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We use extensive new observations of the very rich z [?] 0.4 cluster of galaxies A851 to examine the nature and origin of starburst galaxies in intermediate-redshift clusters. New HST observations, 24 um Spitzer photometry and ground-based spectroscopy cover most of a region of the cluster about 10' across, corresponding to a cluster-centric radial distance of about 1.6 Mpc. This spatial coverage allows us to confirm the existence of a morphology-density relation within this cluster, and to identify several large, presumably infalling, subsystems. We confirm our previous conclusion that a very large fraction of the star-forming galaxies in A851 has recently undergone starbursts. We argue that starbursts are mostly confined to two kinds of sites: infalling groups and the cluster center. At the cluster center, it appears that infalling galaxies are undergoing major mergers, resulting in starbursts whose optical-emission lines are completely buried beneath dust. The aftermath of this process appears to be proto-S0 galaxies devoid of star formation. In contrast, major mergers do not appear to be the cause of most of the starbursts in infalling groups, and fewer of these events result in the transformation of the galaxy into an S0. Some recent theoretical work provides possible explanations for these two distinct processes, but it is not clear whether they can operate with the very high efficiency needed to account for the very large starburst rate observed.

Post-starburst (E+A or k+a) spectra, characterized by their exceptionally strong Balmer lines in absorption and the lack of emission lines, belong to galaxies in which the star formation (SF) activity ended abruptly sometime during the past Gyr. We perform a spectral analysis of galaxies in clusters, groups, poor groups, and the field at z = 0.4-0.8 based on the ESO Distant Cluster Survey. We find that the incidence of k+a galaxies at these redshifts depends strongly on environment. K+a's reside preferentially in clusters and, unexpectedly, in a subset of the s = 200-400 km s-1 groups, those that have a low fraction of O II emitters. In these environments, 20%-30% of the star-forming galaxies have had their SF activity recently truncated. In contrast, there are proportionally fewer k+a galaxies in the field, the poor groups, and groups with a high O II fraction. An important result is that the incidence of k+a galaxies correlates with the cluster velocity dispersion: more massive clusters have higher proportions of k+a's. Spectra of dusty starburst candidates, with strong Balmer absorption and emission lines, present a very different environmental dependence from k+a's. They are numerous in all environments at z = 0.4-0.8, but they are especially numerous in all types of groups, favoring the hypothesis of triggering by a merger. We present the morphological type, stellar mass, luminosity, mass-to-light ratio, local galaxy density, and clustercentric distance distributions of galaxies of different spectral types. These properties are consistent with previous suggestions that cluster k+a galaxies are observed in a transition phase, at the moment they are rather massive S0 and Sa galaxies, evolving from star-forming, recently infallen later types to passively evolving cluster early-type galaxies. The correlation between k+a fraction and cluster velocity dispersion supports the hypothesis that k+a galaxies in clusters originate from processes related to the intracluster medium, while several possibilities are discussed for the origin of the puzzling k+a frequency in low-O II groups.

Spitzer-MIPS 24 um observations and ground-based optical imaging and spectroscopy of the rich galaxy cluster Abell 851 at z = 0.41 are used to derive and compare star formation rates from the mid-IR 24 um and from [O II] ll3727 emission. Many cluster galaxies have star formation rates SFR(24um)/SFR([O II]) [?] 1, indicative of star formation in regions highly obscured by dust. We focus on the substantial minority of A851 cluster members where strong Balmer absorption points to a starburst on a 108-109 year timescale. As is typical, two types of galaxies with strong Balmer absorption are found in A851: with optical emission (starforming), and without optical emission (post-starburst). Our principal result is that the starforming variety, so-called e(a) galaxies, are mostly detected (9 out of 12) at 24 um--for these we find typically SFR(24 um)/SFR([O II]) [?]4. Strong Balmer absorption and high values of SFR(24 um)/SFR([O II]) indicate moderately active starbursts (SB); both observations support the picture that e(a) galaxies are the active starbursts that feed the post-starburst population. While 24 um detections are frequent with Balmer-strong objects (even 6 out of 18 of the supposedly "post-starburst" galaxies are detected), only two out of seven of the continuously starforming `e(c)' galaxies (with weak Balmer absorption) are detected--for them, SFR(24 um)/SFR([O II]) [?]1. Their optical spectra resemble present-epoch spirals that dominate today's universe; we strengthen this association by showing that SFR(24 um)/SFR([O II]) [?] 1 is the norm today. That is, not just the amount of star formation but also its mode has evolved strongly from z [?] 0.4 to the present. We fit spectrophotometric models in order to measure the strength and duration of the bursts and to quantify the evolutionary sequence from active to post-starburst. Our results harden the evidence that moderately active starbursts are the defining feature of starforming cluster galaxies at z [?] 0.4.

