13CO (J=1-0) depression in luminous starburst mergers, revisited.
TANIGUCHI Y., OHYAMA Y. and SANDERS D.B.
Abstract (from CDS):
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.
Galaxies: Interactions - Galaxies: ISM - Galaxies: Starburst - Radio Lines: Galaxies