Anomalous Microwave Emission from the HII region RCW175

TL;DR

This paper provides evidence for anomalous microwave emission in the RCW175 HII region, likely caused by spinning dust grains, based on high-resolution observations at 31 GHz that exceed expected free-free emission levels.

Contribution

The study presents new high-resolution 31 GHz observations of RCW175, confirming excess emission consistent with spinning dust, advancing understanding of anomalous microwave emission sources.

Findings

Detected 31 GHz flux density of 5.97 Jy in RCW175

Observed excess emission 8.6 sigma above free-free predictions

Attributed excess to electric dipole emission from spinning dust

Abstract

We present evidence for anomalous microwave emission in the RCW175 \hii region. Motivated by 33 GHz $13\arcmin$ resolution data from the Very Small Array (VSA), we observed RCW175 at 31 GHz with the Cosmic Background Imager (CBI) at a resolution of $4\arcmin$. The region consists of two distinct components, G29.0-0.6 and G29.1-0.7, which are detected at high signal-to-noise ratio. The integrated flux density is $5.97\pm0.30$ Jy at 31 GHz, in good agreement with the VSA. The 31 GHz flux density is $3.28\pm0.38$ Jy ($8.6\sigma$) above the expected value from optically thin free-free emission based on lower frequency radio data and thermal dust constrained by IRAS and WMAP data. Conventional emission mechanisms such as optically thick emission from ultracompact \hii regions cannot easily account for this excess. We interpret the excess as evidence for electric dipole emission from small spinning dust grains, which does provide an adequate fit to the data.