A Pair of O VI and Broad Ly-alpha Absorbers Probing Warm Gas in a Galaxy Group Environment at z ~ 0.4

Abstract: 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.