THE HETDEX PILOT SURVEY. III. THE LOW METALLICITIES OF HIGH-REDSHIFT Lyα GALAXIES

We present the results of Keck/NIRSPEC spectroscopic observations of three Lyα emitting galaxies (LAEs) at z∼ 2.3 discovered with the HETDEX pilot survey. We detect Hα, [O iii], and Hβ 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 Lyα emission. We find that the redshifts of the Lyα emission from these galaxies are offset redward of the systemic redshifts (derived from the Hα and [O iii] emission) by Δv = 162 ± 37 (photometric) ± 42 (systematic) km s−1 for HPS194 and Δv = 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☉ (1σ). Measuring the stellar masses of these objects via spectral energy distribution (SED) fitting (∼1010 and 6 × 108 M☉, 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 Lyα 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 ∼2. This effect is particularly important at z≳ 7, where similarly strong emission lines may masquerade in the photometry as a 4000 Å break.

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