The Intrinsic Sizes of Odd Radio Circles

TL;DR

This study investigates the nature of Odd Radio Circles (ORCs) by spectroscopically analyzing their central galaxies, revealing their large sizes, galaxy types, and potential origins involving AGN activity or blastwaves.

Contribution

First spectroscopic confirmation of the distances and properties of galaxies associated with ORCs, providing insights into their sizes and possible formation mechanisms.

Findings

ORCs have large intrinsic diameters of 300-500 kpc.

Central galaxies are massive, old ellipticals with LINER spectra and low-luminosity AGN.

Some ORCs may be related to AGN jets or blastwaves from starbursts.

Abstract

A new class of radio source, the so-called Odd Radio Circles (ORCs), have been discovered by recent sensitive, large-area radio continuum surveys. The distances of these sources have so far relied on photometric redshifts of optical galaxies found at the centers of or near the ORCs. Here we present Gemini rest-frame optical spectroscopy of six galaxies at the centers of, or potentially associated with, the first five ORC discoveries. We supplement this with Legacy Survey imaging and Prospector fits to their griz+W1/W2 photometry. Of the three ORCs with central galaxies, all lie at distances (z = 0.27-0.55) that confirm the large intrinsic diameters of the radio circles (300-500 kpc). The central galaxies are massive ($M_*\sim10^{11}M_\odot$), red, unobscured ellipticals with old ($\gtrsim$1~Gyr) stellar populations. They have LINER spectral types that are shock- or AGN-powered. All three host low-luminosity, radio-quiet AGN. The similarity of their central galaxies are consistent with a common origin, perhaps as a blastwave from an ancient starburst. The other two ORCs are adjacent and have no prominent central galaxies. However, the z=0.25 disk galaxy that lies between them hosts a Type 2, moderate-luminosity AGN. They may instead be the lobes of a radio jet from this AGN.