Caught in the Act: A Metal-rich High-velocity Cloud in the Inner Galaxy
Frances H. Cashman, Andrew J. Fox, Bart P. Wakker, Trisha Ashley, D. Massa, E. B. Jenkins, Dhanesh Krishnarao, Robert A. Benjamin, Rongmon Bordoloi, Tae‐Sun Kim
Abstract
Abstract We characterize the chemical and physical conditions in an outflowing high-velocity cloud (HVC) in the inner Galaxy. We report a supersolar metallicity of [O/H] = +0.36 ± 0.12 for the HVC at v LSR = 125.6 km s −1 toward the star HD 156359 ( l = 328.°7, b = −14.°5, d = 9 kpc, z = −2.3 kpc). Using archival observations from the Far-Ultraviolet Spectroscopic Explorer (FUSE), the Hubble Space Telescope Imaging Spectrograph, and the European Southern Observatory Fiber-fed Extended Range Optical Spectrograph 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 ± 0.05 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 ± 0.14, [Fe/O] = −0.30 ± 0.20, and [Ca/O] = −0.56 ± 0.16. Analysis of the high-ion absorption using collisional ionization models indicates that the hot plasma is multiphase, 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 multiphase Galactic outflow and driven out into the halo.