A Pair of O VI and Broad Ly-alpha Absorbers Probing Warm Gas in a Galaxy
  Group Environment at z ~ 0.4
  We report on the detection of two O VI absorbers separated in velocity by 710
km/s at z ~ 0.4 towards the background quasar SBS0957+599. Both absorbers are
multiphase systems tracing substantial reservoirs of warm baryons. The low and
intermediate ionization metals in the first absorber is consistent with an
origin in photoionized gas. The O VI has a velocity structure different from
other metal species. The Ly-alpha shows the presence of a broad feature. The
line widths for O VI and the broad Ly-alpha suggest T = 7.1 x 10^5 K. This warm
medium is probing a baryonic column which is an order of magnitude more than
the total hydrogen in the cooler photoionized gas. The second absorber is
detected only in H I and O VI. Here the temperature of 4.6 x 10^4 K supports O
VI originating in a low-density photoionized gas. A broad component is seen in
the Ly-alpha, offset from the O VI. The temperature in the broad Ly-alpha is T
< 2.1 x 10^5 K. The absorbers reside in a galaxy overdensity region with 7
spectroscopically identified galaxies within ~ 10 Mpc and delta_v ~ 1000 km/s
of the first absorber, and 2 galaxies inside a similar separation from the
second absorber. The distribution of galaxies relative to the absorbers suggest
that the line of sight could be intercepting a large-scale filament connecting
galaxy groups, or the extended halo of a sub-L* galaxy. Though kinematically
proximate, the two absorbers reaffirm the diversity in the physical conditions
of low redshift O VI systems and the galactic environments they inhabit.
