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) have been classified according to morphology, and their photometric properties are presented. There are 269 spiral, 100 elliptical, 114 chain, 126 double- clump, 97 tadpole, and 178 clump- cluster galaxies. We also cataloged 30 B- band and 13 V- band dropouts 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 similar to 10%, 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 similar to 0.55 instead of a nearly flat distribution. The falloff at small axial ratio occurs at a higher value than locally, indicating thicker disks by a factor of similar to 2. The falloff 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.