Using deep near-infrared spectroscopy, Kriek et al. found that ∼45% of massive galaxies at have evolved z ∼ 2.3 stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)-assisted Keck/adaptive optics (AO). Considering that their median stellar mass is , the galaxies are remarkably small, with 11 1.7 # 10 M , a median effective radius kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and r p 0.9 e average stellar densities that are 2 orders of magnitude lower than the galaxies. These results extend earlier z ∼ 2.3 work at and confirm previous studies at that lacked spectroscopic redshifts and imaging of sufficient z ∼ 1.5 z 1 2 resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ∼10% of the population of K-selected quiescent galaxies at , effectively ruling out simple monolithic z ∼ 2.3 models for their formation. The galaxies must evolve significantly after , through dry mergers or other z ∼ 2.3 processes, consistent with predictions from hierarchical models.

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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 ¼ 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 ¼ 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.

We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies at 2 < z < 3:5 in the FIRES MS 1054 field with red and blue rest-frame optical colors. About half of the galaxies are very compact (effective radii r e < 1 kpc) at rest-frame optical wavelengths; the others are extended (1 kpc < r e < 10 kpc). For reference, 1 kpc corresponds to 0.12 00 at z ¼ 2:5 in the adopted cosmology. We separate actively star-forming galaxies from quiescent galaxies by modeling their rest-frame UVYNIR SEDs. The star-forming galaxies span the full range of sizes, while the quiescent galaxies all have r e < 2 kpc. In the redshift range where MIPS 24 m imaging is a sensitive probe of reradiated dust emission (z < 2:5), the 24 m fluxes confirm that the light of the small quiescent galaxies is dominated by old stars, rather than dust-enshrouded star formation or AGN activity. The inferred surface mass densities and velocity dispersions for the quiescent galaxies are very high compared to those in local galaxies. The galaxies follow a Kormendy relation ( between surface brightness and size) with approximately the same slope as locally, but shifted to brighter surface brightnesses, consistent with a mean stellar formation redshift of z f $ 5. This paper demonstrates a direct relation between star formation activity and size at z $ 2:5 and the existence of a significant population of massive, extremely dense, old stellar systems without readily identifiable counterparts in the local universe.

Analyzing 24µm MIPS/Spitzer data and the [O II]3727 line of a sample of galaxies at 0.4 ≤ z ≤ 0.8 from the ESO Distant Cluster Survey (EDisCS), we investigate the ongoing star formation rate (SFR) and the specific star formation rate (SSFR) as a function of stellar mass in galaxy clusters and groups, and compare with field studies. As for the field, we find a decline in SFR with time, indicating that star formation (SF) was more active in the past, and a decline in SSFR as galaxy stellar mass increases, showing that the current SF contributes more to the fractional growth of low-mass galaxies than high-mass galaxies. However, we find a lower median SFR (by a factor of ∼1.5) in cluster star-forming galaxies than in the field. The difference is highly significant when all Spitzer and emission-line galaxies are considered, regardless of color. It remains significant at z > 0.6 after removing red emission-line (REL) galaxies, to avoid possible AGN contamination. While there is overlap between the cluster and field SFR-Mass relations, we find a population of cluster galaxies (10-25%) with reduced SFR for their mass. These are likely to be in transition from star-forming to passive. Comparing separately clusters and groups at z > 0.6, only cluster trends are significantly different from the field, and the average cluster SFR at a given mass is ∼ 2 times lower than the field. We conclude that the average SFR in star-forming galaxies varies with galaxy environment at a fixed galaxy mass.