Discovery of extremely low-metallicity circumgalactic gas at $z = 0.5$
  toward Q0454-220
  We have obtained new observations of the absorption system at
$z_\mathrm{abs}=0.48$ toward QSO Q0454-220, which we use to constrain its
chemical and physical conditions. The system features metal-enriched gas and
previously unknown low-metallicity gas detected $\sim 200 \, \mathrm{km \,
s^{-1}}$ blueward of the metal-enriched gas. The low-metallicity gas is
detected in multiple Lyman series lines but is not detected in any metal lines.
Our analysis includes low-ionization (e.g., Fe II, Mg II) metal lines,
high-ionization (e.g., C IV, O VI, N V) metal lines, and several Lyman series
lines. We use new UV spectra taken with HST/COS along with data taken from
HST/STIS, Keck/HIRES, and VLT/UVES. We find that the absorption system can be
explained with a photoionized low-ionization phase with $\mathrm{[Fe/H]} \sim
-0.5$ and $n_\mathrm{H} \sim 10^{-2.3} \, \mathrm{cm}^{-3}$, a photoionized
high-ionization phase with a conservative lower limit of $-3.3 <
\mathrm{[Fe/H]}$ and $n_\mathrm{H} \sim 10^{-3.8} \, \mathrm{cm}^{-3}$, and a
low-metallicity component with a conservative upper limit of $\mathrm{[Fe/H]} <
-2.5$ that may be photoionized or collisionally ionized. We suggest that the
low-ionization phase may be due to cold-flow accretion via large-scale
filamentary structure or due to recycled accretion while the high-ionization
phase is the result of ancient outflowing material from a nearby galaxy. The
low-metallicity component may come from pristine accretion. The velocity spread
and disparate conditions among the absorption system's components suggest a
combination of gas arising near galaxies along with gas arising from intergroup
material.
