Galaxy Morphology in the GTO-NICMOS Northern Hubble Deep Field

Abstract

The increased incidence of morphologically peculiar galaxies at faint magnitudes in the optical could be attributable either to "morphological k-corrections" (the change in appearance when viewing high-z objects at shorter rest-frame wavelengths), or an increase in the incidence of truly irregular systems with redshift. The deep, high-resolution GTO-NICMOS near-IR imaging of a portion of the northern Hubble Deep Field has been combined with the WFPC2 data and photometric redshift estimates to study the redshift evolution of morphology, comparing galaxy appearance at the same rest-wavelengths (Bunker, Spinrad & Thompson 1999). It appears that morphological k-corrections are only significant in a minority of cases, and that once these are accounted for, evolution is still demanded - galaxies were smaller and more irregular in the past, with some of the peculiarities probably merger-related. This multi-waveband data set also enables a study of the spatially-resolved stellar populations in distant galaxies. A near-infrared analysis of some of the brighter spirals shows more pronounced barred structure than in the optical, indicating that the apparent decline in barred spirals at faint magnitudes in the optical HDF may be due to band-shifting effects at the higher redshifts, rather than intrinsic evolution.