A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the
  Sloan Digital Sky Survey
  We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for
low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is
detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest
absorption. We have developed a novel Locally Calibrated Pixel (LCP) search
method that uses adjacent regions of the spectrum to calibrate interlopers and
spectral artifacts, which would otherwise limit the measurement of OVI
absorption. Despite the challenges presented by searching for weak OVI within
the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,
we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2
(the null hypothesis has a chi^2=80 for 9 data points).
  We interpret our results using synthetic spectra generated from a lognormal
density field assuming a mixed quasar-galaxy photoionizing background (Haardt &
Madau 2001) and that it dominates the ionization fraction of detected OVI. The
LCP search data can be fit by a constant metallicity model with [O/H] =
-2.15_(-0.09)^(+0.07), but also by models in which low-density regions are
unenriched and higher density regions have a higher metallicity. The
density-dependent enrichment model by Aguirre et al. (2008) is also an
acceptable fit. All our successful models have similar mass-weighted oxygen
abundance, corresponding to [<O/H>_MW] = -2.45+-0.06. This result can be used
to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =
1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the
mass-weighted mean oxygen abundance and the cosmic oxygen density in the
Lyman-alpha forest to date and indicates that it contains approximately 16% of
metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.
