We use deep Hubble Space Telescope Advanced Camera for Surveys/High Resolution Channel observations of a field within M32 (F1) and an M31 background field (F2) to determine the star formation history (SFH) of M32 from its resolved stellar population. We find that 2-5 Gyr old stars contribute similar to 40% +/- 17% of M32' s mass, while similar to 55% +/- 21% of M32' s mass comes from stars older than 5 Gyr. The mass-weighted mean age and metallicity of M32 at F1 are < Age > = 6.8 +/- 1.5Gyr and <[M/H]> = -0.01 +/- 0.08 dex. The SFH additionally indicates the presence of young (< 2 Gyr old), metal-poor ([M/H] similar to -0.7) stars, suggesting that blue straggler stars contribute similar to 2% of themass at F1; the remaining similar to 3% of themass is in young metal-rich stars. Line-strength indices computed from the SFH imply a light-weighted mean age and metallicity of 4.9 Gyr and [M/H] = -0.12 dex, and single stellar population-equivalent parameters of 2.9 +/- 0.2 Gyr and [M/H] = 0.02 +/- 0.01 dex at F1 (similar to 2.7 r(e)). This contradicts spectroscopic studies that show a steep age gradient from M32' s center to 1 re. The inferred SFH of the M31 background field F2 reveals that the majority of its stars are old, with similar to 95% of its mass already acquired 5-14 Gyr ago. It is composed of two dominant populations; similar to 30% +/- 7.5% of its mass is in a 5-8 Gyr old population, and similar to 65% +/- 9% of the mass is in an 8-14 Gyr old population. The mass-weighted mean age and metallicity of F2 are < Age > = 9.2 +/- 1.2Gyr and <[M/H]> = -0.10 +/- 0.10 dex, respectively. Our results suggest that the inner disk and spheroid populations of M31 are indistinguishable from those of the outer disk and spheroid. Assuming the mean age of M31' s disk at F2 (similar to 1 disk scale length) to be similar to 5-9 Gyr, our results agree with an inside-out disk formation scenario for M31' s disk.

We present the discovery of three late-type (>= T4.5) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of similar to 400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/dM alpha M-alpha with alpha = 0.0-0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral-type relation than previously reported or an upturn in the number of very-late-type brown dwarfs in the observed volume.

Spectroscopic observations of H alpha and H beta emission lines of 128 star-forming galaxies in the redshift range 0.75 <= z <= 1.5 are presented. These data were taken with slitless spectroscopy using the G102 and G141 grisms of theWide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (H alpha/H beta). We present dust extinction as a function of H alpha luminosity (down to 3 x 10(41) erg s(-1)), galaxy stellar mass (reaching 4 x 10(8) M-circle dot), and rest-frame H alpha equivalent width. The faintest galaxies are two times fainter in H alpha luminosity than galaxies previously studied at z similar to 1.5. An evolution is observed where galaxies of the same H alpha luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower H alpha luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the [O III] lambda 5007/H alpha flux ratio as a function of luminosity where galaxies with L-H alpha < 5 x 10(41) erg s(-1) are brighter in [O III] lambda 5007 than H alpha. This trend is evident even after extinction correction, suggesting that the increased [O III] lambda 5007/H alpha ratio in low-luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters.

We present here a simple model for the star formation history (SFH) of galaxies that is successful in describing both the star formation rate density (SFRD) over cosmic time, as well as the distribution of specific star formation rates (sSFRs) of galaxies at the current epoch, and the evolution of this quantity in galaxy populations to a redshift of z = 1. We show first that the cosmic SFRD is remarkably well described by a simple log-normal in time. We next postulate that this functional form for the ensemble is also a reasonable description for the SFHs of individual galaxies. Using the measured sSFRs for galaxies at z similar to 0 from Paper III in this series, we then construct a realization of a universe populated by such galaxies in which the parameters of the log-normal SFH of each galaxy are adjusted to match the sSFRs at z similar to 0 as well as fitting, in ensemble, the cosmic SFRD from z = 0 to z = 8. This model predicts, with striking fidelity, the distribution of sSFRs in mass-limited galaxy samples to z = 1; this match is not achieved by other models with a different functional form for the SFHs of individual galaxies, but with the same number of degrees of freedom, suggesting that the log-normal form is well matched to the likely actual histories of individual galaxies. We also impose the sSFR versus mass distributions at higher redshifts from Paper III as constraints on the model, and show that, as previously suggested, some galaxies in the field, particularly low mass galaxies, are quite young at intermediate redshifts. As emphasized in Paper III, starbursts are insufficient to explain the enhanced sSFRs in intermediate redshift galaxies; we show here that a model using only smoothly varying log-normal SFHs for galaxies, which allows for some fraction of the population to have peak star formation at late times, does however fully explain the observations. Finally, we show that this model, constrained in detail only at redshifts z < 1, also produces the main sequence of star-formation observed at 1.5 < z < 2.5, again suggesting that the log-normal SFHs are a close approximation to the actual histories of typical galaxies.

