Prospects for studying million-degree gas in the Milky Way halo using the forbidden optical [Fe

Context. The Milky Way is surrounded by large amounts of hot gas at temperatures of T > 106 K, which represents a major baryon reservoir. Aims. We explore the prospects of studying the hot coronal gas in Milky Way halo by analyzing the highly forbidden optical coronal lines of [Fe <ns0:sc>X</ns0:sc>] and [Fe <ns0:sc>XIV</ns0:sc>] in absorption against bright (unrelated) extragalactic background sources. Methods. We used 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 Fe <ns0:sc>X</ns0:sc> and Fe <ns0:sc>XIV</ns0:sc> column densities, as well as the line shapes and strengths for the [Fe <ns0:sc>X</ns0:sc>] λ6374.5 and [Fe <ns0:sc>XIV</ns0:sc>] λ5302.9 transitions. We provide predictions for the signal-to-noise ratio (S/N) required to detect these lines. Using archival optical data from an original sample of 739 high-resolution AGN spectra from VLT/UVES and KECK/HIRES, we generated a stacked composite spectrum to measure an upper limit for the column densities of Fe <ns0:sc>X</ns0:sc> and Fe <ns0:sc>XIV</ns0:sc> in the Milky Way’s coronal gas. Results. We predicted column densities of log N(Fe <ns0:sc>X</ns0:sc>) = 15.40 and log N(Fe <ns0:sc>XIV</ns0:sc>) = 15.23 in the Milky Way’s hot halo, corresponding to equivalent widths of WFe<ns0:sc>X</ns0:sc>, 6347 = 190 μÅ and WFe<ns0:sc>XIV</ns0:sc>, 5302 = 220 μÅ. We estimated that a minimum S/N of ∼50 000(∼25 000) is required to detect [Fe <ns0:sc>X</ns0:sc>] λ6374.5 ([Fe <ns0:sc>XIV</ns0:sc>] λ5302.9) absorption at a 3σ level. There was no [Fe <ns0:sc>X</ns0:sc>] and [Fe <ns0:sc>XIV</ns0:sc>] detected in our composite spectrum, which achieves a maximum S/N = 1240 near 5300 Å. We derived 3σ upper column-density limits of log N(Fe <ns0:sc>X</ns0:sc>) ≤ 16.27 and log N(Fe <ns0:sc>XIV</ns0:sc>) ≤ 15.85, in line with the above-mentioned predictions. Conclusions. While [Fe <ns0:sc>X</ns0:sc>] and [Fe <ns0:sc>XIV</ns0:sc>] absorption is too weak to be detected with current optical data, we outline how upcoming extragalactic spectral surveys with millions of medium- to high-resolution optical spectra will provide the necessary sensitivity and spectral resolution to measure velocity-resolved [Fe <ns0:sc>X</ns0:sc>] and [Fe <ns0:sc>XIV</ns0:sc>] absorption in the Milky Way’s coronal gas (and beyond). This opens up a new prospective window on studies of the dominant baryonic mass component of the Milky Way taking the form of hot coronal gas via optical spectroscopy.