13CO (J = 1-0) depression in luminous starburst mergers.
TANIGUCHI Y. and OHYAMA Y.
Abstract (from CDS):
It is known that the class of luminous starburst galaxies tends to have higher R=12CO (J=1-0)/13CO (J=1-0) integrated line intensity ratios (R>20) than normal spiral galaxies (R∼10). Since most previous studies investigated only R, it remains uncertain whether the luminous starburst galaxies are overabundant in 12CO or underabundant in 13CO. 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 12CO and 13CO [L(12CO) and L(13CO), respectively]. It is shown that there is a very tight correlation between L(12CO) and L(FIR), as found in many previous studies. However, we find that the 13CO luminosities of the high-R galaxies are lower by a factor of 3 on 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 13CO line intensities.