The 21 cm "Outer Arm" and the Outer-galaxy High-velocity Clouds: Connected by Kinematics, Metallicity, and Distance
Abstract
Using high-resolution ultraviolet spectra obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer, we study the metallicity, kinematics, and distance of the gaseous "outer arm" (OA) and the high-velocity clouds (HVCs) in the outer Galaxy. We detect the OA in a variety of absorption lines toward two QSOs, H1821+643 and HS0624+6907. We search for OA absorption toward eight Galactic stars and detect it in one case, which constrains the OA Galactocentric radius to 9 kpc <R G < 18 kpc. We also detect HVC Complex G, which is projected near the OA at a similar velocity, in absorption toward two stars; Complex G is therefore in the same region at R G = 8-10 kpc. HVC Complex C is known to be at a similar Galactocentric radius. Toward H1821+643, the low-ionization absorption lines are composed of multiple narrow components, indicating the presence of several cold clouds and rapid cooling and fragmentation. Some of the highly ionized gas is also surprisingly cool. Accounting for ionization corrections, we find that the OA metallicity is Z = 0.2-0.5 Z ⊙, but nitrogen is underabundant and some species are possibly mildly depleted by dust. The similarity of the OA metallicity, Galactocentric location, and kinematics to those of the adjacent outer-Galaxy HVCs, including high velocities that are not consistent with Galactic rotation, suggests that the OA and outer-Galaxy HVCs could have a common origin.
Based on observations with (1) the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, and (2) the NASA-CNES/ESA Far Ultraviolet Spectroscopic Explorer mission, operated by Johns Hopkins University, supported by NASA contract NAS 5-32985.- Publication:
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The Astrophysical Journal
- Pub Date:
- February 2012
- DOI:
- 10.1088/0004-637X/746/2/173
- arXiv:
- arXiv:1101.1107
- Bibcode:
- 2012ApJ...746..173T
- Keywords:
-
- galaxies: ISM;
- ISM: abundances;
- quasars: absorption lines;
- quasars: individual: H1821+643 HS0624+6907;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication in the Astrophysical Journal