Galaxy Morphologies in the Hubble Ultra Deep Field: Dominance of Linear Structures at the Detection Limit

Galaxies in the Hubble Ultra Deep Field (UDF) larger than 10 pixels (0.3 arcsec) have been classified according to morphology and their photometric properties are presented. There are 269 spirals, 100 ellipticals, 114 chains, 126 double-clump, 97 tadpole, and 178 clump-cluster galaxies. We also catalogued 30 B-band and 13 V-band drop-outs and calculated their star formation rates. Chains, doubles, and tadpoles dominate the other types at faint magnitudes. The fraction of obvious bars among spirals is ~10 percent, a factor of 2-3 lower than in other deep surveys. The distribution function of axial ratios for elliptical galaxies is similar to that seen locally, suggesting that ellipticals relaxed quickly to a standardized shape. The distribution of axial ratios for spiral galaxies is significantly different than locally, having a clear peak at ~0.55 instead of a nearly flat distribution. The fall-off at small axial ratio occurs at a higher value than locally, indicating thicker disks by a factor of ~2. The fall-off at high axial ratio could be from intrinsic triaxial shapes or selection effects. Inclined disks should be more highly sampled than face-on disks near the surface brightness limit of a survey. Simple models and data distributions demonstrate these effects. The decreased numbers of obvious spiral galaxies at high redshifts could be partly the result of surface brightness selection.