Using data from the IMACS Cluster Building Survey and from nearby galaxy surveys, we examine the evolution of the rate of star formation in field galaxies from z = 0.60 to the present. Fitting the luminosity function to a standard Schechter form, we find a rapid evolution of M-B* consistent with that found in other deep surveys; at the present epoch M-B* is evolving at the rate of 0.38 Gyr(-1), several times faster than the predictions of simple models for the evolution of old, coeval galaxies. The evolution of the distribution of specific star formation rates (SSFRs) is also too rapid to explain by such models. We demonstrate that starbursts cannot, even in principle, explain the evolution of the SSFR distribution. However, the rapid evolution of both M-B* and the SSFR distribution can be explained if some fraction of galaxies have star formation rates characterized by both short rise and fall times and by an epoch of peak star formation more recent than the majority of galaxies. Although galaxies of every stellar mass up to 1.4 x 10(11) M-circle dot show a range of epochs of peak star formation, the fraction of "younger" galaxies falls from about 40% at a mass of 4 x 10(10) M-circle dot to zero at a mass of 1.4 x 10(11) M-circle dot. The incidence of younger galaxies appears to be insensitive to the density of the local environment; but does depend on group membership: relatively isolated galaxies are much more likely to be young than are group members.

The IMACS Cluster Building Survey (ICBS) provides spectra of similar to 2200 galaxies 0.31 < z < 0.54 in five rich clusters (R less than or similar to 5 Mpc) and the field. Infalling, dynamically cold groups with tens of members account for approximately half of the supercluster population, contributing to a growth in cluster mass of similar to 100% by the present day. The ICBS spectra distinguish non-star-forming (PAS) and poststarburst (PSB) from star-forming galaxies-continuously star-forming (CSF) or starbursts (SBH or SBO), identified by anomalously strong H delta absorption or [O II] emission. For the infalling cluster groups and similar field groups, we find a correlation between PAS+PSB fraction and group mass, indicating substantial "preprocessing" through quenching mechanisms that can turn star-forming galaxies into passive galaxies without the unique environment of rich clusters. SBH + SBO starburst galaxies are common, and they maintain an approximately constant ratio (SBH+SBO)/CSF approximate to 25% in all environments-from field, to groups, to rich clusters. Similarly, while PSB galaxies strongly favor denser environments, PSB/PAS approximate to 10%-20% for all environments. This result, and their timescale tau similar to 500 Myr, indicates that starbursts are not signatures of a quenching mechanism that produces the majority of passive galaxies. We suggest instead that starbursts and poststarbursts signal minor mergers and accretions, in star-forming and passive galaxies, respectively, and that the principal mechanisms for producing passive systems are (1) early major mergers, for elliptical galaxies, and (2) later, less violent processes-such as starvation and tidal stripping, for S0 galaxies.

We present the first robust measurement of the high redshift mass-metallicity (MZ) relation at 10(8) less than or similar to M/M-circle dot less than or similar to 10(10), obtained by stacking spectra of 83 emission-line galaxies with secure redshifts between 1.3 less than or similar to z less than or similar to 2.3. For these redshifts, infrared grism spectroscopy with the Hubble Space Telescope Wide Field Camera 3 is sensitive to the R-23 metallicity diagnostic: ([OII] lambda lambda 3726, 3729 + [OIII] lambda lambda 4959, 5007)/H beta. Using spectra stacked in four mass quartiles, we find a MZ relation that declines significantly with decreasing mass, extending from 12+log(O/H) = 8.8 at M = 10(9.8) M-circle dot, to 12+log(O/H)= 8.2 at M = 10(8.2)M(circle dot). After correcting for systematic offsets between metallicity indicators, we compare our MZ relation to measurements from the stacked spectra of galaxies with M greater than or similar to 10(9.5)M(circle dot) and z similar to 2.3. Within the statistical uncertainties, our MZ relation agrees with the z similar to 2.3 result, particularly since our somewhat higher metallicities (by around 0.1 dex) are qualitatively consistent with the lower mean redshift (z = 1.76) of our sample. For the masses probed by our data, the MZ relation shows a steep slope which is suggestive of feedback from energy-driven winds, and a cosmological downsizing evolution where high mass galaxies reach the local MZ relation at earlier times. In addition, we show that our sample falls on an extrapolation of the star-forming main sequence (the SFR-M-* relation) at this redshift. This result indicates that grism emission-line selected samples do not have preferentially high star formation rates (SFRs). Finally, we report no evidence for evolution of the mass-metallicity-SFR plane; our stack-averaged measurements show excellent agreement with the local relation.

