Observations of Galactic star-forming regions with the Cosmic Background Imager at 31 GHz

TL;DR

This study uses 31 GHz observations to analyze Galactic star-forming regions, revealing dominant free-free emission, detecting excess and spinning dust emissions, and characterizing anomalous microwave emission properties.

Contribution

First detailed 31 GHz observations of specific Galactic regions, identifying emission components and quantifying anomalous microwave emission characteristics.

Findings

Most emission is due to optically thin free-free

Detected excess emission in NGC 6334 sub-region

Identified spinning dust component in W40

Abstract

Studies of the diffuse Galactic radio emission are interesting both for better understanding the physical conditions in our Galaxy and for minimising the contamination in cosmological measurements. Motivated by this we present Cosmic Background Imager 31 GHz observations of the Galactic regions NGC 6357, NGC 6334, W51 and W40 at $\sim$4$'$.5 resolution and conduct an investigation of the spectral emission process in the regions at 4$'$.5 and 1$^{\circ}$ resolution. We find that most of the emission in the regions is due to optically thin free-free. For 2 sub-regions of NGC 6334 and for a sub-region of W51 though, at 4$'$.5 resolution and at 31 GHz we detect less emission than expected from extrapolation of radio data at lower frequencies assuming a spectral index of $-$0.12 for optically thin free-free emission, at 3.3$\sigma$, 3.7$\sigma$ and 6.5$\sigma$ respectively. We also detect excess emission in a sub-region of NCG 6334 at 6.4$\sigma$, after ruling out any possible contribution from Ultra Compact HII (UCHII) regions. At 1$^{\circ}$ resolution we detect a spinning dust component in the Spectral Energy Distribution (SED) of W40 that accounts for 18$\pm$7 % of the total flux density in the region at the peak frequency of 37 GHz. Comparison with 100 ${\rm \mu m}$ data indicate an average dust emissivity for the sub-regions of $0.5\pm4.4$ $\mu$K(MJy sr$^{-1}$)$^{-1}$. Finally we translate the excess emission in the regions to an Anomalous Microwave Emission (AME) emissivity relative to the optical depth at 250 ${\rm \mu m }$. We find that this form of emissivity is independent of the AME significance and has a value somewhere in the order of 10$^4$ Jy.