90 GHz continuum observations of messier 66

Abstract

Radio emission at around 90GHz from star-forming galaxies is expected to be strongly dominated by the free-free component due to ionizing radiation from massive, short-lived, stars. We present high surface-brightness sensitivity observations at 90GHz of the nearby star-forming galaxy Messier 66 with resolution of about 10arcsec (corresponding to a physical scale of about 500pc) and analyse these observations in combination with archival lower frequency radio and mid-infrared measurements. We detect five regions of emission, four of which are well fitted by the models we adopt. For these four regions, we find that the free-free component indeed dominates the emission at 90GHz, making up 76-90per cent of the luminosity. However, the data are also consistent with all of the emission being due to free-free. The estimates of free-free luminosities agree, within measurement and decomposition errors, with star formation rates derived from lower radio frequencies and mid-infrared observations. In our analysis we consider both power-law and curved spectra for the synchrotron component but do not find evidence to support the curved model in preference to the power law. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.