13CO (J=1-0) depression in luminous starburst mergers

It is known that the class of luminous starburst galaxies tends to have higher R = (CO)-C-12 (J = 1-0)/(CO)-C-13 (J = 1-0) integrated line intensity ratios (R > 20) than normal spiral galaxies (R similar to 10). Since most previous studies investigated only R, it remains uncertain whether the luminous starburst galaxies are overabundant in (CO)-C-12 or underabundant in (CO)-C-13. Here we propose a new observational test to examine this problem. Our new test is to compare far-infrared luminosities [L(FIR)] with those of (CO)-C-12 and (CO)-C-13 [L((CO)-C-12) and L((CO)-C-13), respectively]. It is shown that there is a very tight correlation between L((CO)-C-12) and L(FIR), as found in many previous studies. However, we find that the (CO)-C-13 luminosities of the high-R galaxies are lower by a factor of 3 on average than those expected from the correlation for the remaining galaxies with ordinary R values. Therefore, we conclude that the observed high R values for the luminous starburst galaxies are attributed to their lower (CO)-C-13 line intensities.