Abstract
We analyze 26 luminous compact blue galaxies (LCBGs) in the Hubble Space Telescope ACS Ultra Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these truly are small galaxies or, rather, bright central starbursts within existing or forming large disk galaxies. Surface brightness profiles from UDF images reach fainter than rest-frame 26.5 B mag arcsec-2 even for compact objects at z ~ 1. Most LCBGs show a smaller, brighter component that is likely star-forming, and an extended, roughly exponential component with colors suggesting stellar ages from ≳100 Myr to a few gigayears. Scale lengths of the extended components are mostly ≲2 kpc, more than 1.5-2 times smaller than those of nearby large disk galaxies like the Milky Way. Larger, very low surface brightness disks can be excluded down to faint rest-frame surface brightnesses (≳26 B mag arcsec-2). However, one or two of the LCBGs are large, disklike galaxies that meet LCBG selection criteria because of a bright central nucleus, possibly a forming bulge. These results indicate that ≳90% of high-z LCBGs are small galaxies that will evolve into small disk galaxies, or low-mass spheroidal or irregular galaxies in the local universe, assuming passive evolution and no significant disk growth. The data do not reveal signs of disk formation around small, H II galaxy-like LCBGs, nor do they suggest a simple inside-out growth scenario for larger LCBGs with a disklike morphology. Irregular blue emission in distant LCBGs is relatively extended, suggesting that nebular emission lines from star-forming regions sample a major fraction of an LCBG's velocity field.
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