Constraining the Anomalous Microwave Emission Mechanism in the S140 Star-forming Region with Spectroscopic Observations between 4 and 8 GHz at the Green Bank Telescope

Abstract

Anomalous microwave emission (AME) is a category of Galactic signals that cannot be explained by synchrotron radiation, thermal dust emission, or optically thin free-free radiation. Spinning dust is one variety of AME that could be partially polarized and therefore relevant for ongoing and future cosmic microwave background polarization studies. The Planck satellite mission identified candidate AME regions in approximately 1 • patches that were found to have spectra generally consistent with spinning dust grain models. The spectra for one of these regions, G107.2+5.2, was also consistent with optically thick free-free emission because of a lack of measurements between 2 and 20 GHz. Follow-up observations were needed. Therefore, we used the C-band receiver (4 to 8 GHz) and the VEGAS spectrometer at the Green Bank Telescope to constrain the AME mechanism. For the study described in this paper, we produced three band averaged maps at 4.575, 5.625, and 6.125 GHz and used aperture photometry to measure the spectral flux density in the region relative to the background. We found if the spinning dust description is correct, then the spinning dust signal peaks at 30.9 ± 1.4 GHz, and it explains the excess emission. The morphology and spectrum together suggest the spinning dust grains are concentrated near S140, which is a star forming region inside our chosen photometry aperture. If the AME is sourced by optically thick free-free radiation, then the region would have to contain HII with an emission measure of 5.27 +2.5 −1.5 × 10 8 cm −6 pc and a physical extent of 1.01 +0.21 −0.20 × 10 −2 pc. This result suggests the HII would have to be ultra or hyper compact to remain an AME candidate.

The Simons Observatory: Galactic Science Goals and Forecasts

“…It remains unknown whether AME is polarized (see Dickinson et al 2018, for a recent review), though searches are ongoing (e.g., Abitbol et al 2018). Current observational upper limits of both individual clouds (e.g., Génova-Santos et al 2017) and the large-scale diffuse emission (Macellari et al 2011;Planck Collaboration XXV 2016;Herman et al 2022) suggest it must have a polarization fraction of no more than a few percent.…”