Detection of Low Metallicity Warm Plasma in a Galaxy Overdensity Environment at z ~ 0.2

TL;DR

This study analyzes a complex multiphase absorber at z ~ 0.2, revealing a warm, collisionally ionized plasma phase with low metallicity, associated with galaxy environment, and highlights the utility of O VI-BLA absorbers as probes of warm baryons.

Contribution

It provides detailed ionization modeling of a multiphase absorber, identifying a collisionally ionized warm plasma phase and its galactic environment, advancing understanding of baryon reservoirs.

Findings

Warm plasma at T ~ 3.2 x 10^5 K with low metallicity.

Presence of multiple ionization phases with distinct properties.

Absorber located outside the virial radius of nearby galaxies.

Abstract

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.