Stellar Populations in Ten Clump-Cluster Galaxies of the Ultra Deep Field

Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Ultra Deep Field are made from B,V,i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses. The clump colors suggest declining star formation over the last ~0.3 Gy, while the interclump emission is older. The clump luminous masses are typically 6x10^8 Msun and their diameters average 1.8 kpc. Total galaxy luminous masses average 6.5x10^10 Msun. The distribution of axial ratios is consistent with a thick disk geometry. The ages of the clumps are longer than their internal dynamical times by a factor of ~8, so they are stable clusters, but the clump densities are only ~10 times the limiting tidal densities, so they could be deformed by tidal forces. This is consistent with the observation that some clumps have tails. The clumps could form by gravitational instabilities in accreting disk gas, or they could be captured as gas-rich dwarf galaxies. Support for this second possibility comes from the high abundance of nearly identical bare clumps in the UDF field. Several clump-clusters have disk densities that are much larger than in local disks, suggesting they do not survive but get converted into ellipticals by collisions.