The first galaxies: structure and stellar populations

Abstract

The Hubble deep fields (HDFs) 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 (NIR) imaging survey of the HDFnorth with the Near Infrared Camera and Multi–Object Spectrograph (NICMOS), which provides structural and photometric information in the optical rest frame (λλ₂4000–5500 Å) 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 (LBGs) 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 LBGs have ultraviolet (UV)-optical spectral energy distributions like those of local starburst galaxies. Population synthesis models suggest typical ages of a few x 10⁸ years and moderate UV extinction (ca .1.2 mag at 1700 Å), but the constraints are fairly weak and there may be considerable variety. Considering an NIR 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 LBG candidates at 4.5 < z < 9, as well as one remarkable object that might (or might not) be an LBG 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.