SCORPIO-II: Spectral indices of weak Galactic radio sources

TL;DR

This paper introduces an algorithm to estimate spectral indices of Galactic radio sources from multiwavelength images, applied to the SCORPIO survey, revealing the nature and distribution of different thermal sources in the Galactic Plane.

Contribution

The paper presents a new algorithm for spectral index estimation from radio images and applies it to the SCORPIO survey, providing insights into Galactic source populations.

Findings

Reliable spectral indices are measurable for sources >40 times the noise level.

Galactic sources increase source density by 20% compared to extragalactic fields.

Distribution of spectral indices suggests presence of thermal and optically thick sources.

Abstract

In the next few years the classification of radio sources observed by the large surveys will be a challenging problem, and spectral index is a powerful tool for addressing it. Here we present an algorithm to estimate the spectral index of sources from multiwavelength radio images. We have applied our algorithm to SCORPIO (Umana et al. 2015), a Galactic Plane survey centred around 2.1 GHz carried out with ATCA, and found we can measure reliable spectral indices only for sources stronger than 40 times the rms noise. Above a threshold of 1 mJy, the source density in SCORPIO is 20 percent greater than in a typical extra-galactic field, like ATLAS (Norris et al. 2006), because of the presence of Galactic sources. Among this excess population, 16 sources per square degree have a spectral index of about zero, suggesting optically thin thermal emission such as Hii regions and planetary nebulae, while 12 per square degree present a rising spectrum, suggesting optically thick thermal emission such as stars and UCHii regions.