Widespread Detection of Two Components in the Hot Circumgalactic Medium of the Milky Way

TL;DR

This study uses HaloSat data to reveal that the Milky Way's circumgalactic medium contains at least two distinct hot gas components at different temperatures, indicating a complex, clumpy structure.

Contribution

It extends previous HaloSat analysis to include the northern CGM and demonstrates the necessity of two hot gas components to explain the observed X-ray emission.

Findings

Detection of two hot gas components at different temperatures.

Evidence of a clumpy and patchy CGM structure.

Better fit with a two-component model than a neon-enhanced single-temperature model.

Abstract

Surrounding the Milky Way (MW) is the circumgalactic medium (CGM), an extended reservoir of hot gas that has significant implications for the evolution of the MW. We used the HaloSat all-sky survey to study the CGM's soft X-ray emission in order to better define its distribution and structure. We extend a previous HaloSat study of the southern CGM (Galactic latitude b < -30 deg) to include the northern CGM (b > 30 deg) and find evidence that at least two hot gas model components at different temperatures are required to produce the observed emission. The cooler component has a typical temperature of kT ~ 0.18 keV, while the hotter component has a typical temperature of kT ~ 0.7 keV. The emission measure in both the warm and hot components has a wide range (~ 0.005 - 0.03, ~ 0.0005 - 0.004 cm-6 pc respectively), indicating that the CGM is clumpy. A patch of relatively consistent CGM was found in the north, allowing for the CGM spectrum to be studied in finer detail using a stacked spectrum. The stacked spectrum is well described with a model including two hot gas components at temperatures of kT = 0.166 +/- 0.005 keV and kT = 0.69 +0.04 -0.05 keV. As an alternative to adding a hot component, a neon-enhanced single-temperature model of the CGM was also tested and found to have worse fit statistics and poor residuals.