident among the brightest subset with the highest signal-to-noise ratio spectra. Finally, we present an interpretation of the index-magnitude relations observed. We show that the slopes of the indices/magnitude relations are the consequence of both age and metallicity trends with luminosity: each such trend on its own would be sufficient to produce relations similar to those observed.

The Morphological Content of 10 EDisCS Clusters at 0.5 < z < 0.8

We describe Hubble Space Telescope (HST) imaging of 10 of the 20 ESO Distant Cluster Survey (EDisCS) fields. Each ~40 arcmin^2 field was imaged in the F814W filter with the Advanced Camera for Surveys Wide Field Camera. Based on these data, we present visual morphological classifications for the ~920 sources per field that are brighter than I_(auto) = 23 mag. We use these classifications to quantify the morphological content of 10 intermediate-redshift (0.5 < z < 0.8) galaxy clusters within the HST survey region. The EDisCS results, combined with previously published data from seven higher redshift clusters, show no statistically significant evidence for evolution in the mean fractions of elliptical, S0, and late-type (Sp+Irr) galaxies in clusters over the redshift range 0.5 < z < 1.2. In contrast, existing studies of lower redshift clusters have revealed a factor of ~2 increase in the typical S0 fraction between z = 0.4 and 0, accompanied by a commensurate decrease in the Sp+Irr fraction and no evolution in the elliptical fraction. The EDisCS clusters demonstrate that cluster morphological fractions plateau beyond z ≈ 0.4. They also exhibit a mild correlation between morphological content and cluster velocity dispersion, highlighting the importance of careful sample selection in evaluating evolution. We discuss these findings in the context of a recently proposed scenario in which the fractions of passive (E, S0) and star-forming (Sp, Irr) galaxies are determined primarily by the growth history of clusters.