Abstract
It is known that merging galaxies with luminous starbursts and high far-infrared luminosities tend to have higher R1-0 = 12CO (J = 1-0)/13CO (J = 1-0) integrated line intensity ratios (R1-0 ≃ 20-50) than normal spiral galaxies (R1-0 ≃ 5-15). Comparing far-infrared luminosities [L(FIR)] with those of 12CO (J = 1-0) and 13CO (J = 1-0) for a sample of normal and starburst galaxies, Taniguchi & Ohyama found that the observed high R1-0 values for the luminous starburst mergers are attributed to their lower (by a factor of 3 on average) 13CO line intensities. They suggested the following two possibilities: in the luminous starburst mergers (1) 13CO is underabundant with respect to 12CO, or (2) exitation and/or optical depth effects are responsible for the change in R1-0. In this paper, we investigate the second possibility, using higher transition data of both 12CO and 13CO emission lines. Applying the same method proposed by Taniguchi & Ohyama to both 12CO (J = 2-1) and 13CO (J = 2-1), we find that 13CO (J = 2-1) is also depressed with respect to 12CO (J = 2-1). This suggests that the 13CO gas may be underabundant in the high-R1-0 starburst mergers, although we cannot rule out the possibility that excitation and optical depth effects are still affecting R2-1, for example, as a result of the large velocity widths in the CO emission lines. Additional observations of both 12CO and 13CO lines at J ≥ 3 are required to better constrain the conditions of the molecular gas in luminous starburst galaxies.
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