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 (0.3arcsec) have been classified according to morphology and their photometricproperties are presented. There are 269 spirals, 100 ellipticals, 114 chains,126 double-clump, 97 tadpole, and 178 clump-cluster galaxies. We alsocatalogued 30 B-band and 13 V-band drop-outs and calculated their starformation rates. Chains, doubles, and tadpoles dominate the other types atfaint magnitudes. The fraction of obvious bars among spirals is ~10 percent, afactor of 2-3 lower than in other deep surveys. The distribution function ofaxial ratios for elliptical galaxies is similar to that seen locally,suggesting that ellipticals relaxed quickly to a standardized shape. Thedistribution of axial ratios for spiral galaxies is significantly differentthan locally, having a clear peak at ~0.55 instead of a nearly flatdistribution. The fall-off at small axial ratio occurs at a higher value thanlocally, indicating thicker disks by a factor of ~2. The fall-off at high axialratio could be from intrinsic triaxial shapes or selection effects. Inclineddisks should be more highly sampled than face-on disks near the surfacebrightness limit of a survey. Simple models and data distributions demonstratethese effects. The decreased numbers of obvious spiral galaxies at highredshifts could be partly the result of surface brightness selection.Comment: 29 pages, 12 figures, ApJ in press, 20 Sept 2005, Vol 63