Stacking Analysis of 12CO and 13CO Spectra of NGC3627: Existence of
  non-optically thick 12CO emission?
  We stacked 12CO and 13CO spectra of NGC 3627 after redefining the velocity
axis of each spectrum of the mapping data so that the zero corresponds to the
local mean velocity of 12CO spectra. The signal-to-noise ratios of the
resulting spectra are improved by a factor of up to 3.2 compared to those
obtained with normal stacking analysis. We successfully detect a weak 13CO
emission from the interarm region where the emission was not detected in the
individual pointings. We compare the integrated intensity ratios I12 CO/I13 CO
among six characteristic regions (center, bar, bar-end, offset, arm, and
interarm). We find that I12CO/I13CO in the bar and interarm are higher than
those in the other regions by a factor of ~2 and I12CO/I13CO in the center is
moderately high. These high I12CO/I13CO ratios in the bar and center are
attributed to a high intensity ratio (T12CO/T13CO) and one in the interarm is
attributed to a high ratio of the full width at half maximum of spectra
(FWHM12CO/FWHM13CO). The difference between FWHM12CO and FWHM13CO of the
interarm indicates the existence of two components, one with a narrow line
width (~FWHM13CO) and the other with a broad line width (~FWHM12CO).
Additionally, the T12CO/T13CO ratio in the broad-line-width component of the
interarm is higher than the other regions. The high T12CO/T13CO in the center
and bar and of the broad-line-width component in the interarm suggest the
existence of non-optically thick 12CO components. We find that more than half
of the 12CO emissions of the interarm are likely to be radiated from the
diffuse component. Our result suggests that the use of a universal CO-to-H2
conversion factor might lead to an overestimation of molecular gas mass and
underestimation of star-formation efficiency in the interarm by a factor of a
few.
