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90 GHz continuum observations of Messier 66




Radio emission at around 90 GHz 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 90 GHz of the nearby star-forming galaxy Messier 66 with resolution of about 10 arcsec (corresponding to a physical scale of about 500 pc) 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 90 GHz, making up 76–90 per 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.