Atomic hydrogen produced in M33 photodissociation regions

Abstract

We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far‐ultraviolet (FUV) photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess FUV emission that reaches the GMC from the OB association and of the excess 21‐cm radio H i emission produced after these FUV photons convert H2 into H i on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs and is especially sensitive to a range of densities well below the critical density for CO(1–0) emission. We describe our ‘PDR method’ in more detail and apply it using GALEX FUV and Very Large Array 21‐cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80‐pc resolution. The cloud densities we derive range from 1 to 500 cm−3, with no clear dependence on the galactocentric radius; these results are generally similar to those obtained earlier in the cases of M81, M83 and M101 using the same method.

Applying a one-dimensional PDR model to the Taurus molecular cloud and its atomic envelope

“…Even though the comparison of our method to farinfrared emission shows a clear structure, if we conservatively estimate our results to have an uncertainty comparable to the infrared method, namely 50%, the correlation is significantly degraded. We expect the derived total hydrogen volume densities to have the same or better uncertainties than the ones in the nearest extragalactic case, namely M33 (Heiner et al 2011). The same range of values is obtained with our method as with the far-infrared method, meaning that both methods yield consistent results.…”

“…The above discussion suggests that G0 is in the range of 1 to perhaps several times 10. We consider this quite realistic, considering the extragalactic case -for example see the range of G0 values obtained in M33 (Heiner et al 2011). All that is needed is a cluster of a few dozen O or B stars within a few hundred parsecs distance (further than our own distance from Taurus) and a sufficiently clumpy interstellar medium to let the UV radiation penetrate.…”

“…The notion that a large fraction of extragalactic atomic hydrogen in galactic disks is produced in PDRs has been around for quite some time (Allen et al 1986(Allen et al , 1997Smith et al 2000;Heiner et al 2011). Using PDR-produced atomic hydrogen, it becomes possible to calculate total hydrogen volume densities at selected positions across galactic disks using a simple approximate analytical one-dimensional PDR model (see also e.g.…”

THE H i PROBABILITY DISTRIBUTION FUNCTION AND THE ATOMIC-TO-MOLECULAR TRANSITION IN MOLECULAR CLOUDS

“…Such observations suggest that H I is a necessary ingredient for MC formation. On the other hand, perhaps H I envelopes result from the photodissociation of molecular hydrogen, H 2 (e.g., Allen et al 2004;Heiner et al 2011). It is also possible that such H I envelopes result from a combination of MC formation and evolution processes.…”

H I-TO-H2TRANSITIONS AND H I COLUMN DENSITIES IN GALAXY STAR-FORMING REGIONS

“…Somewhat warmer HI (∼ 100 K, so still "cold") is also observed in dissociation zones surrounding Galactic HII regions associated with individual OB-type stars or clusters, and/or as HI PDRs in molecular cloud envelopes exposed to ambient interstellar radiation (e.g., Sancisi et al 1974;Myers et al 1978;Read 1981;Roger & Pedlar 1981;Wannier et al 1983;Elmegreen & Elmegreen 1987;van der Werf & Goss 1989;Wannier et al 1991;Andersson et al 1992;Gir et al 1994;Reach et al 1994;Williams & Maddalena 1996;Gomez et al 1998;Habart et al 2003;Matthews et al 2003;Roger et al 2004;Lee et al 2007; Lee et al 2012;van der Werf et al 2013). In nearby galaxies, HI has been mapped in spiral arms showing that the atomic gas likely traces outer photodissociated layers in the star-forming giant molecular clouds (Allen et al 1986;Shaya & Federman 1987;Rand et al 1992;Madden et al 1993;Allen et al 1997;Smith et al 2000;Heyer et al 2004;Knapen et al 2006;Schuster et al 2007;Heiner et al 2009Heiner et al , 2011.…”