Observing the Formation of the Hubble Sequence in the Great Observatories Origins Deep Survey

Understanding the physical formation of the Hubble sequence remains one ofthe most important unsolved astrophysical problems. Searches for proto-disksand proto-ellipticals can now be effectively done using deep wide-field HubbleSpace Telescope images taken with the new Advanced Camera for Surveys. Throughan analysis of concentrations (C), asymmetries (A) and clumpiness values (S)(CAS) of galaxies found in the GOODS Field South, we are able to identifyobjects possibly forming onto the Hubble sequence. Using this approach, wedetect a sizeable population of star forming luminous diffuse objects and starforming luminous asymmetric objects between redshifts 0.5 < z < 2. Thesegalaxies have extremely low light concentrations, or high asymmetries, withabsolute magnitudes M_B < -19. The luminous diffuse objects are found inabundance between z = 1 - 2, with fewer objects at z > 2 and z < 1. Theluminous asymmetric objects are found at a similar abundance, with a peak atz~1. We argue that these galaxies are a subset of modern disks and ellipticalsin formation. The co-moving volume density of the luminous diffuse objectsbetween z = 1 - 2 is similar to the local density of bright disk galaxies, withvalues ~5*10^{5} Gpc^{-3}. The SEDs of these objects are mostly consistent withstarbursts, or star-forming normal galaxies, with average uncorrected forextinction star formation rates of ~4 M_0 yr^{-1}. These galaxies also host35-40% of the star formation activity at 1 < z < 2. We briefly discuss theimplications of these objects for understanding the origin of the Hubblesequence.Comment: ApJ letters, accepted: July 22, 200