First results from the Large-Area Lyman Alpha survey

We report on a new survey for z ≈ 4.5 Lyα 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 Lyα galaxies (i.e., observed equivalent width EW > 80 Å; 2.6 × 10^-17 ergs cm^-2 s^-1 < line + continuum flux < 5.2 × 10^-17 ergs cm^-2 s^-1, and a small uncertainty on the equivalent width, δEW < 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 Å filters. Early spectroscopic followup results from the Keck telescope included three sources meeting our criteria for good Lyα 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] λ5007 emitter at z = 0.34. These pilot spectroscopic results suggest that approximately one-third of our good candidates are bona fide Lyα emitters, implying a net density of ∼4000 Lyα emitters per square degree per unit redshift.