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

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Abstract
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 arcmin2. At the redshifts of interest, these flux limits correspond to typical rest-frame line luminosities of ~1042-1043 ergs s-1, unobscured star formation rates of ~1-100 M☉ yr-1, and a sampled comoving volume of several tens of Mpc3. We have found no candidate PGs at a 2 σ confidence level. We demonstrate that for moderate dust absorption (AV 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.
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