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Date and Time of the Query: 2019-05-18 T23:30:36 PDT
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For refcode 2009MNRAS.394.1857O:
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Copyright by Royal Astronomical Society. 2009MNRAS.394.1857O Understanding the H_2_/HI ratio in galaxies Obreschkow, D.; Rawlings, S. Abstract. We revisit the mass ratio {eta}_galaxy_ between molecular hydrogen (H_2_) and atomic hydrogen (HI) in different galaxies from a phenomenological and theoretical viewpoint. First, the local H_2_ mass function (MF) is estimated from the local CO luminosity function (LF) of the FCRAO Extragalactic CO Survey, adopting a variable CO-to-H_2_ conversion fitted to nearby observations. This implies an average H_2_ density and in the local Universe. Secondly, we investigate the correlations between {eta}_galaxy_ and global galaxy properties in a sample of 245 local galaxies. Based on these correlations we introduce four phenomenological models for {eta}_galaxy_, which we apply to estimate H_2_ masses for each HI galaxy in the HIPASS catalogue. The resulting H_2_ MFs (one for each model for {eta}_galaxy_) are compared to the reference H_2_ 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, {eta}_galaxy_ negatively correlates with both galaxy Hubble type and total gas mass. Thirdly, we derive a theoretical model for {eta}_galaxy_ 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 {eta}_galaxy_ on global galaxy properties. Applying the best phenomenological model for {eta}_galaxy_ to the HIPASS sample, we derive the first integral cold gas MF (HI + H_2_ + helium) of the local universe. Key words: ISM: atoms, ISM: clouds, ISM: molecules, radio lines: galaxies
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