Physical Conditions in a Young, Unreddened, Low Metallicity Galaxy at
  High Redshift
  Increasingly large samples of galaxies are now being discovered at redshifts
z~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_star ~ 10^9 Msun),
low in metallicity (Z ~ 1/6 Zsun), and unreddened (E(B-V)~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 O III] 1661, 1666/[O III]
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 contains strong
emission from Lya (with rest-frame equivalent width 54 A), He II 1640 (both
stellar and nebular), C III] 1907, 1909 and O III] 1661, 1666. The C IV/C III]
ratio indicates that the source of ionization is unlikely to be an AGN.
Analysis of the He II, O III] and C III] line strengths indicates a very high
ionization parameter log U ~ -1, while Lya 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.
