We investigate the molecular gas properties of galaxies across the main sequence of star-forming (SF) galaxies in the local Universe using $^{ 12}$CO($J=1-0$) (hereafter $^{ 12}$CO) and $^{ 13}$CO($J=1-0$) ($^{ 13}$CO) mapping data of 147 nearby galaxies obtained in the COMING project, a legacy project of the Nobeyama Radio Observatory. In order to improve signal-to-noise ratios of both lines, we stack all the pixels where $^{ 12}$CO emission is detected after aligning the line center expected from the first-moment map of $^{ 12}$CO. As a result, $^{ 13}$CO emission is successfully detected in 80 galaxies with a signal-to-noise ratio larger than three. The error-weighted mean of integrated-intensity ratio of $^{ 12}$CO to $^{ 13}$CO lines ($R_{ 1213}$) of the 80 galaxies is $10.9$ with a standard deviation of $7.0$. We find that (1) $R_{ 1213}$ positively correlates to specific star-formation rate (sSFR) with a correlation coefficient of $0.46$, and (2) both flux ratio of IRAS 60~$\mu$m to 100~$\mu$m ($f_{ 60}/f_{ 100}$) and inclination-corrected linewidth of $^{ 12}$CO stacked spectra ($\sigma_{ { \rm ^{ 12}CO},i}$) also correlate with sSFR for galaxies with the $R_{ 1213}$ measurement. Our results support the scenario where $R_{ 1213}$ variation is mainly caused by the changes in molecular-gas properties such as temperature and turbulence. The consequent variation of CO-to-H$_2$ conversion factor across the SF main sequence is not large enough to completely extinguish the known correlations between sSFR and $M_{ \rm mol}/M_{ \rm star}$ ($\mu_{ \rm mol}$) or star-formation efficiency (SFE) reported in previous studies, while this variation would strengthen (weaken) the sSFR-SFE (sSFR-$\mu_{ \rm mol}$) correlation.
Categories: astro-ph.GA