A Near-Infrared Search for Line Emission from Protogalaxies Using the W. M. Keck Telescope

We present the first results from a near-infrared narrowband search for primeval galaxies (PGs) using the Keck 10 m telescope. We have targeted our fields for three QSOs and one radio galaxy at redshifts z = 2.28 to z = 4.70. We selected narrowband filters in the K band centered on strong emission lines at the redshift of the targets. We reach limiting fluxes between 1.6 × 10 -17 and 1.0 × 10 -16 ergs s -1 cm -2 , and cover a total area of ~4 arcmin 2 . At the redshifts of interest, these flux limits correspond to typical rest-frame line luminosities of ~10 42 -10 43 ergs s -1 , unobscured star formation rates of ~1-100 M ☉ yr -1 , and a sampled comoving volume of several tens of Mpc 3 . We have found no candidate PGs at a 2 σ confidence level. We demonstrate that for moderate dust absorption ( A V ≳ 2 mag) in a simple dust-screen model, our preliminary survey puts a strong constraint on intrinsic PG luminosities during an initial burst of star formation. In the case of the QSOs, we have used point-spread function subtraction to search for the presence of faint, extended line emission surrounding these radio-quiet active galactic nuclei. We find no evidence for extended emission down to 3.7 × 10 -17 ergs s -1 cm -2 . This imposes limits on the reprocessed fraction of the QSO continuum emission ranging between 0.02% and 0.29%; if beaming effects are not important, then this implies similarly small covering factors for clouds optically thick to the QSO continuum emission.