Deep Near–Infrared Imaging Surveys and the Stellar Content of High Redshift Galaxies

Deep, near–infrared imaging surveys have been motivated by the desire to study the rest–frame optical properties and stellar content of galaxies at high redshift. Here we briefly review their history, and illustrate one application, using HST NICMOS imaging of the Hubble Deep Field North to examine the rest–frame optical luminosities and colors of galaxies at 2 < z < 3, and to constrain their stellar masses. The rest–frame B–band luminosity density at z ≈ 2.5 is similar to that in the local universe, but the galaxies are evidently less massive, with rapid star formation and low mass–to–light ratios. There are few candidates for red, non– star–forming galaxies at these redshifts to the HDF/NICMOS limits. We estimate a stellar mass density at 2 < z < 3 that is ∼ 5% of the present–day value, with an upper bound of ≤ 30%. Future headway will come from wide–field ground–based surveys, multiplexing infrared spectrographs, and new space–based facilities such as HST/WFC3 and SIRTF/IRAC.