Journal Article

Understanding the H<sub>2</sub>/H <span class="smallCaps">i</span> ratio in galaxies

D. Obreschkow and S. Rawlings

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 4, pages 1857-1874
Published in print April 2009 | ISSN: 0035-8711
Published online April 2009 | e-ISSN: 1365-2966 | DOI:
Understanding the H2/H i ratio in galaxies

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We revisit the mass ratio ηgalaxy between molecular hydrogen (H2) and atomic hydrogen (H i) in different galaxies from a phenomenological and theoretical viewpoint. First, the local H2 mass function (MF) is estimated from the local CO luminosity function (LF) of the FCRAO Extragalactic CO Survey, adopting a variable CO-to-H2 conversion fitted to nearby observations. This implies an average H2 density and in the local Universe. Secondly, we investigate the correlations between ηgalaxy and global galaxy properties in a sample of 245 local galaxies. Based on these correlations we introduce four phenomenological models for ηgalaxy, which we apply to estimate H2 masses for each H i galaxy in the HIPASS catalogue. The resulting H2 MFs (one for each model for ηgalaxy) are compared to the reference H2 MF derived from the CO LF, thus allowing us to determine the Bayesian evidence of each model and to identify a clear best model, in which, for spiral galaxies, ηgalaxy negatively correlates with both galaxy Hubble type and total gas mass. Thirdly, we derive a theoretical model for ηgalaxy for regular galaxies based on an expression for their axially symmetric pressure profile dictating the degree of molecularization. This model is quantitatively similar to the best phenomenological one at redshift z= 0, and hence represents a consistent generalization while providing a physical explanation for the dependence of ηgalaxy on global galaxy properties. Applying the best phenomenological model for ηgalaxy to the HIPASS sample, we derive the first integral cold gas MF (H i+ H2+ helium) of the local universe.

Keywords: ISM: atoms; ISM: clouds; ISM: molecules; radio lines: galaxies

Journal Article.  11906 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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