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A.J. Bunker, H. Spinrad (U.C. Berkeley), R.I. Thompson (Steward Obs., Univ. of Arizona)
We present deep near-infrared images obtained with HST/NICMOS of galaxies in the northern Hubble Deep Field, selected for their peculiar morphology in the WFPC\,2 optical data. Half of our sample have spectroscopic redshifts, ranging from z=0.5 to z=3.2.
The optical morphologies range from greatly elongated (including near-linear `chain galaxies') to `bow shock' shapes. We include examples with multiple compact components; one such case (HDF\,4-858), which resembles an Einstein quad, is likely to be a multiple nucleus system rather than the result of lensing of a single source -- the absence of a foreground galaxy in our infrared imaging places stringent limits on the mass of any intervening lens. We highlight examples of spiral galaxies at z\approx 1, where the nuclei are visible only at infrared wavelengths, with \ion{H}{2} regions in the spiral arms dominating the optical images and leading to an irregular appearance. 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. For the galaxies with z\la 1 there is a clear tendency toward increased surface-brightness symmetry and regularity at the NICMOS wavelengths (that is, \lambda\rm rest\ga 5000~Å). For many z>2 galaxies, the peculiar morphology persists through our longest observed wavelengths; many examples are `true' irregulars, with some of the peculiarities probably merger-related.
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