AN ISOPHOT STUDY OF THE DISK OF GALAXY NGC-6946 - 60 MU-M INFRARED AND RADIO-CONTINUUM CORRELATION

We use the ISOPHOT 60 mu m image of Tuffs et al. (1996) to study the 6 0 mu m light distribution and its correlation with radio continuum wit hin the disk of spiral galaxy NGC 6946, and to resolve the long-standi ng IRAS controversy as to whether the infrared-to-radio continuum rati o, Q, decreases radially out in galaxy disks. Our main results are: (1 ) The disk at 60 mu m also appears to follow an exponential fall-off. Its e-folding scale-length lies between 1.3' and an upper limit of 1.9 ', obtained from the mean radial profile between 1.5' and 4' in radius . This scale length is smaller than that derived from the radio contin uum data at either 20 cm or 6 cm, with or without the thermal componen t removed, at a significance level of greater than or similar to 2 sig ma. (2) From 1.5' to 4' in radius, the 60 mu m-to-radio continuum surf ace brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on averag e. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al. (1992) based o n IRAS CPC data. Our results also fit into the picture of the radio di sk as a ''smeared'' version of the infrared disk, but are not consiste nt with the picture in which the variation of Q is a result of the the rmal and non-thermal radio components having two distinct, constant va lues of the infrared-to-radio ratio.