X-Ray Detection of Warm Ionized Matter in the Galactic Halo

TL;DR

This study uses high-resolution X-ray spectra to distinguish and characterize the warm ionized metal components in the Galactic disk and halo, revealing a pervasive halo WIMM with specific temperature, density, and metallicity.

Contribution

First clear differentiation between gaseous metal components in the Galactic disk and halo using X-ray spectroscopy, quantifying their properties and mass estimates.

Findings

Halo WIMM permeates large Galactic volume up to CGM.

Halo WIMM has temperature ~2900 K, density ~0.023 cm^-3, metallicity ~0.4 Z_Solar.

Disk contains cold neutral and warm mildly ionized metal components.

Abstract

We report on a systematic investigation of the cold and mildly ionized gaseous baryonic metal components of our Galaxy, through the analysis of high resolution Chandra and XMM-Newton spectra of two samples of Galactic and extragalactic sources. The comparison between lines of sight towards sources located in the disk of our Galaxy and extragalactic sources, allows us for the first time to clearly distinguish between gaseous metal components in the disk and halo of our Galaxy. We find that a Warm Ionized Metal Medium (WIMM) permeates a large volume above and below the Galaxy's disk, perhaps up to the Circum-Galactic space (CGM). This halo-WIMM imprints virtually the totality of the OI and OII absorption seen in the spectra of our extragalactic targets, has a temperature of T(Halo-WIMM)=2900 +/- 900 K, a density <n_H>(Halo-WIMM) = 0.023 +/- 0.009 cm-3 and a metallicity Z(Halo-WIMM) = (0.4 +/- 0.1) Z_Solar. Consistently with previous works, we also confirm that the disk of the Galaxy contains at least two distinct gaseous metal components, one cold and neutral (the CNMM: Cold Neutral Metal Medium) and one warm and mildly ionized, with the same temperature of the Halo-WIMM, but higher density (<n_H>(Disk-WIMM) = 0.09 +/- 0.03 cm-3) and metallicity (Z(Disk-WIMM) = 0.8 +/- 0.1$ Z_Solar). By adopting a simple disk+sphere geometry for the Galaxy, we estimates masses of the CNMM and the total (disk + halo) WIMM of M(CNMM) <~ 8e8 Solar masses and M(WIMM) ~ 8.2e9 Solar masses.