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

We present the first results from a near-infrared narrow-band search forprimeval galaxies (PGs) using the Keck 10~m telescope. We have targeted ourfields 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 emisionlines at the redshift of the targets. We reach limiting fluxes between$1.6\times 10^{-17}$~erg~s$^{-1}$~cm$^{-2}$ and $1.0\times10^{-16}$~erg~s$^{-1}$~cm$^{-2}$, and cover a total area of $\sim4$~arcmin$^2$. At the redshifts of interest, these flux limits correspond totypical 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 asampled comoving volume of several tens of Mpc$^3$. We have found no candidatePGs at a $2\sigma$ confidence level. We demonstrate that for moderate dustabsorption ($A_V \gsim 2^m$) in a simple dust-screen model, our preliminarysurvey puts a strong constraint on intrinsic PG luminosities during an intialburst of star formation. In the case of the QSOs, we have used PSF-subtractionto search for the presence of faint, extended line emission surrounding theseradio-quiet AGNs. We find no evidence for extended emission down to $3.7 \times10^{-17}$~erg~s$^{-1}$~cm$^{-2}$. This imposes limits on the reprocessedfraction of the QSO continuum emission ranging between $0.02$\% and $0.29$\%;if beaming effects are not important, then this implies similarly smallcovering factors for clouds optically thick to the QSO continuum emission.