Resolved spectral variations of the centimetre-wavelength continuum from the rho Oph W photo-dissociation-region

TL;DR

This study uses high-resolution ATCA observations to analyze spectral variations in the centimetre-wavelength continuum of the rho Oph W PDR, revealing spectral steepening consistent with spinning dust emission and variations in grain size.

Contribution

First resolved spectral analysis of the rho Oph W PDR at centimetre wavelengths, linking spectral variations to dust grain size and emission mechanisms.

Findings

Spectral steepening observed at higher frequencies indicating a minimum grain size of ~6 Å.

Morphological differences in continuum emission correlate with UV source proximity.

Resolved filament width and spectral variations support spinning dust emission models.

Abstract

Cm-wavelength radio continuum emission in excess of free-free, synchrotron and Rayleigh-Jeans dust emission (excess microwave emission, EME), and often called `anomalous microwave emission', is bright in molecular cloud regions exposed to UV radiation, i.e. in photo-dissociation regions (PDRs). The EME correlates with IR dust emission on degree angular scales. Resolved observations of well-studied PDRs are needed to compare the spectral variations of the cm-continuum with tracers of physical conditions and of the dust grain population. The EME is particularly bright in the regions of the rho Ophiuchi molecular cloud (rho Oph) that surround the earliest type star in the complex, HD 147889, where the peak signal stems from the filament known as the rho Oph-W PDR. Here we report on ATCA observations of rho Oph-W that resolve the width of the filament. We recover extended emission using a variant of non-parametric image synthesis performed in the sky plane. The multi-frequency 17 GHz to 39 GHz mosaics reveal spectral variations in the cm-wavelength continuum. At ~30 arcsec resolutions, the 17-20 GHz intensities follow tightly the mid-IR, Icm propto I(8 um), despite the breakdown of this correlation on larger scales. However, while the 33-39 GHz filament is parallel to IRAC 8 mum, it is offset by 15-20 arcsec towards the UV source. Such morphological differences in frequency reflect spectral variations, which we quantify spectroscopically as a sharp and steepening high-frequency cutoff, interpreted in terms of the spinning dust emission mechanism as a minimum grain size a_cutoff ~ 6 +- 1A that increases deeper into the PDR.