Single- and double-headed odd radio circles in the LOFAR surveys

TL;DR

This study expands the catalog of odd radio circles (ORCs) using LOFAR data, revealing their diverse morphologies, associations with ellipticals, and size-spectral index relations, aiding understanding of their origins.

Contribution

The paper significantly increases the number of known ORCs, including new single- and double-headed examples, and analyzes their properties to understand their nature.

Findings

18 ORCs identified, including 4 double-headed and 2 new discoveries.

Confirmed association of ORCs with large elliptical galaxies.

Found a relation between ORC size and spectral index, with small ORCs avoiding steep spectra.

Abstract

Deep radio surveys are now producing catalogs with millions of radio sources. Radio sources can have complex morphologies that depend on both the production mechanisms and the local environment. Recently, an unusual class of circular radio sources named "odd radio circles" (ORCs) were identified. They have sizes of about 1 arcmin, a circular/elliptical shape and appear edge-brightened. Subsequent observations have suggested that this class may comprise a variety of sources. Despite various attempts to explain them, their origin remains unclear. The main goal of this work is to increase the number of known ORCs and derive common characteristics that can help identify the origin of these sources. We searched the LOFAR Two Metre Sky Survey (LoTSS) Data Release 3 (DR3) at 144 MHz for ORCs using a combination of parameter filtering on catalog entries and visual inspection. We then identified possible optical counterparts and derived information such as redshift, physical size, and spectral index using further radio data at 54 and 1400 MHz. We isolated 18 sources with ORC structures. Four of these are double-headed ORCs (ORCs with two rings), and two are new discoveries. We also found five new single-headed ORCs and nine candidate ORCs. With this work we significantly expand the population of known ORCs. Our findings confirm that ORCs are a rare and heterogenous population of radio sources. We confirm the association with large ellipticals in most cases, and we note a relation between the ORCs' physical sizes and their integrated spectral indexes, with small ORCs avoiding steep spectra.