By exploiting the data base of early-type galaxy (ETG) members of the WINGS survey of nearby clusters, we address here the long debated question of the origin and shape of the Fundamental Plane (FP). Our data suggest that different physical mechanisms concur in shaping and 'tilting' the FP with respect to the virial plane (VP) expectation. In particular, a 'hybrid solution' in which the structure of galaxies and their stellar population are the main contributors to the FP tilt seems to be favoured. We find that the bulk of the tilt should be attributed to structural non-homology, while stellar population effects play an important but less crucial role. In addition, our data indicate that the differential FP tilt between the V and K band is due to a sort of entanglement between structural and stellar population effects, for which the inward steepening of colour profiles (V - K) tends to increase at increasing the stellar mass of ETGs. The same kind of analysis applied to the ATLAS3D and Sloan Digital Sky Survey (SDSS) data in common with WINGS (WSDSS throughout the paper) confirms our results, the only remarkable difference being the less important role that our data attribute to the stellar mass-to-light-ratio (stellar populations) in determining the FP tilt. The ATLAS3D data also suggest that the FP tilt depends as well on the dark matter (DM) fraction and on the rotational contribution to the kinetic energy (V-rot/sigma), thus again pointing towards the above-mentioned 'hybrid solution'. We show that the global properties of the FP, i.e. its tilt and tightness, can be understood in terms of the underlying correlation among mass, structure and stellar population of ETGs, for which, at increasing the stellar mass, ETGs become (on average) 'older' and more centrally concentrated. Finally, we show that a Malmquist-like selection effect may mimic a differential evolution of the mass-to-light ratio for galaxies of different masses. This should be taken into account in the studies investigating the amount of the so-called 'downsizing' phenomenon.

Using J- and Ks-band imaging obtained as part of the IMACS Cluster Building Survey (ICBS), we measure Sersic indices for 2160 field and cluster galaxies at 0.31 < z < 0.54. Using both mass-and magnitude-limited samples, we compare the distributions for spectroscopically determined passive, continuously star-forming, starburst, and post-starburst systems and show that previously established spatial and statistical connections between these types extend to their gross morphologies. Outside of cluster cores, we find close structural ties between starburst and continuously star-forming, as well as post-starburst and passive types, but not between starbursts and post-starbursts. These results independently support two conclusions presented in Paper II of this series: (1) most starbursts are the product of a non-disruptive triggering mechanism that is insensitive to global environment, such as minor mergers; (2) starbursts and post-starbursts generally represent transient phases in the lives of "normal" star-forming and quiescent galaxies, respectively, originating from and returning to these systems in closed "recycling" loops. In this picture, spectroscopically identified post-starbursts constitute a minority of all recently terminated starbursts, largely ruling out the typical starburst as a quenching event in all but the densest environments.

We combine Hubble Space Telescope (HST) G102 and G141 near-IR (NIR) grism spectroscopy with HST/WFC3UVIS, HST/WFC3-IR, and Spitzer/IRAC [3.6 mu m] photometry to assemble a sample ofmassive (log(M-star/M-circle dot) similar to 11.0) and quenched (specific star formation rate < 0.01 Gyr-1) galaxies at z similar to 1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy for quenched sources at these redshifts. In contrast to the local universe, z similar to 1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties. We find that their spectral energy distributions (SEDs) are well fitted with exponentially decreasing star formation histories and short star formation timescales (tau <= 100 Myr). Quenched galaxies also show a wide distribution in ages, between 1 and 4 Gyr. In the (u - r)(0)-versus-mass space quenched galaxies have a large spread in rest-frame color at a given mass. Most quenched galaxies populate the z similar to 1.5 red sequence (RS), but an important fraction of them (32%) have substantially bluer colors. Although with a large spread, we find that the quenched galaxies on the RS have older median ages (3.1 Gyr) than the quenched galaxies off the RS (1.5 Gyr). We also show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies off the RS. We derive the upper limit on the fraction of massive galaxies on the RS at z similar to 1.5 to be < 43%. We speculate that the young quenched galaxies off the RS are in a transition phase between vigorous star formation at z > 2 and the z similar to 1.5 RS. According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z similar to 1.5 RS is of the order of similar to 1Gyr.