Too large and overlooked? Extended free-free emission towards massive
  star formation regions
  We present Australia Telescope Compact Array observations towards 6 massive
star formation regions which, from their strong 24 GHz continuum emission but
no compact 8 GHz continuum emission, appeared good candidates for hyper-compact
HII regions. However, the properties of the ionised gas derived from the 19 to
93 GHz continuum emission and H70 alpha + H57 alpha radio recombination line
data show the majority of these sources are, in fact, regions of
spatially-extended, optically-thin free-free emission. These extended sources
were missed in the previous 8 GHz observations due to a combination of
spatial-filtering, poor surface brightness sensitivity and primary beam
attenuation.
  We consider the implications that a significant number of these extended HII
regions may have been missed by previous surveys of massive star formation
regions. If the original sample of 21 sources is representative of the
population as a whole, the fact that 6 contain previously undetected extended
free-free emission suggests a large number of regions have been mis-classified.
Rather than being very young objects prior to UCHII region formation, they are,
in fact, associated with extended HII regions and thus significantly older. In
addition, inadvertently ignoring a potentially substantial flux contribution
(up to ~0.5Jy) from free-free emission has implications for dust masses derived
from sub-mm flux densities. The large spatial scales probed by single-dish
telescopes, which do not suffer from spatial filtering, are particularly
susceptible and dust masses may be overestimated by up to a factor of ~2.
