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 (λλ04000–5500Å) 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×108 years and moderate UV extinction (∼ 1.2 mag at 1700Å), 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.