The Hot Circumgalactic Medium of the Milky Way: Evidence for Supervirial, Virial, and Subvirial Temperatures; Nonsolar Chemical Composition; and Nonthermal Line Broadening

For the first time, we present the simultaneous detection and characterization of three distinct phases at >10⁵ 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 $ 10⁷ K coexisting with a warm-hot phase at 1.5 ± 0.1 × 10⁶ K and a warm phase at 3.0 ± 0.4 × 10⁵ K. The warm-hot phase is at the virial temperature of the Galaxy, and the warm phase may have cooled from the warm-hot phase, but the supervirial hot phase remains a mystery. We find that [C/O] in the warm and warm-hot phases, [Mg/O] in the warm-hot phase, and [Ne/O] in the hot phase are supersolar, and the hot and the warm-hot phases are α-enhanced. Nonthermal line broadening is evident in the warm-hot and the hot phases, and it dominates the total line broadening. Our results indicate that the >10⁵ 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.