13CO ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $J=1- 0$ \end{document} ) Depression in Luminous Starburst Mergers, Revisited

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.