  Radio emission at around 90 GHz from star-forming galaxies is expected to be
strongly dominated by the free-free component due to ionising 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 9 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. For the four regions for which the
observations support our models we find that the free-free component indeed
dominates the emission at 90 GHz, making up 76--90 per cent of the luminosity
at this frequency but with the data also consistent with all of the emission
being due to free-free. The estimates of free-free luminosities are also
consistent, 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.
