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

We selected 40 candidate Lyman Alpha Emitting galaxies (LAEs) at z ~=3.1 withobserved frame equivalent widths >150A and inferred emission line fluxes>2.5x10^-17 ergs/cm^2/s from deep narrow-band and broad-band MUSYC images ofthe Extended Chandra Deep Field South. Covering 992 sq. arcmin, this is thelargest ``blank field'' surveyed for LAEs at z ~3, allowing an improvedestimate of the space density of this population of 3+-1x10^-4 h_70^3/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 Lymanbreak galaxy spectroscopic limit of R= 25.5 (AB), with a median continuummagnitude R ~=27 and very blue continuum colors, (V-z) ~=0. Over 80% of theLAEs have the right UVR colors to be selected as Lyman break galaxies, but only10% also have R<=25.5. Stacking the UBVRIzJK fluxes reveals that LAEs havestellar masses ~=5x10^8 h_70^-2 M_sun and minimal dust extinction, A_V < ~ 0.1.Inferred star formation rates are ~=6 h_70^-2 M_sun/yr, yielding a cosmic starformation rate density of 2x10^-3 h_70 M_sun/yr/Mpc^3. None of our LAEcandidates show evidence for rest-frame emission line equivalent widthsEW_rest>240A which might imply a non-standard IMF. One candidate is detected byChandra, implying an AGN fraction of 2+-2% for LAE candidate samples. Insummary, LAEs at z ~ 3 have rapid star formation, low stellar mass, little dustobscuration and no evidence for a substantial AGN component.Comment: ApJ Letters, in press, very minor revisions to match accepted version, 4 pages with 2 color figure