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

TL;DR

This study uses spectroscopic observations between 4 and 8 GHz at the Green Bank Telescope to constrain the mechanism behind anomalous microwave emission in the S140 star forming region, supporting the spinning dust model.

Contribution

First detailed spectroscopic measurements in this frequency range for S140, providing evidence favoring spinning dust as the source of AME in this region.

Findings

Spinning dust peaks at approximately 31 GHz, explaining the excess emission.

Morphology indicates spinning dust grains are concentrated near S140.

If free-free emission explains AME, the region would contain ultra or hyper compact HII regions.

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 $1^\circ$ 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 $30.9 \pm 1.4$ 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 $5.27^{+2.5}_{-1.5}\times 10^8$ $\rm{cm^{-6}\,pc}$ and a physical extent of $1.01^{+0.21}_{-0.20} \times 10^{-2}\,\rm{pc}$. This result suggests the HII would have to be ultra or hyper compact to remain an AME candidate.