The ICO/TNT relation in the Galaxy and in other Galaxies

The relation between the projected face‐on velocity‐integrated CO(1‐0) brightness ICO and the 20‐cm non‐thermal radio continuum brightness T20 is shown to be virtually constant as a function of radius in the Galactic disk. Averaged in 1 kpc annuli, the ratio ICO/T20 has 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 8 normal spiral galaxies, where 〈ICO/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.An explanation of this correlation is given in the framework of the hypothesis that both ICO and TNT have a common origin in the local density of cosmic rays in galaxy disks. The inner disk of M31 is apparently deficient in cosmic rays; the molecular clouds there are likely to be very cold, and may have temperatures close to the cosmic microwave background, rendering them nearly invisible in emission.