CO Multi-line Imaging of Nearby Galaxies (COMING): VI. Radial variations
  in star formation efficiency
  We examined radial variations in molecular-gas based star formation
efficiency (SFE), which is defined as star formation rate per unit molecular
gas mass, for 80 galaxies selected from the CO Multi-line Imaging of Nearby
Galaxies project (Sorai et al. 2019). The radial variations in SFE for
individual galaxies are typically a factor of 2 -- 3, which suggests that SFE
is nearly constant along galactocentric radius. We found the averaged SFE in 80
galaxies of $(1.69 \pm 1.1) \times 10^{-9}$ yr$^{-1}$, which is consistent with
Leroy et al. 2008 if we consider the contribution of helium to the molecular
gas mass evaluation and the difference in the assumed initial mass function
between two studies. We compared SFE among different morphological (i.e., SA,
SAB, and SB) types, and found that SFE within the inner radii ($r/r_{25} <
0.3$, where $r_{25}$ is $B$-band isophotal radius at 25 mag arcsec$^{-2}$) of
SB galaxies is slightly higher than that of SA and SAB galaxies. This trend can
be partly explained by the dependence of SFE on global stellar mass, which
probably relates to the CO-to-H$_2$ conversion factor through the metallicity.
For two representative SB galaxies in our sample, NGC 3367 and NGC 7479, the
ellipse of $r/r_{25}$ = 0.3 seems to cover not only the central region but also
the inner part of the disk, mainly the bar. These two galaxies show higher SFE
in the bar than in spiral arms. However, we found an opposite trend in NGC
4303; SFE is lower in the bar than in spiral arms, which is consistent with
earlier studies (e.g., Momose et al. 2010). These results suggest diversity of
star formation activities in the bar.
