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

Abstract

We present the first results from a near-infrared narrow-band 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 $4.70$. We selected narrow-band filters in the $K$-band centered on strong emision lines at the redshift of the targets. We reach limiting fluxes between $1.6\times 10^{-17}$~erg~s${}^{-1}$~cm${}^{-2}$ and $1.0\times 10^{-16}$~erg~s${}^{-1}$~cm${}^{-2}$, and cover a total area of $\sim 4$~arcmin${}^2$. At the redshifts of interest, these flux limits correspond to typical restframe line luminosities of $\sim 10^{42}-10^{43}$~erg~s${}^{-1}$, unobscured star formation rates of $\sim 1-100$ $M_{\odot }$~yr${}^{-1}$, and a sampled comoving volume of several tens of Mpc${}^3$. We have found no candidate PGs at a $2\sigma$ confidence level. We demonstrate that for moderate dust absorption ($A_V \gsim 2^m$) in a simple dust-screen model, our preliminary survey puts a strong constraint on intrinsic PG luminosities during an intial burst of star formation. In the case of the QSOs, we have used PSF-subtraction to search for the presence of faint, extended line emission surrounding these radio-quiet AGNs. We find no evidence for extended emission down to $3.7\times 10^{-17}$~erg~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.

9 pages (text) plus 4 figures, uuencoded compressed postscript format, to appear in The Astrophysical Journal (Letters); also available in same format from http://astro.caltech.edu/~map/papers/pgapjl/pgpreprint.uu

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