The HETDEX Pilot Survey V: The Physical Origin of Lyman-alpha Emitters Probed by Near-infrared Spectroscopy

摘要

We present the results from a VLT/SINFONI and Keck/NIRSPEC near-infraredspectroscopic survey of 16 Lyman-alpha emitters (LAEs) at $z$ = 2.1 - 2.5 inthe COSMOS and GOODS-N fields discovered from the HETDEX Pilot Survey. Wedetect rest-frame optical nebular lines (H$\alpha$ and/or [OIII]$\lambda$5007)for 10 of the LAEs and measure physical properties, including the starformation rate (SFR), gas-phase metallicity, gas-mass fraction, and Ly$\alpha$velocity offset. We find that LAEs may lie below the mass-metallicity relationfor continuum-selected star-forming galaxies at the same redshift. The LAEs allshow velocity shifts of Ly$\alpha$ relative to the systemic redshift rangingbetween +85 and +296 km s$^{-1}$ with a mean of +180 km s$^{-1}$. This value issmaller than measured for continuum-selected star-forming galaxies at similarredshifts. The Ly$\alpha$ velocity offsets show a moderate correlation with themeasured star formation rate (2.5$\sigma$), but no significant correlations areseen with the SFR surface density, specific SFR, stellar mass, or dynamicalmass ($\lesssim$ 1.5$\sigma$). Exploring the role of dust, kinematics of theinterstellar medium (ISM), and geometry on the escape of Ly$\alpha$ photons, wefind no signature of selective quenching of resonantly scattered Ly$\alpha$photons. However, we also find no evidence that a clumpy ISM is enhancing theLy$\alpha$ equivalent width. Our results suggest that the low metallicity inLAEs may be responsible for yielding an environment with a low neutral hydrogencolumn density as well as less dust, easing the escape of Ly$\alpha$ photonsover that in continuum-selected star-forming galaxies.