Too large and overlooked? Extended free–free emission towards massive star formation regions

Abstract

We present Australia Telescope Compact Array observations towards six massive star formationregions, which, from their strong 24 GHz continuum emission but no compact 8 GHzcontinuum emission, appeared good candidates for hypercompact HII regions. However, theproperties of the ionized gas derived from the 19 to 93 GHz continuum emission and H70a+ H57a radio recombination line data show the majority of these sources are, in fact, regionsof spatially extended, optically thin free–free emission. These extended sources were missedin the previous 8 GHz observations due to a combination of spatial filtering, poor surfacebrightness sensitivity and primary beam attenuation.We consider the implications that a significant number of these extended HII regions mayhave been missed by previous surveys of massive star formation regions. If the originalsample of 21 sources is representative of the population as a whole, the fact that six containpreviously undetected extended free–free emission suggests a large number of regions havebeen 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 ~0.5 Jy) from free–free emission has implications for dust masses derived from sub-mm flux densities. The largespatial scales probed by single-dish telescopes, which do not suffer from spatial filtering, areparticularly susceptible and dust masses may be overestimated by up to a factor of ~2.