The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations

TL;DR

This study compares models of hot gas in the galactic halo with XMM-Newton X-ray observations, finding that a galactic fountain driven by supernovae likely explains most of the observed emission.

Contribution

It provides observational constraints that favor a supernova-driven galactic fountain model over isolated supernova remnants for the origin of hot halo gas.

Findings

Observed halo temperature is fairly constant (~1.8-2.3 million K).

Supernova-driven fountain model matches observed surface brightness well.

Isolated supernova remnant model underpredicts the emission by an order of magnitude.

Abstract

We compare the predictions of three physical models for the origin of the hot halo gas with the observed halo X-ray emission, derived from 26 high-latitude XMM-Newton observations of the soft X-ray background between $l=120\degr$ and $l=240\degr$. These observations were chosen from a much larger set of observations as they are expected to be the least contaminated by solar wind charge exchange emission. We characterize the halo emission in the XMM-Newton band with a single-temperature plasma model. We find that the observed halo temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc). When we compare our observations with the model predictions, we find that most of the hot gas observed with XMM-Newton does not reside in isolated extraplanar supernova remnants -- this model predicts emission an order of magnitude too faint. A model of a supernova-driven interstellar medium, including the flow of hot gas from the disk into the halo in a galactic fountain, gives good agreement with the observed 0.4-2.0 keV surface brightness. This model overpredicts the halo X-ray temperature by a factor of ~2, but there are a several possible explanations for this discrepancy. We therefore conclude that a major (possibly dominant) contributor to the halo X-ray emission observed with XMM-Newton is a fountain of hot gas driven into the halo by disk supernovae. However, we cannot rule out the possibility that the extended hot halo of accreted material predicted by disk galaxy formation models also contributes to the emission.