The first galaxies: structure and stellar populations

The Hubble Deep Fields continue to be a valuable resource for studying the distant universe, particularly at z>2 where their comoving volume becomes large enough to encompass several hundred L* galaxies or their progenitors. Here I present recent results from a near-infrared imaging survey of the HDF-North with NICMOS, which provides structural and photometric information in the optical rest frame (4000-5500A) for hundreds of ordinary galaxies at 2<z<3, and which offers the means to search for still more distant objects at z>>5. Lyman break galaxies at 2<z<3 are compact and often irregular in the NICMOS images; ordinary Hubble sequence spirals and ellipticals seem to be largely absent at these redshifts, and apparently reached maturity at 1<z<2. The Lyman break galaxies have UV-optical spectral energy distributions like those of local starburst galaxies. Population synthesis models suggest typical ages ~few x 10^8 years and moderate UV extinction (~1.2 mag at 1700A), but the constraints are fairly weak and there may be considerable variety. Considering a near-IR selected galaxy sample, there is little evidence for a significant number of galaxies at z~3 that have been missed by UV-based Lyman break selection. Using the well-characterized z~3 galaxy population as a point of reference, I consider Lyman break galaxy candidates at 4.5<z<9, as well as one remarkable object which might (or might not) be at z>12. The space density of UV-bright galaxies in the HDF appears to thin out toward larger redshifts, although surface brightness selection effects may play an important role.