Metal-enriched Galaxies in the First ~1 Billion Years: Evidence of a
  Smooth Metallicity Evolution at z ~ 5
  We present seven new abundance measurements of the elements O, C and Si at z
> 4.5, doubling the existing sample of weakly depleted elements in gas-rich
galaxies, in order to constrain the first ~1 billion years of cosmic metal
evolution. These measurements are based on quasar spectra of damped Lyman-alpha
absorbers (DLAs) and sub-DLAs obtained with the Magellan Inamori Kyocera
Echelle (MIKE) and Magellan Echellette (MagE) spectrographs on Magellan-South,
and the X-Shooter spectrograph on the Very Large Telescope. We combine these
new measurements with those drawn from the literature to estimate the
NHI-weighted binned mean metallicity of -1.51 +\- 0.18 at z = 4.8. This
metallicity value is in excellent agreement with the prediction from lower
redshift DLAs, supporting the interpretation that the metallicity evolution is
smooth at z ~ 5, rather than showing a sudden decline at z > 4.7. Furthermore,
the metallicity evolution trends for the DLAs and sub-DLAs are similar within
our uncertainties. We also find that the [C/O] ratios for z ~ 5 DLAs are
consistent with those of the very metal-poor DLAs. Additionally, using [C/O]
and [Si/O] to constrain the nucleosynthesis models, we estimate that the
probability distributions of the progenitor star masses for three relatively
metal-poor DLAs are centered around 12 M_{\odot} to 17 M_{\odot}. Finally, the
z ~ 5 absorbers show a different metallicity-velocity dispersion relation than
lower redshift DLAs, suggesting that they may be tracing a different population
of galaxies.
