A large amount of diffuse molecular gases in the bar of the strongly
  barred galaxy NGC1300: Cause of the low star formation efficiency
  In many barred galaxies, star formation efficiency (SFE) in the bar is lower
than those in the arm and bar-end, and its cause has still not been clear.
Focusing on the strongly barred galaxy NGC 1300, we investigate the possibility
that the presence of a large amount of diffuse molecular gas, which would not
contribute to the SF, makes the SFE low in appearance. We examine the relation
between the SFE and the diffuse molecular gas fraction ($f_{\rm dif}$), which
is derived using the $^{12}$CO($1-0$) flux obtained from the interferometer of
ALMA 12-m array, which has no sensitivity on diffuse (extended; FWHM
$\gtrapprox 700$ pc) molecular gases due to the lack of ACA, and the total
$^{12}$CO($1-0$) flux obtained from Nobeyama 45-m single-dish telescope. We
find that the SFE decreases with increasing $f_{\rm dif}$. The $f_{\rm dif}$
and ${\rm SFE}$ are $0.74 - 0.91$ and $(0.06 - 0.16) ~\rm Gyr^{-1}$ in the bar
regions, and $0.28 - 0.65$ and $(0.23 - 0.96) ~\rm Gyr^{-1}$ in the arm and
bar-end regions. This result supports the idea that the presence of a large
amount of diffuse molecular gas makes the SFE low. The suppression of the SFE
in the bar has also been seen even when we exclude the diffuse molecular gas
components. This suggests that the low SFE appears to be caused not only by a
large amount of diffuse molecular gases but also by other mechanisms such as
fast cloud-cloud collisions.
