Star-forming Galaxies at Intermediate Redshifts: Morphology, Ages, and Sizes

We present an analysis of the deepest near-UV image obtained with the Hubble Space Telescope using the WFPC2 (F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign. The U-band 10 σ limiting magnitude measured over 0.2 arcsec2 is mAB = 27.5, which is 0.5 mag deeper than that in the Hubble Deep Field–North. We matched the U-band catalogs with those from the ACS (BViz) taken during the Great Observatories Origins Deep Survey observations of the Chandra Deep Field–South and obtained photometric redshifts for 306 matched objects. We find that the UV-selected galaxies span all the major morphological types at 0.2 < zphot < 1.2. However, disks are more common at lower redshifts, 0.2 < zphot < 0.8. Higher redshift objects (0.7 < zphot < 1.2) are on average bluer than lower z and have spectral types typical of starbursts. They have compact, peculiar, or low surface brightness morphologies. Despite the selection by UV, 13 objects have spectral types of early-type galaxies; two of them are spheroids with blue cores. The evolutionary synthesis code Starburst99 was used to age-date the UV-selected galaxies, which were found to have rest-frame colors typical of stellar populations with mean ages >100 Myr. The average half-light radius (rest-frame 1200–1800 Å) of the UV-selected galaxies at 0.66 < zphot < 1.5 is 0.″26 ± 0.″01 (2.07 ± 0.08 kpc). The UV-selected galaxies are on average fainter (MB = -18.43 ± 0.13) than Lyman break galaxies (LBGs; MB = -23 ± 1). Our sample includes early-type galaxies that are presumably massive and forming stars only in their cores, as well as starburst-type systems that are more similar to the LBGs, although much less luminous. This implies that even the starbursts in our sample are either much less massive than LBGs or forming stars at a much lower rate, or both. The low surface brightness galaxies have no overlap with the LBGs and form an interesting new class of their own.