The 60 Micron to 20 Centimeter Infrared-to-Radio Ratio within Spiral Galaxies

October 1990 • 1990ApJ...362...59B

Authors • Bicay, M. D. • Helou, G.

Abstract • A detailed comparison of the distributions of 60 {micron} infrared and 20 cm radio continuum emission within 25 galaxies, mostly disk spirals, has been performed. Local maxima in the thermal infrared and predominantly nonthermal radio maps of the nearest galaxies (D < 4 Mpc) are found to be spatially coincident on scales <~ 0.4 kpc (where H_0_ = 50 km s^-1^ Mpc^-1^ is assumed throughout). Superposed on this broad correlation, we observe in the disks of most sample galaxies a slow decrease in the 60 micron-to-20 cm ratio Q_60_ with increasing radius. Values of Q_60_ within the central regions are often enhanced by a factor of 2-3 with respect to values in the outer disks, whereas the corresponding enhancement in radio surface brightness is greater by at least an order of magnitude. The radial gradient in Q_60_ is most easily identified in nearby, face-on galaxies (e.g., M83, NGC 6946) due to the limited IRAS angular resolution. However, the gradient is also observed along the major axis of highly inclined systems (e.g., NGC 55). To account for the observations, it is proposed that spiral galaxies are characterized by an infrared disk with a shorter scale length than that of the radio continuum disk, the latter being smeared as a result of cosmic-ray propagation. We introduce a model that successfully accounts for the observed radio disks and the Q_60_ radial gradients based on the assumptions of steady-state star formation activity within the disk on kilo-parsec scales and a tight coupling between the origins of the dust-heating radiation and the radio-emitting cosmic- ray electrons (as suggested by the integrated infrared-radio correlation). The underlying source distribution is described as an exponential disk and is smoothed with a smearing function to obtain the observed radio continuum disk. It is found that a Gaussian smearing function yields excessively broad radio disks, indicating that diffusion dominated by a random walk process cannot by itself describe the temporal evolution of cosmic rays. An exponential smearing function provides a better fit, suggesting that escape of cosmic rays from the disk in a stochastic manner may be an important process. Exponential scale lengths for the best-fit smearing functions are typically 1-5 kpc, similar to scale lengths derived for the 60 micron infrared disks.

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George Helou

IPAC Executive Director