THE MILKY WAY’S HOT GAS KINEMATICS: SIGNATURES IN CURRENT AND FUTURE O vii ABSORPTION LINE OBSERVATIONS

Abstract

ABSTRACT Detections of z ≈ ?> 0 oxygen absorption and emission lines indicate the Milky Way hosts a hot ( ∼ 10 6 ?> K), low-density plasma extending ≳ 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 O vii Kα 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 O vii equivalent width to show the average gas metallicity is ≳ 0.6 Z ⊙ ?> and b ≳ 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.