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

Abstract

We present an analysis of the deepest near-UV image obtained with theHubble Space Telescope using the WFPC2 (F300W) as part of the parallelobservations of the Hubble Ultra Deep Field campaign. The U-band 10{σ} limiting magnitude measured over 0.2 arcsec^{2} ism_{AB}=27.5, which is 0.5 mag deeper than that in the HubbleDeep Field-North. We matched the U-band catalogs with those from the ACS(BViz) taken during the Great Observatories Origins Deep Surveyobservations of the Chandra Deep Field-South and obtained photometricredshifts for 306 matched objects. We find that the UV-selected galaxiesspan all the major morphological types at0.2{\lt}z_{phot}{\lt}1.2. However, disks are more common at lowerredshifts, 0.2{\lt}z_{phot}{\lt}0.8. Higher redshift objects(0.7{\lt}z_{phot}{\lt}1.2) are on average bluer than lower z andhave spectral types typical of starbursts. They have compact, peculiar,or low surface brightness morphologies. Despite the selection by UV, 13objects have spectral types of early-type galaxies; two of them arespheroids with blue cores. The evolutionary synthesis code Starburst99was used to age-date the UV-selected galaxies, which were found to haverest-frame colors typical of stellar populations with mean ages {\gt}100Myr. The average half-light radius (rest-frame 1200-1800 {Å}) of theUV-selected galaxies at 0.66{\lt}z_{phot}{\lt}1.5 is 0.26''+/-0.01''(2.07+/-0.08 kpc). The UV-selected galaxies are on average fainter(M_{B}=-18.43+/-0.13) than Lyman break galaxies (LBGs;M_{B}=-23+/-1). Our sample includes early-type galaxies that arepresumably massive and forming stars only in their cores, as well asstarburst-type systems that are more similar to the LBGs, although muchless luminous. This implies that even the starbursts in our sample areeither much less massive than LBGs or forming stars at a much lowerrate, or both. The low surface brightness galaxies have no overlap withthe LBGs and form an interesting new class of their own.