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

Understanding the physical formation of the Hubble sequence remains one of the most important unsolved astrophysical problems. Searches for protodisks and proto-elliptical galaxies can now be effectively done using deep wide-field Hubble Space Telescope images taken with the new Advanced Camera for Surveys. Through an analysis of the concentrations (C), asymmetries (A), and clumpiness values (S) of galaxies found in the Great Observatories Origins Deep Survey Field South, we are able to identify objects possibly forming onto the Hubble sequence. Using this approach, we detect a sizeable population of star-forming luminous diffuse objects and star-forming luminous asymmetric objects between redshifts 0.5 < z < 2. These galaxies have extremely low light concentrations, or high asymmetries, with absolute magnitudes MB < -19. The luminous diffuse objects are found in abundance between z = 1 and 2, with fewer objects at z > 2 and z < 1. The luminous asymmetric objects are found at a similar abundance, with a peak at z ~ 1. We argue that these galaxies are a subset of modern disks and elliptical galaxies in formation. The comoving volume density of the luminous diffuse objects between z = 1 and 2 is similar to the local density of bright disk galaxies, with values ~5 × 105 Gpc-3. The spectral energy distributions of these objects are mostly consistent with starbursts, or star-forming normal galaxies, with average uncorrected for extinction star formation rates of ~4 M☉ yr-1. These galaxies also host 35%-40% of the star formation activity at 1 < z < 2. We briefly discuss the implications of these objects for understanding the origin of the Hubble sequence.