First results from the Large-Area Lyman Alpha survey

We report on a new survey for z approximate to 4.5 Ly alpha sources, the Large-Area Lyman Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume and sensitivity by using narrowband filters on the new 8192(2) pixel CCD Mosaic camera at the 4 m Mayall telescope of Kitt Peak National Observatory. Well-detected sources with flux and equivalent width matching previously known high-redshift Ly alpha galaxies (i.e., observed equivalent width EW > 80 Angstrom; 2.6 x 10(-17) ergs cm(-2) s(-1) < line + continuum flux < 5.2 x 10(-17) ergs cm(-2) s(-1), and a small uncertainty on the equivalent width, delta EW < EW/4) have an observed surface density corresponding to 11,000 +/- 700 deg(-2) per unit redshift at z = 4.5. Variations in this surface density are apparent on comparison between counts in 6561 +/- 40 and 6730 +/- 40 filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Ly alpha candidates. Of these, one is confirmed as a z = 4.52 source, another remains consistent with either z = 4.55 or z = 0.81, and the third is an [O III] lambda 5007 emitter at z = 0.34. These pilot spectroscopic results suggest that approximately one-third of our good candidates are bona fide Ly alpha emitters, implying a net density of similar to 4000 Ly alpha emitters per square degree per unit redshift.