A SURFACE BRIGHTNESS CORRELATION BETWEEN CARBON-MONOXIDE AND NONTHERMAL RADIO-CONTINUUM EMISSION IN THE GALAXY

The relation between the projected face-on velocity-integrated CO (1-0) brightness I(CO) and the 20 cm nonthermal radio continuum brightness T20 is examined as a function of radius in the Galactic disk. Averaged in 1 kpc annuli, the ratio I(CO)/T20 is nearly constant with a mean value of 1.51 +/- 0.34 km s-1 from 2 to 10 kpc. This value is very close to that reported recently for the disks of eight normal spiral galaxies (Adler, Allen, & Lo), where [I(CO)/T20] = 1.3 +/- 0.6 km s-1 in spite of the fact that the values of surface brightness in CO and radio continuum in the sample vary by more than a factor of 100. The manner in which I(CO) and T20 are derived for the Galaxy is different in several significant respects from the more direct observational determinations possible in nearby galaxies. The fact that the Galaxy also follows this correlation further strengthens the generality of the result.