The HETDEX Pilot Survey. III. The Low Metallicities Of High-Redshift Ly Alpha Galaxies

We present the results of Keck/NIRSPEC spectroscopic observations of three Ly alpha emitting galaxies (LAEs) at z similar to 2.3 discovered with the HETDEX pilot survey. We detect H alpha, [OIII], and H beta emission from two galaxies at z = 2.29 and 2.49, designated HPS194 and HPS256, respectively, representing the first detection of multiple rest-frame optical emission lines in galaxies at high redshift selected on the basis of their Ly alpha emission. We find that the redshifts of the Ly alpha emission from these galaxies are offset redward of the systemic redshifts (derived from the H alpha and [OIII] emission) by Delta v = 162 +/- 37 (photometric) +/- 42 (systematic) km s(-1) for HPS194 and Delta v = 36 +/- 35 +/- 18 km s-1 for HPS256. An interpretation for HPS194 is that a large-scale outflow may be occurring in its interstellar medium. This outflow is likely powered by star-formation activity, as examining emission line ratios implies that neither LAE hosts an active galactic nucleus. Using the upper limits on the [N II] emission, we place meaningful constraints on the gas-phase metallicities in these two LAEs of Z < 0.17 and < 0.28 Z(circle dot) (1 sigma). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (similar to 10(10) and 6 x 10(8) M-circle dot, respectively), we study the nature of LAEs in a mass-metallicity plane. At least one of these two LAEs appears to be more metal poor than continuum-selected star-forming galaxies at the same redshift and stellar mass, implying that objects exhibiting Ly alpha emission may be systematically less chemically enriched than the general galaxy population. We use the SEDs of these two galaxies to show that neglecting the contribution of the measured emission line fluxes when fitting stellar population models to the observed photometry can result in overestimates of the population age by orders of magnitude and the stellar mass by a factor of similar to 2. This effect is particularly important at z greater than or similar to 7, where similarly strong emission lines may masquerade in the photometry as a 4000 angstrom break.