The molecular connection to the FIR-radio continuum correlation in galaxies

We have studied the relationships between the radio continuum (RC) and CO emission for a set of galaxies selected from the BIMA Survey of Nearby Galaxies. We find that the global CO-RC correlation is as tight as the global FIR-RC correlation for the 24 galaxies studied. Within 9 galaxies with ∼6″ CO and RC data available, the CO and RC emission is as tightly correlated as its global value; the radially averaged correlation is nearly linear, extends over four order of magnitude and holds down to the smallest linear resolution of the observations, which is ∼100 pc. We define q_CO/RC as the log of the ratio of the CO to RC flux as a way to characterize the CO-RC correlation. Combining 6″ pixel-by-pixel comparisons across all sources yields an average small-scale correlation of q_CO/RC = 1.1±0.28; that is, the spatially resolved correlation has a dispersion that is less than a factor of 2. There are however systematic variations in the CO/RC ratio; the strongest organized structures in q_CO/RC tend to be found along spiral arms and on size scales much larger than the resolution of the observations. We do not measure any systematic trend in CO/RC ratio as a function of radius in galaxies. The constancy of the CO/RC ratio stands in contrast to the previously measured decrease in the FIR/RC ratio as a function of radius in galaxies. We suggest that the excellent correlation between the CO, RC and FIR emission in galaxies is a consequence of regulation by hydrostatic pressure; this model links all three emissions without invoking an explicit dependence on a star formation scenario.