Caught in the Act: A Metal-Rich High-Velocity Cloud in the Inner Galaxy

Abstract

We characterize the chemical and physical conditions in an outflowing high-velocity cloud in the inner Galaxy. We report a super-solar metallicity of [O/H] = $+0.36\pm0.12$ for the high-velocity cloud at $v_\mathrm{LSR}$ = 125.6 km s$^{-1}$ toward the star HD 156359 ($l$ = 328.$^{\circ}$7, $b$ = $-$14.$^{\circ}$5, $d$ = 9 kpc, $z$ = $-$2.3 kpc). Using archival observations from FUSE, HST STIS, and ESO FEROS we measure high-velocity absorption in H I, O I, C II, N II, Si II, Ca II, Si III, Fe III, C IV, Si IV, N V, and O VI. We measure a low H I column density of log $N$(H I) = $15.54\pm0.05$ in the HVC from multiple unsaturated H I Lyman series lines in the FUSE data. We determine a low dust depletion level in the HVC from the relative strength of silicon, iron, and calcium absorption relative to oxygen, with [Si/O]=$-0.33\pm0.14$, [Fe/O]=$-0.30\pm0.20$, and [Ca/O] =$-0.56\pm0.16$. Analysis of the high-ion absorption using collisional ionization models indicates that the hot plasma is multi-phase, with the C IV and Si IV tracing 10$^{4.9}$ K gas and N V and O VI tracing 10$^{5.4}$ K gas. The cloud's metallicity, dust content, kinematics, and close proximity to the disk are all consistent with a Galactic wind origin. As the HD 156359 line of sight probes the inner Galaxy, the HVC appears to be a young cloud caught in the act of being entrained in a multi-phase Galactic outflow and driven out into the halo.