Observations of Anomalous Microwave Emission from HII Regions

I give a summary of the observations of Anomalous Microwave Emission (AME) from HII regions. AME has been detected in, or in the vicinity of, HII regions. Given the difficulties in measuring accurate SEDs over a wide range of frequencies and in complex environments, many of these detections require more data to confirm them as emitting significant AME. The contribution from optically thick free-free emission from UCHII regions may be also be significant in some cases. The AME emissivity, defined as the ratio of the AME brightness to the 100？μm brightness, is comparable to the value observed in high-latitude diffuse cirrus in some regions, but is significantly lower in others. However, this value is dependent on the dust temperature. More data, both at high frequencies (>~5？GHz) and high resolution (~1′ or better) is required to disentangle the emission processes in such complex regions. 1. Introduction HII regions refer to the environment around the most (O and B type) massive stars, which are hot enough to produce intense UV radiation that can ionize the gas around them. HII regions typically form within large molecular clouds, often in clusters (due to triggered star formation), and are therefore are also associated with significant amounts of dust grains. Anomalous Microwave Emission (AME), if due to electric dipole radiation from spinning dust [1], requires a large column of dust grains (with a population of the smallest dust grains or PAHs) and a mechanism for rotationally exciting these grains, for example, plasma drag and photons. For these reasons, HII regions may be a good place to look for AME. In fact, there is evidence that photodissociation regions (PDRs) typically found around the edges of HII regions/molecular clouds might be good AME emitters [2, 3]. Counterarguments include the depletion of PAHs close in the centre of HII regions and the fact that they strongly emit in other forms of continuum emission, notably free-free (thermal bremmsstrahlung) and thermal dust radiation. In this paper, I give an overview of the the continuum radiation and current observations of AME from HII regions. I will discuss some issues with measuring AME from HII regions, including calibration, the contribution from ultracompact (UCHII) regions, and the definition of emissivity. 2. Observations of HII Regions 2.1. The SEDs of Classical HII Regions The general form of HII region SEDs (radio to the far infrared continuum) is thought to be well understood. Figure 1 shows the SED of the well-known Orion nebula (M42) HII region, measured by a number of different