Prospects for studying million-degree gas in the Milky Way halo using the forbidden optical [Fe X] and [Fe XIV] intersystem lines

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Golm, Germany
² Department of Physics and Astronomy, University of Notre Dame, Notre Dame, IN 46556, USA
³ European Southern Observatory (ESO), Karl-Schwarzschildstrasse 2, 85748 Garching, Germany
⁴ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

^★ Corresponding author: prichter@astro.physik.uni-potsdam.de

Received: 2 June 2025
Accepted: 16 July 2025

Abstract

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 X] and [Fe XIV] 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 X and Fe XIV column densities, as well as the line shapes and strengths for the [Fe X] λ6374.5 and [Fe XIV] 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 X and Fe XIV in the Milky Way’s coronal gas.

Results. We predicted column densities of log N(Fe X) = 15.40 and log N(Fe XIV) = 15.23 in the Milky Way’s hot halo, corresponding to equivalent widths of W_{FeX, 6347} = 190 μÅ and W_{FeXIV, 5302} = 220 μÅ. We estimated that a minimum S/N of ∼50 000(∼25 000) is required to detect [Fe X] λ6374.5 ([Fe XIV] λ5302.9) absorption at a 3σ level. There was no [Fe X] and [Fe XIV] 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 X) ≤ 16.27 and log N(Fe XIV) ≤ 15.85, in line with the above-mentioned predictions.

Conclusions. While [Fe X] and [Fe XIV] 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 X] and [Fe XIV] 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.