Hst and Spitzer Imaging of Red and Blue Galaxies at Z ∼ 2.5: a Correlation between Size and Star Formation Activity from Compact Quiescent Galaxies to Extended Star Forming Galaxies

We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies at 2 < z < 3.5 in the FIRES MS1054 field with red and blue rest-frame optical colors. About half of the galaxies are very compact (effective radii re < 1 kpc) at rest-frame optical wavelengths, the others are extended (1 < re < 10 kpc). For reference, 1 kpc corresponds to 0. 12 at z = 2.5 in the adopted cosmology. We separate actively star forming galaxies from quiescent galaxies by modeling their rest-frame UVNIR SEDs. The star forming galaxies span the full range of sizes, while the quiescent galaxies all have re < 2kpc. In the redshift range where MIPS 24μm imaging is a sensitive probe of re-radiated dust emission (z < 2.5), the 24μm 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 zf ∼ 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. Subject headings: galaxies: formation — galaxies: evolution — galaxies: high redshift — galaxies: fundamental parameters — galaxies: structure — infrared: galaxies