Discovery of Peculiar Radio Morphologies with ASKAP using Unsupervised Machine Learning

TL;DR

This paper uses an unsupervised machine learning approach with ASKAP radio data to identify rare and peculiar radio source morphologies, including new odd radio circle candidates, by analyzing complex radio sources with a trained self-organizing map.

Contribution

It introduces an extension of the SOM algorithm that is rotation and flipping invariant for astronomical sources, enabling effective identification of rare radio morphologies in large surveys.

Findings

Discovered two new odd radio circle candidates.

Identified five other peculiar radio morphologies.

Demonstrated the effectiveness of an invariant SOM in classifying complex radio sources.

Abstract

We present a set of peculiar radio sources detected using an unsupervised machine learning method. We use data from the Australian Square Kilometre Array Pathfinder (ASKAP) telescope to train a self-organizing map (SOM). The radio maps from three ASKAP surveys, Evolutionary Map of Universe pilot survey (EMU-PS), Deep Investigation of Neutral Gas Origins pilot survey (DINGO) and Survey With ASKAP of GAMA-09 + X-ray (SWAG-X), are used to search for the rarest or unknown radio morphologies. We use an extension of the SOM algorithm that implements rotation and flipping invariance on astronomical sources. The SOM is trained using the images of all "complex" radio sources in the EMU-PS which we define as all sources catalogued as "multi-component". The trained SOM is then used to estimate a similarity score for complex sources in all surveys. We select 0.5\% of the sources that are most complex according to the similarity metric, and visually examine them to find the rarest radio morphologies. Among these, we find two new odd radio circle (ORC) candidates and five other peculiar morphologies. We discuss multiwavelength properties and the optical/infrared counterparts of selected peculiar sources. In addition, we present examples of conventional radio morphologies including: diffuse emission from galaxy clusters, and resolved, bent-tailed, and FR-I and FR-II type radio galaxies. We discuss the overdense environment that may be the reason behind the circular shape of ORC candidates.