CORRELATIONS BETWEEN FAR-INFRARED, RADIO, AND BLUE LUMINOSITIES OF SPIRAL GALAXIES

Accurate far-infrared (FIR) and 1.49 GHz radio flux densities of optically selected spiral and irregular galaxies brighter than B(T) = +12 and of infrared-selected galaxies stronger than S = 5.24 Jy at 60-mu-m were used to derive FIR-radio luminosity correlations for two statistically complete samples spanning a wide luminosity range. The observed correlations are tight but not linear, and the observed scatter is primarily intrinsic, with negligible broadening from measurement errors. Published methods to improve (linearize and tighten) the observed FIR-radio correlation by subtracting the (presumed) radio-quiet infrared cirrus or by compensating for cosmic-ray diffusion losses do not work for both samples simultaneously. An empirical correction to the radio luminosities based on the observed blue luminosities was found that makes the corrected FIR-radio correlation linear and eliminates most of the residual variance. We propose a two-component model for the heating luminosity within galaxies consistent with the improved correlation: (1) a radio-loud component containing young (less-than-or-similar-to 3 x 10(7) yr) massive (M greater-than-or-similar-to 8 M.) stars that heats the warm dust and contributes significantly to the diffuse interstellar radiation field that heats the cool cirrus dust plus (2) a radio-quiet component comprised of older stars with M < 8 M.. The present data cannot rule out either of the two physical interpretations of this improved correlation, but suggested radio flux-density measurements of selected galaxies at higher frequencies should be able to.