The SAMI Galaxy Survey: a new method to estimate molecular gas surface densities from star formation rates


Federrath, Christoph; Salim, Diane M.; Medling, Anne M.; Davies, Rebecca L.; Yuan, Tiantian; Bian, Fuyan; Groves, Brent A.; Ho, I.-Ting; Sharp, Robert; Kewley, Lisa J.; Sweet, Sarah M.; Richards, Samuel N.; Bryant, Julia J.; Brough, Sarah; Croom, Scott; Scott, Nicholas; Lawrence, Jon; Konstantopoulos, Iraklis; Goodwin, Michael


Stars form in cold molecular clouds. However, cold gas is difficult to observe because the most abundant molecule ($\mathrm{H_2}$) lacks a permanent dipole moment so does not typically emit radiation. Less abundant rotational transitions of CO are often to trace $\mathrm{H_2}$, but this relies on conversion factors that are often highly uncertain. Here we present a new method for estimating the column density of cold molecular gas ($\Sigma_\mathrm{gas}$) using optical spectroscopy, utilising the spatially resolved H$\alpha$ maps of flux and velocity dispersion from the Sydney-AAO Multi-object Integral-field spectograph (SAMI) Galaxy Survey. We derive maps of $\Sigma_\mathrm{gas}$ by inverting the multi-freefall star formation relation, which connects the star formation rate surface density ($\Sigma_\mathrm{SFR}$) with $\Sigma_\mathrm{gas}$ and the turbulent Mach number ($\mathcal{M}$). We predict $\Sigma_\mathrm{gas}=7$--$200,\mathrm{M_{\odot}},\mathrm{pc}^{-2}$ in the star-forming regions of our sample of 260 galaxies. These values are close to previously measured $\Sigma_\mathrm{gas}$ 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 $\Sigma_\mathrm{SFR}$, $\mathcal{M}$, and $\Sigma_\mathrm{gas}$ are enhanced by factors of $2.0$, $1.6$, and $1.3$, respectively, compared to Star-forming galaxies. Assuming that the inversion of the multi-freefall star formation relation remains a valid approximation at high redshift, our method of estimating $\Sigma_\mathrm{gas}$ may be applied more generally to any set of data with measurements of $\Sigma_\mathrm{SFR}$ and gas velocity dispersion.

Publication Date: 
June 2016
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