Ezequiel Treister

The Physical Nature of Lyman Alpha 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 A 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 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 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 UBVRIzJK fluxes reveals that LAEs have stellar masses 5 × 108 h M☉ and minimal dust extinction, AV 0.1. Inferred star formation rates are 6 h M☉ yr-1, yielding a cosmic star formation rate density of 2 × 10-3 h70 M☉ yr-1 Mpc-3. None of our LAE candidates show evidence for rest-frame emission-line equivalent widths EWrest > 240 A 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.