THE EVOLUTION OF Lyα-EMITTING GALAXIES BETWEEN z=2.1 AND z=3.1

We describe the results of a new, wide-field survey for z = 3.1 Ly alpha emitters (LAEs) in the Extended Chandra Deep Field South (ECDF-S). By using a nearly top-hat 5010 angstrom filter and complementary broadband photometry from the MUSYC survey, we identify a complete sample of 141 objects with monochromatic fluxes brighter than 2.4 x 10(-17) erg cm(-2) s(-1) and observers-frame equivalent widths (EWs) greater than similar to 80 angstrom (i.e., 20 angstrom in the rest frame of Ly alpha). The bright end of this data set is dominated by X-ray sources and foreground objects with Galaxy Evolution Explorer detections, but when these interlopers are removed, we are still left with a sample of 130 LAE candidates, 39 of which have spectroscopic confirmations. This sample overlaps the set of objects found in an earlier ECDF-S survey, but due to our filter's redder bandpass, it also includes 68 previously uncataloged sources. We confirm earlier measurements of the z = 3.1 LAE emission-line luminosity function and show that an apparent anticorrelation between EW and continuum brightness is likely due to the effect of correlated errors in our heteroskedastic data set. Finally, we compare the properties of z = 3.1 LAEs to LAEs found at z = 2.1. We show that in the similar to 1 Gyr after z similar to 3, the LAE luminosity function evolved significantly, with L* fading by similar to 0.4 mag, the number density of sources with L > 1.5 x 10(42) erg s(-1) declining by similar to 50%, and the EW scale length contracting from 70(-5)(+7) angstrom to 50(-6)(+9) angstrom. When combined with literature results, our observations demonstrate that over the redshift range z similar to 0 to z similar to 4, LAEs contain less than similar to 10% of the star formation rate density of the universe.