Anomalous Microwave Emission in HII regions: is it really anomalous? The
  case of RCW 49
  The detection of an excess of emission at microwave frequencies with respect
to the predicted free-free emission has been reported for several Galactic HII
regions. Here, we investigate the case of RCW 49, for which the Cosmic
Background Imager tentatively (~ 3 sigma) detected Anomalous Microwave Emission
at 31 GHz on angular scales of 7'. Using the Australia Telescope Compact Array,
we carried out a multi-frequency (5 GHz, 19 GHz and 34 GHz) continuum study of
the region, complemented by observations of the H109$\alpha$ radio
recombination line. The analysis shows that: 1) the spatial correlation between
the microwave and IR emission persists on angular scales from 3.4' to 0.4",
although the degree of the correlation slightly decreases at higher frequencies
and on smaller angular scales, 2) the spectral indices between 1.4 and 5 GHz
are globally in agreement with optically thin free-free emission, however, ~
30% of these are positive and much greater than -0.1, consistently with a
stellar wind scenario, 3) no major evidence for inverted free-free radiation is
found, indicating that this is likely not the cause of the Anomalous Emission
in RCW 49. Although our results cannot rule out the spinning dust hypothesis to
explain the tentative detection of Anomalous Microwave emission in RCW 49, they
emphasize the complexity of astronomical sources very well known and studied
such as HII regions, and suggest that, at least in these objects, the reported
excess of emission might be ascribed to alternative mechanisms such as stellar
winds and shocks.
