THE PHYSICAL NATURE OF Lya-EMITTING GALAXIES AT zp 3.1

Abstract

We selected 40 candidate Lya-emitting galaxies (LAEs) at z 3.1 with observed-frame equivalent widths greater than 150 and inferred emission-line fluxes above 2.5#10 17 ergs cm 2 s 1 from deep narrowband and °A broadband MUSYC images of the Extended Chandra Deep Field South. Covering 992 arcmin2, 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 h3 Mpc 3. Spectroscopic follow-up of 23 candidates yielded 18 redshifts, all at z 3.1. Over 70 80% of the LAEs are dimmer in continuum magnitude than the typical Lyman break galaxy (LBG) spectroscopic limit of Rp25.5 (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 UBVRIzJK fluxes reveals that LAEs have stellar masses 5#108 M, h 2 andminimal dust extinction, 70 AV 0.1. Inferred star formation rates are 6 M, h 2 yr 1, yielding a cosmic star formation rate density of 70 2#10 3 M, h yr 1 Mpc 3. None of our LAE candidates show evidence for rest-frame emission-line equivalent 70 widths EWrest 1 240 that might imply a nonstandard initial mass function. One candidate is detected by Chandra, °A 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.