We present the results of \atom C12 \atom O ( J = 1–0 ) and \atom C13 \atom O ( J = 1–0 ) simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as a part of the CO Multi-line Imaging of Nearby Galaxies ( COMING ) project .
Barred spiral galaxies often show lower star-formation efficiency ( SFE ) in their bar region compared to the spiral arms .
In this paper , we examine the relation between the SFEs and the volume densities of molecular gas n ( \mathrm { H _ { 2 } } ) in the eight different regions within the galactic disk with \atom C \atom O data combined with archival far-ultraviolet and 24 \ > \micron data .
We confirmed that SFE in the bar region is lower by 39 % than that in the spiral arms .
Moreover , velocity-alignment stacking analysis was performed for the spectra in the individual regions .
The integrated intensity ratios of \atom C12 \atom O to \atom C13 \atom O ( R _ { 12 / 13 } ) range from 10 to 17 as the results of stacking .
Fixing a kinetic temperature of molecular gas , n ( { H _ { 2 } } ) was derived from R _ { 12 / 13 } via non-local thermodynamic equilibrium ( non-LTE ) analysis .
The density n ( \mathrm { H _ { 2 } } ) in the bar is lower by 31–37 % than that in the arms and there is a rather tight positive correlation between SFEs and n ( \mathrm { H _ { 2 } } ) , with a correlation coefficient of \sim 0.8 .
Furthermore , we found a dependence of n ( H _ { 2 } ) on the velocity dispersion of inter-molecular clouds ( \Delta V / \sin i ) .
Specifically , n ( \mathrm { H _ { 2 } } ) increases as \Delta V / \sin i increases when \Delta V / \sin i < 100 km s ^ { -1 } .
On the other hand , n ( \mathrm { H _ { 2 } } ) decreases as \Delta V / \sin i increases when \Delta V / \sin i > 100 km s ^ { -1 } .
These relations indicate that the variations of SFE could be caused by the volume densities of molecular gas , and the volume densities could be governed by the dynamical influence such as cloud-cloud collisions , shear and enhanced inner-cloud turbulence .