Probing the structure of the gas in the Milky Way through X-ray high resolution spectroscopy

TL;DR

This study introduces a new X-ray absorption model, IONeq, to analyze the Milky Way's interstellar medium, revealing the distribution and mass fractions of its neutral, warm, and hot phases through high-resolution spectroscopy.

Contribution

The paper presents the novel IONeq model for simultaneous ion optical depth calculations and applies it to characterize the ISM's multi-phase structure in the Milky Way.

Findings

Neutral component accounts for ~89% of the ISM mass in the disk.

Hot component has comparable disk and halo column densities (~6-7×10^{18} cm^{-2}).

Spatial distribution of neutral and warm phases is highly nonuniform.

Abstract

We have developed a new X-ray absorption model, called {\tt IONeq}, which computes the optical depth $\tau(E)$ simultaneously for ions of all abundant elements, assuming ionization equilibrium and taking into account turbulent broadening. We use this model to analyze the interstellar medium (ISM) absorption features in the Milky Way for a sample of 18 galactic (LMXBs) and 42 extragalactic sources (mainly Blazars). The absorbing ISM was modeled as a combination of three components/phases - neutral ($T\lesssim1\times10^{4}$ K), warm ($T\sim5\times 10^{4}$ K) and hot ($T\sim2\times10^{6}$ K). We found that the spatial distribution of both, neutral and warm components, are difficult to describe using smooth profiles due to nonuniform distribution of the column densities over the sky. For the hot phase we used a combination of a flattened disk and a halo, finding comparable column densities for both spatial components, in the order of $\sim 6-7\times10^{18}\;{\rm cm^{-2}}$, although this conclusion depends on the adopted parametrization. If the halo component has sub-solar abundance $Z$, then the column density has be scaled up by a factor $\frac{Z_\odot}{Z}$. The vertically integrated column densities of the disk components suggests the following mass fractions for these three ISM phases in the Galactic disk: neutral $\sim~89\%$, warm $\sim 8\%$ and hot $\sim 3\%$ components, respectively. The constraints on the radial distribution of the halo component of the hot component are weak.