The HETDEX Pilot Survey. V. The Physical Origin Of Ly Alpha Emitters Probed By Near-Infrared Spectroscopy

We present the results from a Very Large Telescope/SINFONI and Keck/NIRSPEC near-infrared spectroscopic survey of 16 Ly alpha emitters (LAEs) at z = 2.1-2.5 in the COSMOS and GOODS-N fields discovered from the Hobby Eberly Telescope Dark Energy Experiment Pilot Survey. We detect rest-frame optical nebular lines (H alpha and/or [O III] lambda 5007) for 10 of the LAEs and measure physical properties, including the star formation rate (SFR), gas-phase metallicity, gas mass fraction, and Ly alpha velocity offset. We find that LAEs may lie below the mass-metallicity relation for continuum-selected star-forming galaxies at the same redshift. The LAEs all show velocity shifts of Ly alpha relative to the systemic redshift ranging between +85 and +296 km s(-1) with a mean of +180 km s(-1). This value is smaller than measured for continuum-selected star-forming galaxies at similar redshifts. The Ly alpha velocity offsets show a moderate correlation with the measured SFR (2.5 sigma), but no significant correlations are seen with the SFR surface density, specific SFR, stellar mass, or dynamical mass (less than or similar to 1.5 sigma). Exploring the role of dust, kinematics of the interstellar medium (ISM), and geometry on the escape of Ly alpha photons, we find no signature of selective quenching of resonantly scattered Ly alpha photons. However, we also find no evidence that a clumpy ISM is enhancing the Ly alpha equivalent width. Our results suggest that the low metallicity in LAEs may be responsible for yielding an environment with a low neutral hydrogen column density and less dust, easing the escape of Ly alpha photons over that in continuum-selected star-forming galaxies.