The Large Area Lyman Alpha Survey

Abstract

The Lyman-$α$ line is expected to be strong in the presence of active star formation and the absence of dust, making it a good tool for finding chemically primitive galaxies in the early universe. We report on a new survey for high redshift Lyman-$α$ sources, the Large Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrow-band filters on the new $8192^2$ pixel CCD Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching known high redshift Lyman-$α$ galaxies have an observed surface density corresponding to $11000 \pm 700$ per square degree per unit redshift at $z=4.5$. Early spectroscopic followup from the Keck telescope suggests that $\sim 1/3$ of these are actually at $z\approx 4.5$, and has confirmed five $z > 4$ Lyman-$α$ emitters so far. Combining our photometric survey with spectroscopic results, we estimate a net density of $\sim 4000 $ Lyman-$α$ emitters per square degree per unit redshift at $z\approx 4.5$. The star formation rate density (estimated both from UV continuum and from line emission) is comparable to that of the Lyman break galaxy population within present uncertainties. The most extreme Lyman-$α$ emitters in our sample have rest frame equivalent widths $> 100Å$, consistent with the expectations for the first burst of star formation in a primitive, dust-free galaxy.