SIMBAD references

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.