The Milky Way's Hot Gas Kinematics: Signatures in Current and Future OVII Absorption Line Observations

TL;DR

This paper models the kinematics of the Milky Way's hot halo gas using OVII absorption lines, revealing how line profiles encode gas dynamics and improving interpretation methods for current and future X-ray observations.

Contribution

It introduces models of absorption line profiles accounting for optical depth and velocity effects, enabling better extraction of hot gas properties from observations.

Findings

Line profiles can be asymmetric and broader than Doppler width.

Line of sight velocity effects significantly impact column density estimates.

Estimated hot gas metallicity is at least 0.6 Z_sun with velocity dispersion over 100 km/s.

Abstract

Detections of $z \approx$ 0 oxygen absorption and emission lines indicate the Milky Way hosts a hot ($\sim 10^6$ K), low-density plasma extending $\gtrsim$50 kpc into the Mily Way's halo. Current X-ray telescopes cannot resolve the line profiles, but the variation of their strengths on the sky constrains the radial gas distribution. Interpreting the OVII K$\alpha$ absorption line strengths has several complications, including optical depth and line of sight velocity effects. Here, we present model absorption line profiles accounting for both of these effects to show the lines can exhibit asymmetric structures and be broader than the intrinsic Doppler width. The line profiles encode the hot gas rotation curve, the net inflow or outflow of hot gas, and the hot gas angular momentum profile. We show how line of sight velocity effects impact the conversion between equivalent width and the column density, and provide modified curves of growth accounting for these effects. As an example, we analyze the LMC sight line pulsar dispersion measure and OVII equivalent width to show the average gas metallicity is $\gtrsim 0.6 Z_{\odot}$ and $b$ $\gtrsim$ 100 km s$^{-1}$. Determining these properties offers valuable insights into the dynamical state of the Milky Way's hot gas, and improves the line strength interpretation. We discuss future strategies to observe these effects with an instrument that has a spectral resolution of about 3000, a goal that is technically possible today.