Radio emission during the formation of stellar clusters in M33
  We investigate thermal and non-thermal radio continuum associated with the
early formation and evolution of Young Stellar Clusters (YSCs) selected by
their MIR emission in M33. For the first time in an external galaxy it has been
possible to identify radio counterparts to more than 300 star forming regions.
We proof the nature of candidate YSCs fully embedded in molecular clouds, by
recovering their associated faint radio continuum luminosities. Using the
Halpha line to identify free-free radio emission at 5 GHz in the more evolved,
partially exposed YSCs, we retrieve information on the relevance of magnetic
fields and cosmic rays across the M33 disk at 25 pc spatial scales. A
cross-correlation of MIR and radio continuum luminosities is established from
bright to very faint YSCs, with MIR-to-radio emission ratio showing a gradual
decline towards the outer disk, while the magnetic field is pervasive at all
radii. We establish and discuss the tight relation between radio continuum and
other star formation indicators, such as Halpha. This relation holds for
individual YSCs over four orders of magnitude as well as for molecular clouds
hosting YSCs. On average about half of radio emission at 5 GHz in YSCs is
non-thermal. For exposed but compact YSCs the non-thermal radio fraction
increases with source brightness, while for large HII regions the fraction is
lower and shows no clear trend. This has been found for YSCs with and without
identified SNRs and underlines the possible role of massive stars in triggering
particle acceleration through winds and shocks: these particles diffuse
throughout the native molecular cloud prior to cloud dispersal.
