A Spectroscopic Survey of Lyα Emitters and Lyα Luminosity Function at Redshifts 3.7 and 4.8

We present a spectroscopic survey of Ly alpha emitters (LAEs) at z similar to 3.7 and z similar to 4.8. The LAEs are selected using the narrowband technique based on the combination of deep narrowband and broadband imaging data in two deep fields, and then spectroscopically confirmed with the MMT multifiber spectrograph Hectospec. The sample consists of 71 LAEs at z similar to 3.7 and 69 LAEs at z similar to 4.8 over similar to 1.5 deg(2), making it one of the largest spectroscopically confirmed samples of LAEs at the two redshifts. Their Ly alpha luminosities are measured using the secure redshifts and deep photometric data, and span a range of similar to 10(42.5)-10(43.6) erg s(-1), so these LAEs represent the most luminous galaxies at the redshifts in terms of Ly alpha luminosity. We estimate and correct sample incompletenesses and derive reliable Ly alpha luminosity function (LFs) at z similar to 3.7 and 4.8 based on the two spectroscopic samples. We find that our Ly alpha LFs are roughly consistent (within a factor of 2-3) with previous measurements at similar redshifts that were derived from either photometric samples or spectroscopic samples. By comparing with previous studies in different redshifts, we find that the Ly alpha LFs decrease mildly from z similar to 3.1 to z similar to 5.7, supporting the previous claim of the slow LF evolution between z similar to 2 and z similar to 6. At z > 5.7, the LF declines rapidly toward higher redshift, partly due to the effect of cosmic reionization.