On the origin of the warm-hot absorbers in the Milky Way's halo

TL;DR

This study models the origin of warm-hot absorbers in the Milky Way's halo as cooling gas from the galactic corona, driven by supernova feedback, matching observed absorption features and suggesting ongoing accretion of fresh gas.

Contribution

It combines hydrodynamical simulations with a 3D galactic fountain model to explain the distribution and origin of warm-hot absorbers in the Milky Way's halo, a novel integrated approach.

Findings

Model reproduces position-velocity distribution of warm absorbers.

Approximately half of OVI absorbers are explained by the model.

Warm-hot gas within a few kiloparsecs from the Galactic plane accounts for most detections.

Abstract

Disc galaxies like the Milky Way are expected to be surrounded by massive coronae of hot plasma that may contain a significant fraction of the so-called missing baryons. We investigate whether the local (|vLSR|<400 km/s) warm-hot absorption features observed towards extra-Galactic sources or halo stars are consistent with being produced by the cooling of the Milky Way's corona. In our scheme, cooling occurs at the interface between the disc and the corona and it is triggered by positive supernova feedback. We combine hydrodynamical simulations with a dynamical 3D model of the galactic fountain to predict the all-sky distribution of this cooling material, and we compare it with the observed distribution of detections for different `warm' (SiIII, SiIV, CII, CIV) and `hot' (OVI) ionised species. The model reproduces the position-velocity distribution and the column densities of the vast majority of warm absorbers and about half of OVI absorbers. We conclude that the warm-hot gas responsible for most of the detections lies within a few kiloparsecs from the Galactic plane, where high-metallicity material from the disc mixes efficiently with the hot corona. This process provides an accretion of a few Mo/yr of fresh gas that can easily feed the star formation in the disc of the Galaxy. The remaining OVI detections are likely to be a different population of absorbers, located in the outskirts of the Galactic corona and/or in the circumgalactic medium of nearby galaxies.