SIMBAD references

Stars form in cold molecular clouds. However, molecular gas is difficult to observe because the most abundant molecule (H₂) lacks a permanent dipole moment. Rotational transitions of CO are often used as a tracer of H₂, but CO is much less abundant and the conversion from CO intensity to H₂ mass is often highly uncertain. Here we present a new method for estimating the column density of cold molecular gas (Σ_gas) using optical spectroscopy. We utilize the spatially resolved Hα maps of flux and velocity dispersion from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. We derive maps of Σ_gas by inverting the multi-freefall star formation relation, which connects the star formation rate surface density (Σ_SFR) with Σ_gas and the turbulent Mach number ( M). Based on the measured range of Σ_SFR = 0.005-1.5 M_☉ yr^–1 kpc^–2 and M=18-130, we predict Σ_gas = 7-200 M_☉ pc^–2 in the star-forming regions of our sample of 260 SAMI galaxies. These values are close to previously measured Σ_gas obtained directly with unresolved CO observations of similar galaxies at low redshift. We classify each galaxy in our sample as 'star-forming' (219) or 'composite/AGN/shock' (41), and find that in 'composite/AGN/shock' galaxies the average Σ_SFR, M and Σ_gas are enhanced by factors of 2.0, 1.6 and 1.3, respectively, compared to star-forming galaxies. We compare our predictions of Σ_gas with those obtained by inverting the Kennicutt-Schmidt relation and find that our new method is a factor of 2 more accurate in predicting Σ_gas, with an average deviation of 32 per cent from the actual Σ_gas.