• Mckee, Christopher F.
• Krumholz, Mark R.
• Bolatto, Alberto D.
• Tumlinson, Jason
• Roman-Duval, Julia
• Gordon, Karl D.

Abstract

Theoretical and observational evidence suggests that molecular gas (H2) is not always correlated with CO emission, because CO is photo-dissociated more easily than H2. The mass of CO-poor H2 present in the envelopes of giant molecular clouds (GMCs) is expected to be higher in low metallicity GMCs, which are more exposed to dissociating radiation due to their lower dust abundance. Since we cannot detect H2 directly, the mass of these CO-poor envelopes is effectively "hidden" from radio telescopes. As a result, theoretical predictions of the CO/H2 abundance, are difficult to test and validate, and the amount of molecular gas present in low metallicity galaxies is not well constrained. The most sensitive and unambiguous way to determine CO and H2 column densities is to derive them from UV absorption spectroscopy. We have acquired high S/N spectra of the 4th positive absorption band of CO with HST/COS toward 9 translucent (A 1) Magellanic Cloud sight-lines probing the edges of GMCs, for which previous FUSE spectra and H2 column density determinations are available from the FUSE catalogs and archive. After removing spectral features associated with the stellar sources, we find CO column densities as low as ~1013 cm-2 toward those translucent sight-lines. We have computed the CO and H2 abundances as a function of total gas column and extinction, and have found this relation to be compatible with predictions from numerical models of CO and H2 formation and dissociation. The cooling rate associated with CO rotational emission appears to be negligible compared to cooling by ionized carbon (C II) in this density regime.

Topics

• density
• impedance spectroscopy
• Carbon