Far Ultraviolet Observations of Molecular Hydrogen in the Diffuse Interstellar Medium of Starburst Galaxies
Abstract
(Abridged) We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to search for H2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H2 in M83 and NGC 5253, and set upper limits on the H2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H2 mass inferred from CO emission. This indicates that almost all of the H2 is in the form of clouds with N(H2)>1020 cm-2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the dense clouds, which have a covering factor <1. The complex observational biases related to varying extinction across the extended UV emission in the FUSE apertures prevent an unambiguous characterization of the diffuse H2 in these starbursts. However, the evidence is suggestive that there is less H2 in the diffuse interstellar medium between the dense clouds compared to similarly reddened sight lines in the Milky Way. This holds with the expectation that the destructive UV radiation field is stronger in starbursts. However, previous UV observations of these starbursts have shown that there is reddening caused by the diffuse interstellar medium. This suggests that while diffuse H2 may be destroyed in the starburst, dust still exists.
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