${}^{13}CO(J=1--0)$ Depression in Luminous Starburst Mergers

Abstract

It is known that the class of luminous starburst galaxies tends to have higher $R{=}^{12}CO(J=1--0){/}^{13}CO(J=1--0)$ integrated line intensity ratios ($R>20$) than normal spiral galaxies ($R\sim 10$). Since most previous studies investigated only $R$, it remains uncertain whether the luminous starburst galaxies are overabundant in ${}^{12}$CO or underabundant in ${}^{13}$CO. 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 ${}^{12}$CO and ${}^{13}CO[L{(}^{12}CO)$ and $L{(}^{13}CO)$, respectively]. It is shown that there is a very tight correlation between $L{(}^{12}CO)$ and L(FIR), as found in many previous studies. However, we find that the ${}^{13}$CO luminosities of the high-R galaxies are lower by a factor of three on the 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 ${}^{13}$CO line intensities.

9 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for publication in Astrophysical Journal Letters

References in corpus (1)

• Variation of Molecular Line Ratios and Cloud Properties in the Arp 299 Galaxy Merger

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