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We stacked 12CO and 13CO spectra of NGC 3627 after redefining the velocityaxis of each spectrum of the mapping data so that the zero corresponds to thelocal mean velocity of 12CO spectra. The signal-to-noise ratios of theresulting spectra are improved by a factor of up to 3.2 compared to thoseobtained with normal stacking analysis. We successfully detect a weak 13COemission from the interarm region where the emission was not detected in theindividual pointings. We compare the integrated intensity ratios I12 CO/I13 COamong six characteristic regions (center, bar, bar-end, offset, arm, andinterarm). We find that I12CO/I13CO in the bar and interarm are higher thanthose in the other regions by a factor of ~2 and I12CO/I13CO in the center ismoderately high. These high I12CO/I13CO ratios in the bar and center areattributed to a high intensity ratio (T12CO/T13CO) and one in the interarm isattributed to a high ratio of the full width at half maximum of spectra(FWHM12CO/FWHM13CO). The difference between FWHM12CO and FWHM13CO of theinterarm indicates the existence of two components, one with a narrow linewidth (~FWHM13CO) and the other with a broad line width (~FWHM12CO).Additionally, the T12CO/T13CO ratio in the broad-line-width component of theinterarm is higher than the other regions. The high T12CO/T13CO in the centerand bar and of the broad-line-width component in the interarm suggest theexistence of non-optically thick 12CO components. We find that more than halfof the 12CO emissions of the interarm are likely to be radiated from thediffuse component. Our result suggests that the use of a universal CO-to-H2conversion factor might lead to an overestimation of molecular gas mass andunderestimation of star-formation efficiency in the interarm by a factor of afew.

Stacking Analysis of 12CO and 13CO Spectra of NGC3627: Existence of non-optically thick 12CO emission?