The Origin of the Hot Gas in the Galactic Halo: Testing Galactic Fountain Models' X-ray Emission

TL;DR

This study compares galactic fountain model predictions with XMM-Newton X-ray observations of the Milky Way's hot halo, finding significant underprediction of surface brightness and exploring possible reasons for this discrepancy.

Contribution

It evaluates the effectiveness of magnetohydrodynamical galactic fountain models in reproducing observed X-ray emission from the Milky Way's hot halo and discusses potential missing physics.

Findings

Model reproduces halo temperature but underpredicts surface brightness by two orders of magnitude.

Overionization and thermal conduction are unlikely to explain the brightness shortfall.

Charge exchange and cosmic ray effects may be significant but are not included in current models.

Abstract

We test the X-ray emission predictions of galactic fountain models against XMM-Newton measurements of the emission from the Milky Way's hot halo. These measurements are from 110 sight lines, spanning the full range of Galactic longitudes. We find that a magnetohydrodynamical simulation of a supernova-driven interstellar medium, which features a flow of hot gas from the disk to the halo, reproduces the temperature but significantly underpredicts the 0.5-2.0 keV surface brightness of the halo (by two orders of magnitude, if we compare the median predicted and observed values). This is true for versions of the model with and without an interstellar magnetic field. We consider different reasons for the discrepancy between the model predictions and the observations. We find taking into account overionization in cooled halo plasma, which could in principle boost the predicted X-ray emission, is unlikely in practice to bring the predictions in line with the observations. We also find that including thermal conduction, which would tend to increase the surface brightnesses of interfaces between hot and cold gas, would not overcome the surface brightness shortfall. However, charge exchange emission from such interfaces, not included in the current model, may be significant. The faintness of the model may also be due to the lack of cosmic ray driving, meaning that the model may underestimate the amount of material transported from the disk to halo. In addition, an extended hot halo of accreted material may be important, by supplying hot electrons that could boost the emission of the material driven out from the disk. Additional model predictions are needed to test the relative importance of these processes in explaining the observed halo emission.