We study how the proportion of star-forming galaxies evolves between z = 0.8 and 0 as a function of galaxy environment, using the O II line in emission as a signature of ongoing star formation. Our high-z data set comprises 16 clusters, 10 groups, and another 250 galaxies in poorer groups and the field at z = 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 = 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 star-forming galaxies than clusters at z = 0.4-0.8 and, in contrast with the distant clusters, show a plateau for velocity dispersions [?]550 km s-1, where the fraction of galaxies with O II emission does not vary systematically with velocity 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 (s ~ 500-600 km s-1 at z = 0). To understand the origin of the observed trends, we use the Press-Schechter formalism and the Millennium Simulation and show that galaxy star formation histories may be closely related to the growth history of clusters and groups. If the scenario we 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.

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.

We investigate the radial dependence of the spectroscopic properties, in particular the Mg2, [?]Fe[?], and Hb spectroscopic indices, in a sample of galaxies spanning a wide range of absolute luminosity in the Coma Cluster. After allowing for the magnitude dependence of these indices, we find a significant gradient in Mg2, in the sense that galaxies in the core of the cluster have stronger Mg2. We find only weak gradients in [?]Fe[?] and Hb. Using the model grids presented in an earlier paper in this series, we attribute the Mg2 gradient to changes in metal abundance. One possible mechanism to create this abundance gradient is pressure confinement by the intracluster medium of material from supernova-driven winds early in the history of the galaxies.

The processes that form and shape galaxy clusters, such as infall, mergers, and dynamical relaxation, tend to generate distinguishable differences between the distributions of a cluster's giant and dwarf galaxies. Thus, the dynamics of dwarf galaxies in a cluster can provide valuable insights into its dynamical history. With this in mind, we look for differences between the spatial and velocity distributions of giant (b < 18) and dwarf (b > 18) galaxies in the Coma cluster. Our redshift sample contains new measurements from the 2dF and WYFFOS spectrographs, making it more complete at faint magnitudes than any previously studied sample of Coma galaxies. It includes 745 cluster members--452 giants and 293 dwarfs. We find that the line-of-sight velocity distribution of the giants is significantly non-Gaussian, but not that for the dwarfs. A battery of statistical tests of both the spatial and localized velocity distributions of the galaxies in our sample finds no strong evidence for differences between the giant and dwarf populations. These results rule out the cluster as a whole having moved significantly toward equipartition, and they are consistent with the cluster having formed via mergers between dynamically relaxed subclusters.

The ages of stellar populations in 52 elliptical and S0 galaxies in the Coma Cluster are investigated, using a new spectroscopic sample. More than 40% of the S0's are found to have undergone star formation in their central regions during the last ~5 Gyr, while such activity is absent in the ellipticals. Galaxies in this sample have absolute magnitudes in the range -20.5 < MB < -17.5, and the fraction of S0 galaxies with recent star formation is higher at fainter luminosities. The observed luminosity range of S0 galaxies with signs of recent star formation activity is consistent with them being the descendants of typical star-forming spirals at intermediate redshift whose star formation has been halted as a consequence of the dense environment.

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 x 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 l = 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 Hb, HgF, and HdF) 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.

We investigate the star formation and dust extinction properties of very luminous infrared galaxies whose spectra display a strong Hd line in absorption and a moderate [O II] emission [e(a) spectrum]. This spectral combination has been suggested to be a useful method to identify dusty starburst galaxies at any redshift on the basis of optical data alone. We compare the average e(a) optical spectrum with synthetic spectra that include both the stellar and the nebular contribution, allowing dust extinction to affect differentially the stellar populations of different ages. We find that reproducing the e(a) spectrum requires the youngest stellar generations to be significantly more extinguished by dust than older stellar populations and implies a strong ongoing star formation activity at a level higher than in quiescent spirals. A model fitting the optical spectrum does not necessarily produce the observed FIR luminosity, and this can be explained by the existence of stellar populations which are practically obscured at optical wavelengths. Models in which dust and stars are uniformly mixed yield a reddening of the emerging emission lines which is too low compared to observations: additional foreground reddening is required.

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 Hd 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]/Ha 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 Ha line and find that the values obtained from the optical emission lines are a factor of 10-70 (Ha) 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.

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.

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~ 0.5) clusters observed with WFPC2 on the HubbleSpaceTelescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the "morphology-density" relationship, T- ?, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the T- ? relation is strong in centrally concentrated "regular" clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low-redshift clusters, where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction.