The Warm Gaseous Disk and the Anisotropic Circumgalactic Medium of the Milky Way

Abstract

Warm ( ) gas is an important gaseous component in the galaxy baryonic cycle, which is important for understanding both gas accretion and galactic feedback processes. We built a two-dimensional disk–circumgalactic medium (CGM) model to study the warm gas distribution of the Milky Way (MW) using the absorption-line surveys of Si iv and O vi . In this model, the disk component of both ions has the same density profile ( ) with a scale height of z 0  = 2.6 ± 0.4 kpc and a scale length of r 0  = 6.1 ± 1.2 kpc. For this disk component, we calculate the warm gas mass of . The similar disk density profiles and total masses of the Si iv- and O vi -bearing gas set constraints on the ionization mechanisms. We suggest that the warm gas disk might be dominated by the Galactic fountain mechanism, which ejects and recycles gas to set both the scale height and the scale length of the warm gas disk. The CGM component in our model has a dependence on Galactic latitude, with a higher column density along the direction perpendicular to the Galactic plane ( b  = 90°) than along the radial direction ( b  = 0°). The column density difference between these two directions is 0.82 ± 0.32 dex at 6.3 σ for both ions. This difference may be due to either the enrichment of Galactic feedback to the entire CGM or an additional interaction layer between the warm gas disk and the CGM; existing data cannot distinguish between these two scenarios. If this higher column density at b  = 90° is for the entire CGM, the total warm CGM mass is within the MW virial radius of 250 kpc.