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arXiv · 1703.09224

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

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

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 (Sigma_gas) using optical spectroscopy. We utilise the spatially resolved H-alpha maps of flux and velocity dispersion from the Sydney-AAO Multi-object Integral-field spectrograph (SAMI) Galaxy Survey. We derive maps of Sigma_gas by inverting the multi-freefall star formation relation, which connects the star formation rate surface density (Sigma_SFR) with Sigma_gas and the turbulent Mach number (Mach). Based on the measured range of Sigma_SFR = 0.005-1.5 M_sol/yr/kpc^2 and Mach = 18-130, we predict Sigma_gas = 7-200 M_sol/pc^2 in the star-forming regions of our sample of 260 SAMI galaxies. These values are close to previously measured Sigma_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_SFR, Mach, and Sigma_gas are enhanced by factors of 2.0, 1.6, and 1.3, respectively, compared to Star-forming galaxies. We compare our predictions of Sigma_gas with those obtained by inverting the Kennicutt-Schmidt relation and find that our new method is a factor of two more accurate in predicting Sigma_gas, with an average deviation of 32% from the actual Sigma_gas.

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Christoph Federrath, Diane M. Salim, Anne M. Medling, Rebecca L. Davies, Tiantian Yuan, Fuyan Bian, Brent A. Groves, I-Ting Ho, Robert Sharp, Lisa J. Kewley, Sarah M. Sweet, Samuel N. Richards, Julia J. Bryant, Sarah Brough, Scott Croom, Nicholas Scott, Jon Lawrence, Iraklis Konstantopoulos, Michael Goodwin. 2017-03-27. The SAMI Galaxy Survey: a new method to estimate molecular gas surface densities from star formation rates. https://doi.org/10.1093/mnras%2Fstx727

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