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 +1.5 −0.5 × 10 7 K, coexisting with a warm-hot phase at 1.5±0.1×10 6 K and a warm phase at 3.0±0.4×10 5 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 super-virial 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 super-solar, 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 > 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.