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We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxiesat 2<z<3.5 in the FIRES MS1054 field with red and blue rest-frame opticalcolors. About half of the galaxies are very compact (effective radii r_e < 1kpc) at rest-frame optical wavelengths, the others are extended (1< r_e < 10kpc). For reference, 1 kpc corresponds to 0.12 arcsec at z=2.5 in the adoptedcosmology. We separate actively star forming galaxies from quiescent galaxiesby modeling their rest-frame UV-NIR SEDs. The star forming galaxies span thefull range of sizes, while the quiescent galaxies all have r_e<2kpc. In theredshift range where MIPS 24 micron imaging is a sensitive probe of re-radiateddust emission (z<2.5), the 24 micron fluxes confirm that the light of the smallquiescent galaxies is dominated by old stars, rather than dust-enshrouded starformation or AGN activity. The inferred surface mass densities and velocitydispersions for the quiescent galaxies are very high compared to those in localgalaxies. The galaxies follow a Kormendy relation (between surface brightnessand size) with approximately the same slope as locally, but shifted to brightersurface brightnesses, consistent with a mean stellar formation redshift ofz_f~5. This paper demonstrates a direct relation between star formationactivity and size at z~2.5, and the existence of a significant population ofmassive, extremely dense, old stellar systems without readily identifiablecounterparts in the local universe.

astrophysical journal/the astrophysical journal

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