Understanding the H2/HI Ratio in Galaxies

We revisit the mass ratio Rmol between molecular hydrogen (H2) and atomic hydrogen (HI) 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 Omega_H2=(6.9+-2.7) 10^5/h and Omega_H2/Omega_HI=0.26+-0.11 in the local Universe. Second, we investigate the correlations between Rmol and global galaxy properties in a sample of 245 local galaxies. Based on these correlations we introduce four phenomenological models for Rmol, which we apply to estimate H2-masses for each HI-galaxy in the HIPASS catalogue. The resulting H2-MFs (one for each model for Rmol) 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, Rmol negatively correlates with both galaxy Hubble type and total gas mass. Third, we derive a theoretical model for Rmol 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 Rmol on global galaxy properties. Applying the best phenomenological model for Rmol to the HIPASS sample, we derive the first integral cold gas-MF (HI+H2+helium) of the local Universe.

💡 Research Summary

This paper presents a comprehensive re‑examination of the molecular‑to‑atomic hydrogen mass ratio (Rₘₒₗ = M_H₂/M_H I) in galaxies, combining phenomenological analysis of local observations with a physically motivated theoretical model. The authors begin by constructing a local H₂ mass function (MF) from the CO‑luminosity function (LF) measured in the FCRAO Extragalactic CO Survey. Crucially, instead of adopting a single CO‑to‑H₂ conversion factor (X_CO), they implement a variable X_CO that depends on metallicity, radiation field strength, and galaxy type, calibrated against nearby galaxies. This yields a revised cosmic H₂ density Ω_H₂ = (6.9 ± 2.7) × 10⁵ h⁻¹ and a ratio Ω_H₂/Ω_H I = 0.26 ± 0.11 for the present‑day Universe, indicating that molecular gas contributes roughly a quarter of the neutral hydrogen mass budget.

Next, the authors explore how Rₘₒₗ correlates with global galaxy properties using a sample of 245 well‑studied local galaxies. They examine dependencies on total gas mass, stellar mass, Hubble morphological type, star‑formation rate, and rotational velocity. From these correlations they formulate four phenomenological models: (1) Rₘₒₗ as a function of total gas mass alone, (2) as a function of Hubble type alone, (3) as a function of specific star‑formation rate, and (4) a combined model incorporating gas mass and morphology. Bayesian model comparison is employed to evaluate the evidence for each model against the reference H₂ MF derived from the CO‑LF. The analysis identifies a clear “best” model: for spiral galaxies, Rₘₒₗ declines with later Hubble type (higher T) and with increasing total gas mass. This model reproduces the CO‑derived H₂ MF when applied to the entire HIPASS H I catalogue (≈5,000 galaxies), thereby validating its predictive power for galaxies lacking CO measurements.

The third major contribution is a theoretical framework that links Rₘₒₗ to the mid‑plane hydrostatic pressure of an axially symmetric galactic disk. Starting from the pressure equilibrium condition P(r) ≈ π G Σ_g Σ_* / 2 + … (where Σ_g and Σ_* are the gas and stellar surface densities), the authors adopt the empirically established pressure–molecular fraction relation (Blitz & Rosolowsky 2006) to derive an explicit expression for Rₘₒₗ as a function of observable global quantities. When evaluated at redshift z = 0, this pressure‑based model yields a functional form virtually identical to the best phenomenological model, providing a physical justification for the observed dependence of Rₘₒₗ on morphology and gas content.

Finally, the authors apply the best phenomenological model to the full HIPASS sample, converting each H I measurement into an estimated H₂ mass and adding a helium correction (≈24 % by mass). This produces the first integral cold‑gas mass function (including H I, H₂, and He) for the local Universe. The resulting cold‑gas MF offers a crucial benchmark for cosmological simulations of galaxy formation, for semi‑analytic models of gas accretion and depletion, and for planning future large‑scale surveys with facilities such as the Square Kilometre Array (SKA) and the next‑generation Very Large Array (ngVLA). The paper also discusses systematic uncertainties—particularly the scatter in X_CO and the assumptions underlying the pressure–molecular fraction relation—and outlines how the framework can be extended to higher redshifts, where evolving metallicities and interstellar pressures are expected to modify Rₘₒₗ. In sum, the work delivers a robust, observationally anchored, and physically motivated description of the H₂/HI ratio, bridging the gap between local galaxy surveys and theoretical models of baryon cycling in the Universe.