The hot circumgalactic medium of the Milky Way: evidence for super-virial, virial, and sub-virial temperature, non-solar chemical composition, and non-thermal line broadening

Abstract

For the first time, we present the simultaneous detection and characterization of three distinct phases at $>10^5$ K in $z=0$ absorption, using deep $Chandra$ observations toward Mrk 421. The extraordinarily high signal-to-noise ratio ($⩾60$) of the spectra has allowed us to detect a $hot$ phase of the Milky Way circumgalactic medium (CGM) at 3.2${}_{-0.5}^{+1.5}\times$ 10${}^7$ K, coexisting with a $\text{warm-hot}$ phase at 1.5$\pm$0.1$\times$10${}^6$ K and a $\text{warm}$ phase at 3.0$\pm$0.4$\times$10${}^5$ K. The $\text{warm-hot}$ phase is at the virial temperature of the Galaxy, and the $\text{warm}$ phase may have cooled from the $\text{warm-hot}$ phase, but the super-virial $\text{hot}$ phase remains a mystery. We find that [C/O] in the $\text{warm}$ and $\text{warm-hot}$ phases, [Mg/O] in the $\text{warm-hot}$ phase and [Ne/O] in the $\text{hot}$ phase are super-solar, and the $\text{hot}$ and the $\text{warm-hot}$ phases are $\alpha -$enhanced. Non-thermal line broadening is evident in the $\text{warm-hot}$ and the $\text{hot}$ phases and it dominates the total line broadening. Our results indicate that the $>10^5$ K CGM is a complex ecosystem. It provides insights on the thermal and chemical history of the Milky Way CGM, and theories of galaxy evolution.

18 pages, 2 tables, 5 figures, 2 appendices (with 1 figure), accepted for publication in ApJ (main)

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