Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

6.5 CO-IR Luminosity Correlation

A large number of CO-IR comparisons have been reported in the literature (Rickard & Harvey 1983, Young et al 1984, Sanders & Mirabel 1985, Young et al 1986a, b, Sanders et al 1986, 1987, Tacconi & Young 1987, Solomon & Sage 1988, Kenney & Young 1988b, Young et al 1989, Tinney et al 1990, Sanders et al 1991). These correlations exhibit at least an order of magnitude of scatter, which is much more than that expected on the basis of measurement uncertainties.

Figure 7 shows a plot of the infrared (IR) and CO luminosities for more than 200 galaxies from several studies. The CO luminosities were derived as described in Kenney & Young (1988a), whereas the infrared luminosities are based on coadded IRAS data and computed following the method of Lonsdale et al (1985). The values of the ratio L_IR / M (H₂) range from 2 to 200 L M^-1 with the higher values occurring in interacting and merging galaxies (see Sections 8-9 and Figure 11) and in starburst irregular galaxies. In Figure 7, where the galaxies are coded by dust temperature, it is clear that the ratio L_IR / M (H₂) is closely correlated with dust temperature. Such a separation by dust temperature is not found for the comparison of IR luminosities and HI masses in the IR bright galaxies, thereby strongly suggesting that the dust emission in these galaxies is more closely tied to the molecular than to the atomic gas content. The correlation of the T_d with the ratio L_IR / M (H₂) can be understood, since a higher radiation energy density (i.e. L/M) leads to a higher radiative equilibrium dust temperature.

Figure 7. Comparison of total H2 masses and IR luminosities for galaxies from several studies (Stark 1979, Young et al 1986a, Sanders et al 1986, Tacconi & Young 1987, Solomon & Sage 1988, Kenney & Young 1988a, Young et al 1989, Thronson et al 1989a, Tinney et al 1990, Sanders et al 1991). The H2 measurements included consist of major axis maps for galaxies larger than 3', and single observations for the smaller galaxies. Points are coded by dust temperature, with stars for Tdust > 40K; triangles for Tdust between 30 and 40K; and circles for Tdust < 30K. The diagonal lines shown represent LIR / M (H2) = 1 and 100 L / M.