NICMOS Imaging of DRGs in the HDF‐S: A Relation between Star Formation and Size atz∼ 2.5

We present deep, high angular-resolution HST/NICMOS imaging in the HubbleDeep Field South (HDF-S), focusing on a subset of 14 Distant Red Galaxies(DRGs) at z ~ 2.5 galaxies that have been pre-selected to have J-K > 2.3. Wefind a clear trend between the rest-frame optical sizes of these sources andtheir luminosity-weighted stellar ages as inferred from their broad-bandspectral energy distributions (SEDs). Galaxies whose SEDs are consistent withbeing dusty and actively star forming generally show extended morphologies inthe NICMOS images (r_e >~ 2 kpc), while the 5 sources which are not vigorouslyforming stars are extremely compact (r_e <~ 1 kpc). This trend suggests adirect link between the mean ages of the stars and the size and density of thegalaxies and supports the conjecture that early events quench star-formationand leave compact remnants. Furthermore, the compact galaxies have stellarsurface mass densities which exceed those of local galaxies by more than anorder of magnitude. The existence of such massive dense galaxies presents aproblem for models of early-type galaxy formation and evolution. Larger samplesof DRGs and higher spatial resolution imaging will allow us to determine theuniversality of the results presented here for a small sample.Comment: Accepted to The Astrophysical Journal, 8 pages, 5 figure