Stellar populations in 10 clump-cluster galaxies of the hubble ultra deep field

Abstract

Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Hubble Ultra Deep Field (UDF) are made from B,V, i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses. These galaxies are members of a class dominated by 5-10 giant clumps, with no exponential disk or bulge. The redshifts are found to be in the range from 1.6 to 3. The clump emission is typically 40% of the total galaxy emission, and the luminous clump mass is 19% of the total galaxy mass. The clump colors suggest declining star formation over the last ∼0.3 Gyr, while the interclump emission is redder than the clumps, corresponding to a greater age. The clump luminous masses are typically 6 × 108 M⊙, and their diameters average 1.8 kpc, making their average density ∼0.2 M⊙ pc-3. Including the interclump populations, assumed to begin forming at z = 6, the total galaxy luminous masses average 6.5 × 1010 M⊙ and their diameters average 19 kpc to the 2 σ noise level. The expected galaxy rotation speeds average ∼150 km s-1 if they are uniformly rotating disks. The ages of the clumps are longer than their internal dynamical times by a factor of ∼8, so they are stable star 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 and then disperse on a ∼1 Gyr timescale, building up the interclump disk emission, or they could be captured as gas-rich dwarf galaxies, flaring up with star formation at first and then dispersing. Support for this second possibility comes from the high abundance of nearly identical clumps in the UDF, smaller than 6 pixels, whose distributions on color-magnitude and color-color plots are the same as the galaxy clumps studied here. The distribution of axial ratios for the combined population of chain and clump-cluster galaxies in the UDF is compared with models and shown to be consistent with a thick-disk geometry. If these galaxies evolve into today's disk galaxies, then we are observing a stage in which accretion and star formation are extremely clumpy and the resulting high velocity dispersions form thick disks. Several clump-clusters have disk densities that are much larger than in local disks, however, suggesting an alternate model in which they do not survive until today, but get converted into ellipticals by collisions. © 2005. The American Astronomical Society. All rights reserved.