Clumping and the Interpretation of kpc-scale Maps of the Interstellar Medium: Smooth H I and Clumpy, Variable H2 Surface Density

Abstract : Many recent models consider the structure of individual interstellar medium (ISM) clouds as a way to explain observations of large parts of galaxies. To compare such models to observations, one must understand how to translate between surface densities observed averaging over large (~kpc) scales and surface densities on the scale of individual clouds (~pc scale), which are treated by models. We define a "clumping factor" that captures this translation as the ratio of the mass-weighted surface density, which is often the quantity of physical interest, to the area-weighted surface density, which is observed. We use high spatial resolution (sub-kpc) maps of CO and H I emission from nearby galaxies to measure the clumping factor of both atomic and molecular gas. The molecular and atomic ISM exhibit dramatically different degrees of clumping. As a result, the ratio H2/H I measured at ~kpc resolution cannot be trivially interpreted as a cloud-scale ratio of surface densities. H I emission appears very smooth, with a clumping factor of only ~1.3. Based on the scarce and heterogeneous high-resolution data available, CO emission is far more clumped with a widely variable clumping factor, median ~7 for our heterogeneous data. Our measurements do not provide evidence for a universal mass-weighted surface density of molecular gas, but also cannot conclusively rule out such a scenario. We suggest that a more sophisticated treatment of molecular ISM structure, one informed by high spatial resolution CO maps, is needed to link cloud-scale models to kpc-scale observations of galaxies.
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Contributor : Jérôme Pety <>
Submitted on : Sunday, May 18, 2014 - 7:00:30 PM
Last modification on : Friday, August 2, 2019 - 2:30:11 PM

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Adam K. Leroy, Cheoljong Lee, Andreas Schruba, Alberto Bolatto, Annie Hughes, et al.. Clumping and the Interpretation of kpc-scale Maps of the Interstellar Medium: Smooth H I and Clumpy, Variable H2 Surface Density. The Astrophysical journal letters, Bristol : IOP Publishing, 2013, 769, pp.L12. ⟨10.1088/2041-8205/769/1/L12⟩. ⟨hal-00992507⟩

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