ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

Abstract

In front of the LMC molecular hydrogen is found in absorption near 0 km/s, being local disk gas, near +60 km/s in an intermediate velocity cloud, and near +120 km/s, in a high velocity halo cloud. The nature of the gas is discussed based on four ORFEUS far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only 2.5 deg, rather fluffy. The fractional abundance of H_2 varies from log(f)=-5.4 to -3.3. Metal depletions (up to -1.7 dex for Fe) are typical for galactic disk gas. In the IV and HV gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90%. H_2 is detected in IV and HV gas toward HD 269546. In the IV gas we find an H_2 column density of log(N)\simeq15.6. The H_2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H_2 (log(N)\simeq15.6) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H_2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.

12 pages, 5 figures, accepted for publication in A&A

References in corpus (5)

• High Velocity Clouds: Building Blocks of the Local Group
• The Definitive Abundance of Interstellar Oxygen
• A confirmed location in the Galactic halo for the high-velocity cloud 'chain A'
• Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo
• The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction

Cited by in corpus (16)

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• The Parkes HI Survey of the Magellanic System
• The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch
• An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way's circumgalactic medium and the Local Group
• A FUSE Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo
• Galactic fountains and their connection with high and intermediate velocity clouds
• High-Velocity Clouds and the Local Intergalactic Medium
• H_2 absorption in a dense interstellar filament in the Milky Way halo
• The contribution of halo red giant mass loss to the high-velocity gas falling onto the Milky Way disk
• H2, HD, and CO at the edge of 30 Dor in the LMC: the line of sight to Sk-69 246
• Large and small-scale structure of the Intermediate and High Velocity Clouds towards the LMC and SMC
• Multiwavelength observations of the M15 intermediate velocity cloud
• A detection of H${}_2$ in a high velocity cloud toward the Large Magellanic Cloud
• Low and high velocity clouds produced by young stellar clusters
• Molecular Gas in the Outskirts of Galaxies