Detection of low-metallicity warm plasma in a galaxy overdensity environment at z ∼ 0.2

Abstract

We present results from the analysis of a multiphase O VI-broad Ly α (BLA) absorber at z = 0.19236 in the HubbleSpaceTelescope/Cosmic Origins Spectrograph spectrum of PG 1121 + 422. The low and intermediate ionization metal lines in this absorber have a single narrow component, whereas the Ly α has a possible broad component with b({H {I}}) ∼ 71 km s^-1. Ionization models favour the low and intermediate ions coming from a T ∼ 8500 K, moderately dense (n _H ∼ 10 ^{- 3} cm^-3) 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 ∼ 3.2 × 10⁵ K, with significantly lower metal abundance and ∼1.8 orders of magnitude higher total hydrogen column density compared to the photoionized phase. Sloan Digitial Sky Survey (SDSS) shows 12 luminous (>L^*) galaxies in the ρ ≤ 5 Mpc, |Δv| ≤ 800 km s^-1 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 ≲L^* galaxy. The absorber highlights the advantage of O VI-BLA absorbers as ionization model independent probes of warm baryon reserves.