Supervirial hot phase in Milky Way circumgalactic medium: further evidences

Abstract

Recent discoveries of a super-virial hot phase of the Milky Way circumgalactic medium (CGM) has launched new questions regarding the multi-phase structure of the CGM around the Galaxy. We use 1.05 Ms of archival Chandra/HETG observations to characterize highly ionized metal absorption at z=0 along the line of sight of the quasar NGC 3783. We detect two distinct temperature phases with T$_1 = 5.83^{+0.15}_{-0.07}$ K, warm-hot virial temperature, and T$_2=6.61^{+0.12}_{-0.06}$ K, hot super-virial temperature. The super-virial hot phase coexisting with the warm-hot virial phase has been detected in absorption along only two other sightlines and in one stacking analysis. There is scatter in temperature of the hot as well as warm-hot gas. Similar to previous observations, we detect super-solar abundance ratios of metals in the hot phase, with a Ne/O ratio 2σ above solar mixtures. These new detections continue the mystery of the mechanism behind the super-virial hot phase, but provide evidence that this is a true property of the CGM rather than an isolated observation. The super-virial CGM could hold the key to understanding the physical and chemical history of the Milky Way.

Detecting the Effect of Nonthermal Sources on the Warm-hot Galactic Halo

“…Based on the oxygen-only analysis, we cannot test the possibility of deviation from a single phase. However, by simultaneously studying multiple metal ions (e.g., N, Ne, and Si) in addition to oxygen, a supervirial ≈10 7 K hot phase coexisting with the ≈10 6 K warm-hot CGM has been identified toward several individual high S/N sightlines (Das et al 2019b(Das et al , 2021McClain et al 2024; highlighted in Figure 2) and stacking of many low S/N sightlines (Lara-DI et al 2023). In those scenarios, O VII comes predominantly from the warm-hot phase, but O VIII comes from both the hot and warm-hot phases in a comparable amount.…”