A FUSE Survey of Molecular Hydrogen in Intermediate-Velocity Clouds in the Milky Way Halo

Abstract

Far Ultraviolet Spectroscopic Explorer (FUSE) data is used to investigate the molecular hydrogen (H_2) content of intermediate-velocity clouds (IVCs) in the lower halo of the Milky Way. We analyze interstellar absorption towards 56 (mostly extragalactic) background sources to study H_2 absorption in the Lyman- and Werner bands in 61 IVC components at H I column densities >10^19 cm^-2. For data with good S/N (~9 per resolution element and higher), H_2 in IVC gas is convincingly detected in 14 cases at column densities varying between ~10^14 and ~10^17 cm^-2. We find an additional 17 possible H_2 detections in IVCs in FUSE spectra with lower S/N. The molecular hydrogen fractions, f, vary between 10^-6 and 10^-3, implying a dense, mostly neutral gas phase that is probably related to the Cold Neutral Medium (CNM) in these clouds. If the H_2 stays in formation-dissociation equlibrium, the CNM in these clouds can be characterized by compact (D~0.1 pc) filaments with volume densities on the order of n_H~30 cm^-3. The relatively high detection rate of H_2 in IVC gas implies that the CNM in these clouds is ubiquitous. More dense regions with much higher molecular fractions may exist, but it would be difficult to detect them in absorption because of their small size.

arXiv comment36 pages, 11 figures; accepted for publication in ApJ

References in corpus (20)

• Structure of Stationary Photodissociation Fronts
• Overview of the Far Ultraviolet Spectroscopic Explorer Mission
• On-Orbit Performance of the Far Ultraviolet Spectroscopic Explorer (FUSE) Satellite
• Distances and Metallicities of High- and Intermediate-Velocity Clouds
• A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds
• The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch
• Extraplanar Dust in the Edge-On Spiral NGC 891
• The Metallicity and Dust Content of HVC 287.5+22.5+240: Evidence for a Magellanic Clouds Origin
• Metal Abundances in the Magellanic Stream
• FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream
• Molecular Hydrogen in High-Velocity Clouds
• Far Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption Toward 3C 273
• High-Velocity Cloud Complex C: Galactic Fuel or galactic Waste?
• The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond
• HI spectra and column densities toward HVC and IVC probes
• Discovery of molecular hydrogen in a high-velocity cloud of the Galactic halo
• A Na I Absorption Map of the Small-Scale Structure in the Interstellar Gas Toward M15
• Small-area molecular clouds without shielding
• H_2 absorption in a dense interstellar filament in the Milky Way halo
• ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

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