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

• Main Authors: 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
• Format: Text
• Language: unknown
• Published: arXiv 2017
• Subjects: Astrophysics of Galaxies astro-ph.GA; Cosmology and Nongalactic Astrophysics astro-ph.CO; Instrumentation and Methods for Astrophysics astro-ph.IM; Solar and Stellar Astrophysics astro-ph.SR; FOS Physical sciences; sami
• Online Access: https://dx.doi.org/10.48550/arxiv.1703.09224; https://arxiv.org/abs/1703.09224

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. : 15 pages, 8 figures, 2 tables (online material), accepted for publication in MNRAS, more info: https://www.mso.anu.edu.au/~chfeder/pubs/sami_gas/sami_gas.html