Star--forming galaxies at intermediate redshifts: morphology, ages and sizes

Abstract

We present the analysis of the deepest near-UV image obtained with HST using the WFPC2(F300W) as part of the parallel observations of the Ultra Deep Field campaign. The U-band 10sigma limiting magnitude measured over 0.2 arcsec square is m(AB)=27.5 which is 0.5 magnitudes deeper than that in the HDF-North. We matched the U-band catalog with those in the ACS images taken during the GOODS observations of the CDF-South and obtained photometric-z for 306 matched objects. We find that the UV-selected galaxies span all the major morphological types at 0.2<z_phot<1.2. However, disks are more common at lower redshifts, 0.2<z_phot<0.8. Higher-z objects (0.7<z_phot<1.2) are on average bluer than lower-z and have spectral type typical of starbursts. Their morphologies are compact, peculiar or low surface brightness galaxies. The average half-light radius (rest-frame 1200--1800 A) of the UV-selected galaxies at 0.66<z_ phot<1.5 is 0.26 +- 0.01 arcsec (2.07 +- 0.08 kpc). The UV-selected galaxies are on average fainter (M_B=-18.43+-0.13) than Lyman Break Galaxies (M_B=-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 are 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.