Ultraviolet Properties of Multiphase Gas toward the Inner Galaxy

Abstract We present a systematic study of the multiphase interstellar gas in the inner Galaxy using Hubble Space Telescope/STIS absorption spectroscopy of 16 massive stars located at spectroscopic distances between 1.3 and 10 kpc in the region −30° ≲ l ≲ +30° and −15° ≲ b ≲ +15°. These sight lines probe gas above and below the Sagittarius Carina, Scutum Crux-Centaurus, Norma, and Near 3-kpc spiral arms in a range of z -height from 0 to 1.5 kpc. Along the 16 sight lines, we measure velocity centroids for 800 UV absorption-line components across multiple gas phases (molecular CO, neutral, low ion, and high ion). We find that 619/800 components have velocities that are consistent with a simple model of corotation with the disk, indicating that multiphase gas with disk-like kinematics extends at least 1 kpc into the halo. We present a database of absorption-line parameters that can be used for kinematic modeling of gas flows into and out of the Galactic disk.