Prospects for studying million-degree gas in the Milky Way halo using the forbidden optical [FeX] and [FeXIV] intersystem lines
The Milky Way is surrounded by large amounts of hot gas at temperatures T>10^6 K, which represents a major baryon reservoir. We here explore the prospects of studying the hot coronal gas in Milky Way halo by analyzing the highly forbidden optical coronal lines of [FeX] and [FeXIV] in absorption against bright extragalactic background sources. We use a semi-analytic model of the Milky Way's coronal gas distribution together wih HESTIA simulations of the Local Group and observational constraints to predict the expected FeX and FeXIV column densities as well as the line shapes and strengths. We predict column densities of log N(FeX)=15.40 and log N(FeXIV)=15.23 in the Milky Way's hot halo and estimate that a minimum S/N of 50,000 (25,000) is required to detect [FeX] l6374.5 ([FeXIV] l5302.9) absorption at a 3sigma level. Using archical optical data from an original sample of 739 high resolution AGN spectra from VLT/UVES and KECK/HIRES, we generate a stacked composite spectrum to measure an upper limit for the column densities of FeX and FeXIV in the Milky Way's coronal gas. No [FeX] and [FeXIV] is detected in our composite spectrum, which achieves a maximum S/N= 1,240 near 5300 A. We derive 3sigma upper column-density limits of log N(FeX)<16.27 and log N(FeXIV)<15.85, in line with the above-mentioned predictions. While [FeX] and [FexIX] absorption is too weak to be detected with current optical data, we outline how up-coming extragalactic spectral surveys with millions of medium- to high-resolution optical spectra will provide the necessary sensitivity and spectral resolution to measure velocity-resolved [FeX] and [FeXIV] absorption in the Milky Way's coronal gas (and beyond). This gives the prospect of opening a new window for studying the dominant baryonic mass component of the Milky Way in the form of hot coronal gas via optical spectroscopy.