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We present the results of $^{12}$CO($J$=1-0) and $^{13}$CO($J$=1-0)simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as a partof the CO Multi-line Imaging of Nearby Galaxies (COMING) project. Barred spiralgalaxies often show lower star-formation efficiency (SFE) in their bar regioncompared to the spiral arms. In this paper, we examine the relation between theSFEs and the volume densities of molecular gas $n(\rm{H}_2)$ in the eightdifferent regions within the galactic disk with CO data combined with archivalfar-ultraviolet and 24 $\mu$m data. We confirmed that SFE in the bar region islower by 39% than that in the spiral arms. Moreover, velocity-alignmentstacking analysis was performed for the spectra in the individual regions. Theintegrated intensity ratios of $^{12}$CO to $^{13}$CO ($R_{12/13}$) range from10 to 17 as the results of stacking. Fixing a kinetic temperature of moleculargas, $n(\rm{H}_2)$ was derived from $R_{12/13}$ via non-local thermodynamicequilibrium (non-LTE) analysis. The density $n(\rm{H}_2)$ in the bar is lowerby 31-37% than that in the arms and there is a rather tight positivecorrelation between SFEs and $n(\rm{H}_2)$, with a correlation coefficient of$\sim 0.8$. Furthermore, we found a dependence of $n(\rm{H}_2)$ on the velocitydispersion of inter-molecular clouds ($\Delta V/ \sin i$). Specifically,$n(\rm{H}_2)$ increases as $\Delta V/ \sin i$ increases when $\Delta V/ \sin i< 100$ km s$^{-1}$. On the other hand, $n(\rm{H}_2)$ decreases as $\Delta V/\sin i$ increases when $\Delta V/ \sin i > 100$ km s$^{-1}$. These relationsindicate that the variations of SFE could be caused by the volume densities ofmolecular gas, and the volume densities could be governed by the dynamicalinfluence such as cloud-cloud collisions, shear and enhanced inner-cloudturbulence.

publications of the astronomical society of japan

CO Multi-line Imaging of Nearby Galaxies (COMING). III. Dynamical effect  on molecular gas density and star formation in the barred spiral galaxy NGC  4303