Annu. Rev. Astron. Astrophys. 1982. 20: 517-45 Copyright © 1982 by Annual Reviews. All rights reserved

5.2. Interrelations of Far-Infrared, Nonthermal Continuum, and CO Emission

Galaxies emitting bright nonthermal radio continuum emission from their disks also tend to be strong class I CO galaxies (Rickard et al. 1977c). This relationship presumably reflects the fact that the massive stars heating the gas to produce the CO emission are the same ones that become supernovae, which presumably provide the relativistic particles that, in turn emit the nonthermal radiation. If the formation of OB stars is thus responsible for the nonthermal emission, then the thermal radio emission from HII regions about such stars should also be coupled to the CO emission (e.g. Israel & Rowan-Robinson 1982). This expectation is indeed borne out by the available data. Figure 5 shows that the total radio continuum intensity from a galaxy correlates well with the CO emission intensity measured toward the center of the galaxy. which is roughly proportional to the total CO emission for class 1 galaxies. This relationship was examined in detail by Israel & Rowan-Robinson through point-by-point comparisons in several galaxies of the 21 -cm surface brightness and P(CO). For the most extensively mapped galaxies. the two quantities are well correlated. The mean ratio of the two. however, varies from galaxy to galaxy.

Heating of gas by luminous young stars should be accompanied by heating of the associated dust. Searches for extended far-infrared emission associated with the molecular clouds in galactic disks have been rather difficult, generally because infrared telescopes usually nutate by only a few arcmin, leaving the off-source beam still within the optical disk, and presumably within the infrared disk as well.

Smith (1982) recently constructed a 170µ map of M51, using a seven-element array detector flown aboard the NASA Gerard P. Kuiper Airborne Observatory. The map is shown in Figure 6. Emission is clearly detected over a region ~ 5' in diameter, with a surface brightness 15% of the central peak. The ratio of central-to-disk brightness thus matches the CO. The integrated disk emission accounts for two thirds of the total 170µ flux of the galaxy. The characteristic dust temperature is quite low (~ 17 K). The 170µ disk resembles the CO disk in its extent and general features (e.g. the northeast enhancement). However, specific structures in the two maps do not agree in detail (compare Figures 1 and 6). Presumably these structures occur where the gas content varies between being atomic and molecular, or where the gas and dust temperatures differ significantly.

Figure 6. Map of the 170µ continuum emission from M51, taken from Smith (1982). The dashed line indicates the extent of the mapped region; the contours are logarithmic, spaced by factors of 2, and range from 7 to 40 x 10-19 W m2 Hz-1 sr-1.