1999ApJ...522..214T

It is known that merging galaxies with luminous starbursts and high far-infrared luminosities tend to have higher R_1–0=¹²CO (J=1-0)/¹³CO (J=1-0) integrated line intensity ratios (R_1–0≃20-50) than normal spiral galaxies (R_1–0≃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 R_1–0 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 R_1–0. 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-R_1–0 starburst mergers, although we cannot rule out the possibility that excitation and optical depth effects are still affecting R_2–1, 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.