The Physical Nature of Lyα-emitting Galaxies at z  = 3.1

We selected 40 candidate Lyα-emitting galaxies (LAEs) at z ≃ 3.1 with observed-frame equivalent widths greater than 150 Å and inferred emission-line fluxes above 2.5 × 10 -17 ergs cm -2 s -1 from deep narrowband and broadband MUSYC images of the Extended Chandra Deep Field South. Covering 992 arcmin 2 , this is the largest "blank field" surveyed for LAEs at z ~ 3, allowing an improved estimate of the space density of this population of (3 ± 1) × 10 -4 h Mpc -3 . Spectroscopic follow-up of 23 candidates yielded 18 redshifts, all at z ≃ 3.1. Over 80% of the LAEs are dimmer in continuum magnitude than the typical Lyman break galaxy (LBG) spectroscopic limit of R = 25.5 (AB), with a median continuum magnitude R ≃ 27 and very blue continuum colors, V - z ≃ 0. Over 80% of the LAEs have the right UVR colors to be selected as LBGs, but only 10% also have R ≤ 25.5. Stacking the UBVRI z JK fluxes reveals that LAEs have stellar masses ≃5 × 10 8 h M ☉ and minimal dust extinction, A V ≲ 0.1. Inferred star formation rates are ≃6 h M ☉ yr -1 , yielding a cosmic star formation rate density of 2 × 10 -3 h 70 M ☉ yr -1 Mpc -3 . None of our LAE candidates show evidence for rest-frame emission-line equivalent widths EW rest > 240 Å that might imply a nonstandard initial mass function. One candidate is detected by Chandra , implying an AGN fraction of 2% ± 2% for LAE candidate samples. In summary, LAEs at z ~ 3 have rapid star formation, low stellar mass, little dust obscuration, and no evidence for a substantial AGN component.