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 r _ { e } < 1 kpc ) at rest-frame optical wavelengths , the others are extended ( 1 < r _ { e } < 10 kpc ) .
For reference , 1 kpc corresponds to 0 \farcs 12 at z = 2.5 in the adopted cosmology .
We separate actively star forming galaxies from quiescent galaxies by modeling their rest-frame UV-NIR 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 \micron imaging is a sensitive probe of re-radiated dust emission ( z < 2.5 ) , the 24 \micron 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 } \sim 5 .
This paper demonstrates a direct relation between star formation activity and size at z \sim 2.5 , and the existence of a significant population of massive , extremely dense , old stellar systems without readily identifiable counterparts in the local universe .