A pair of O VI and broad Ly α absorbers probing warm gas in a galaxy group environment at z ∼ 0.4

Abstract

We report the detection of two O VI absorbers at z = 0.416 14 and 0.419 50 (|Δv| = 710 km s^-1), towards SBS 0957+599. Both absorbers are multiphase systems tracing substantial reservoirs of warm baryons. The low- and intermediate-ionization metals in the z = 0.416 14 absorber are consistent with an origin in photoionized gas. O VI has a velocity structure different from other metal species. Ly α shows the presence of a broad feature. The linewidths for O VI and the broad Ly α suggest T = 7.1 × 10⁵ 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 a temperature of 4.6 × 10⁴ K supports O VI originating in a low-density photoionized gas. A broad component is seen in Ly α, offset from O VI. The temperature in the broad Ly α is T ≲ 2.1 × 10⁵ K. The absorbers reside in a galaxy overdensity region with seven spectroscopically identified galaxies within ∼10 Mpc and Δv ∼ 1000 km s^-1 of the z = 0.416 14 absorber, and two galaxies inside a similar separation from the z = 0.419 50 absorber. The distribution of galaxies relative to the absorbers suggests 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 red-shift O VI systems and the galactic environments they inhabit.