Hot Gas in the Galactic Thick Disk and Halo Near the Draco Cloud

TL;DR

This study analyzes ultraviolet and X-ray emissions from hot gas in the Galactic thick disk and halo near the Draco Cloud, revealing higher-than-expected pressure and complex heating processes involving shocks, conduction, and cooling.

Contribution

It provides new measurements of ion intensities and pressures in the Galactic halo, and compares these with models to suggest multiple heating mechanisms are involved.

Findings

Measured high thermal pressure in the thick disk/halo gas.

Identified discrepancies between observed ion ratios and simple heating models.

Suggested multiple heating processes are necessary to explain observations.

Abstract

This paper examines the ultraviolet and X-ray photons generated by hot gas in the Galactic thick disk or halo in the Draco region of the northern hemisphere. Our analysis uses the intensities from four ions, C IV, O VI, O VII, and O VIII, sampling temperatures of ~100,000 to ~3,000,000 K. We measured the O VI, O VII and O VIII intensities from FUSE and XMM-Newton data and subtracted off the local contributions in order to deduce the thick disk/halo contributions. These were supplemented with published C IV intensity and O VI column density measurements. Our estimate of the thermal pressure in the O VI-rich thick disk/halo gas, p_{th}/k = 6500^{+2500}_{-2600} K cm^{-3}, suggests that the thick disk/halo is more highly pressurized than would be expected from theoretical analyses. The ratios of C IV to O VI to O VII to O VIII, intensities were compared with those predicted by theoretical models. Gas which was heated to 3,000,000 K then allowed to cool radiatively cannot produce enough C IV or O VI-generated photons per O VII or O VIII-generated photon. Producing enough C IV and O VI emission requires heating additional gas to 100,000 < T < 1,000,000 K. However, shock heating, which provides heating across this temperature range, overproduces O VI relative to the others. Obtaining the observed mix may require a combination of several processes, including some amount of shock heating, heat conduction, and mixing, as well as radiative cooling of very hot gas.