Detection of 'super-virial' gas in the Circumgalactic medium of the Milky Way towards PKS 2155-304

Abstract

We present the first simultaneous detection of four distinct highly ionized $z=0$ absorbing phases using Chandra and XMM-Newton grating spectra toward the blazar PKS 2155$-$304. We detect the MgXII K$\hat{I}\pm$ absorption line for the first time in the circumgalactic medium (CGM) of the Milky Way. Along with MgXII K$\hat{I}\pm$, we detect SiXIV K$\hat{I}\pm$ absorption, which are the tell-tale signatures of the hot 'super-virial' gas in the CGM. Both from the model-independent calculations and hybrid-ionization modeling, we infer four phases at distinct temperatures, hot 'super-virial' ($5.4_{-0.8}^{+1.9}\times 10^7$ K), warm-hot 'virial' ($1.8_{-0.2}^{+0.3}\times 10^6$ K), warm 'sub-virial' ($2.2\pm 0.5\times 10^5$ K), and cool phase ($<1.7\times 10^5$ K). The warm-hot and hot phases are $\hat{I}\pm$-enhanced, and [C/O] and [Ne/O] are super-solar in the warm-hot phase, while [Mg/O] and [Si/O] are super-solar in the hot phase. The low-ionization lines are blue-shifted (v${}_{\mathrm{los}}\approx -100$ km s${}^{-1}$), whereas the high-ionization lines are red-shifted. It suggests a scenario of infalling sub-virial, quasi-static virial, and outflowing super-virial phases along this sightline. Earlier studies on individual sightlines were confined to the Northern Hemisphere. Our sightline is located in the Southern hemisphere, demonstrating that hot super-virial gas is also present at Southern Galactic latitudes as well. This confirms a more widespread distribution of the super-virial gas across both hemispheres.

15 pages, Submitted to ApJ