A PHYSICAL MODEL OF THE INFRARED-TO-RADIO CORRELATION IN GALAXIES

We explore the implications of the infrared-radio correlation in star-forming galaxies, using a simple physical model constrained by the constant global ratio q of infrared to radio emission and by the radial falloff of this ratio in disks of galaxies. The modeling takes into account the diffusion, radiative decay, and escape of cosmic-ray electrons responsible for the synchrotron emission, and the full range of optical depths to dust-heating photons (tau < 1 to tau much greater than 1). We introduce two assumptions: that dust-heating photons and radio-emitting cosmic-ray electrons are created in constant proportion to each other as part of the star formation activity, and that ps and magnetic field are well coupled locally, expressed as B is-proportional-to n(beta), with 1/3 less-than-or-equal-to beta less-than-or-equal-to 2/3. We conclude that disk galaxies would maintain the observed constant ratio q under these assumptions if the disk scale height ho and the escape scale length l(esc) for cosmic-ray electrons followed a relation of the form l(esc) is-proportional-to h0(1/2); the infrared-to-radio ratio will then depend very weakly on interstellar density, and therefore magnetic field strength or mean optical depth. Scale heights for each phase of the interstellar medium (ISM) are observed to vary, little among disk galaxies, while a scaling like I(esc) is-proportional-to h0(1/2) can be reasonably expected to apply to the disks associated with the various phases of the ISM. In support of this expectation, we propose a specific confinement scheme for cosmic-ray electrons which is consistent with the physical model, and has an escape scale length practically independent of density, but scaling with h0. While more subtle effects may enter the picture and in the end determine the behavior of q, this treatment identifies the various terms that should contribute to the scatter in q in a simple physical picture.