An ISOPHOT study of the disk of galaxy NGC 6946: 60 μm infrared and radio continuum correlation

We use the ISOPHOT 60 μm image of Tuffs et al. (1996) to study the 60/μm light distribution and its correlation with radio continuum within the disk of spiral galaxy NGC 6946, and to resolve the long-standing IRAS controversy as to whether the infrared-to-radio continuum ratio, Q, decreases radially out in galaxy disks. Our main results are: (1) The disk at 60 μm 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 continuum data at either 20 cm or 6 cm, with or without the thermal component removed, at a significance level of ≳ 2σ. (2) From 1′.5 to 4′ in radius, the 60/μm-to-radio continuum surface brightness ratio, Q, decreases by a factor of 1.5 to 1.7 on average. These quantitative results agree with that of Marsh & Helou (1995) using IRAS HiRes data, but not with that of Fitt et al. (1992) based on IRAS CPC data. Our results also fit into the picture of the radio disk as a "smeared" version of the infrared disk, but are not consistent with the picture in which the variation of Q is a result of the thermal and non-thermal radio components having two distinct, constant values of the infrared-to-radio ratio.