PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT

Increasingly large samples of galaxies are now being discovered at redshifts z similar to 5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (< 100 Myr), low mass (M-* similar to 10(9) M-circle dot), low in metallicity (Z similar to 1/6 Z(circle dot)), and unreddened (E(B - V) similar or equal to 0.02, UV continuum slope beta = -2.1). We infer a metallicity 12 + log(O/H) = 7.9 +/- 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio OIII] lambda lambda 1661, 1666/[O III] lambda 5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 +/- 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents similar to 12 hr of integration with the Keck telescope, contains strong emission from Ly alpha (with rest-frame equivalent width 54 angstrom), He II lambda 1640 (both stellar and nebular), C III] lambda lambda 1907, 1909 and O III] lambda lambda 1661, 1666. The C IV/C III] ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He II, O III], and C III] line strengths indicates a very high ionization parameter log U similar to -1, while Ly alpha and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.