Detection of Low Metallicity Warm Plasma in a Galaxy Overdensity
  Environment at z ~ 0.2
  We present results from the analysis of a multiphase O VI - broad Ly-alpha
absorber at z = 0.19236 in the HST/COS spectrum of PG 1121+422. The low and
intermediate ionization metal lines in this absorber have a single narrow
component, whereas the Ly-alpha has a possible broad component with b(HI) $\sim
71$ km/s. Ionization models favor the low and intermediate ions coming from a
$T \sim 8,500$ K, moderately dense photoionized gas with near solar
metallicities. The weak O VI requires a separate gas phase that is
collisionally ionized. The O VI coupled with BLA suggests $T \sim 3.2 \times
10^5$ K, with significantly lower metal abundance and $\sim 1.8$ orders of
magnitude higher total hydrogen column density compared to the photoionized
phase. SDSS shows 12 luminous ($> L^*$) galaxies in the $\rho \leq 5$ Mpc,
$|\Delta v| \leq 800$ km/s region surrounding the absorber, with the absorber
outside the virial bounds of the nearest galaxy. The warm phase of this
absorber is consistent with being transition temperature plasma either at the
interface regions between the hot intragroup gas and cooler photoionized clouds
within the group, or associated with high velocity gas in the halo of a
$\lesssim L^*$ galaxy. The absorber highlights the advantage of O VI-BLA
absorbers as ionization model independent probes of warm baryon reserves.
