Circular Mystery: Exploring Diffuse Emission Surrounding a Radio Galaxy with uGMRT and VLA Multiwavelength Observations

TL;DR

This study reports the discovery and multiwavelength analysis of a circular diffuse radio source, J1218+1813, associated with an elliptical galaxy, revealing steep spectral indices indicative of optically thin synchrotron emission and discussing its possible formation mechanisms.

Contribution

First detailed multi-frequency radio and optical analysis of the circular diffuse radio source J1218+1813, exploring its nature and formation scenarios.

Findings

Steep spectral index of 0.8 in the core, indicating optically thin synchrotron radiation.

Spectral index varies from 1.1 to 1.8 from inner to outer regions.

Identification of the host galaxy with a black hole mass of approximately 2.8×10^8 M⊙.

Abstract

We report the discovery of J1218+1813, a circular diffuse radio source detected in the Very Large Array (VLA) FIRST survey at 1400 MHz. The source has an angular diameter of approximately one arcminute, corresponding to a physical size of ~180 kpc, and is associated with an elliptical galaxy with a redshift of z = 0.139635. To investigate its nature, we conducted a comprehensive multiwavelength analysis spanning both high and low radio frequencies, utilizing the VLA in C-configuration at L, C and X bands, along with the upgraded Giant Metrewave Radio Telescope (uGMRT) at bands 3 (250-500 MHz), 4 (550-850 MHz), and 5 (1000-1460 MHz). A spectral study based on these multiwavelength observations reveals that the core of J1218+1813 exhibits a steep spectral index of 0.8, indicating that the emission is dominated by optically thin synchrotron radiation. The spectral index varies between 1.1 and 1.8, from the inner to the outer structure, with the steepest values observed at the periphery of the diffuse emission. An optical analysis of the central host galaxy using spectroscopic data is also performed. The estimated black hole mass is $2.8 \pm 0.8 \times 10^{8}$ M${_\odot}$, while the host galaxy has a stellar mass of 2.9 $\times$ 10$^{11}$ M${_\odot}$ and a stellar age of 8.58 Gyr. The identification of J1218+1813 is particularly significant because it provides insight into the mechanisms responsible for the formation of circular diffuse radio structures surrounded by an elliptical galaxy. Potential formation scenarios for J1218+1813 are discussed in this paper.