The Far Ultraviolet Spectroscopic Explorer Survey of OVI Absorption in the Disk of the Milky Way

TL;DR

This study uses FUSE observations of OVI absorption toward 148 stars to map the distribution, physical characteristics, and kinematics of hot interstellar gas in the Milky Way's disk, revealing a broad, turbulent, and multiphase medium.

Contribution

It provides the first comprehensive survey of OVI in the Galactic disk, quantifying its density distribution, scale height, and velocity characteristics, highlighting the turbulent nature of hot interstellar plasma.

Findings

Average OVI mid-plane density n_0 = 1.3e-8 cm^-3

Exponential scale height of 3.2 to 4.6 kpc

Broad dispersion in OVI volume densities and velocities

Abstract

To probe the distribution and physical characteristics of interstellar gas at temperatures T ~ 3e5 K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of OVI toward 148 early-type stars situated at distances 1 kpc. After subtracting off a mild excess of OVI arising from the Local Bubble, combining our new results with earlier surveys of OVI, and eliminating stars that show conspicuous localized X-ray emission, we find an average OVI mid-plane density n_0 = 1.3e-8 cm^-3. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of OVI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(OVI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the OVI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.