J1420--0545: The radio galaxy larger than 3C236

TL;DR

The paper reports the discovery of J1420-0545, the largest known giant radio galaxy at 4.69 Mpc, with detailed radio and optical observations revealing its properties and suggesting a highly inhomogeneous intergalactic medium.

Contribution

This study presents the discovery and detailed analysis of the largest giant radio galaxy, including its size, spectral properties, and environmental conditions, surpassing previous records.

Findings

Largest giant radio galaxy at 4.69 Mpc

Low density of surrounding intergalactic medium

High expansion speed along jet axis

Abstract

We report the discovery of the largest giant radio galaxy, J1420-0545: a FR type II radio source with an angular size of 17.4' identified with an optical galaxy at z=0.3067. Thus, the projected linear size of the radio structure is 4.69 Mpc (if we assume that H_{0}=71 km\s\Mpc, Omega_{m}=0.27, and Omega_{\Lambda}=0.73). This makes it larger than 3C236, which is the largest double radio source known to date. New radio observations with the 100 m Effelsberg telescope and the Giant Metrewave Radio Telescope, as well as optical identification with a host galaxy and its optical spectroscopy with the William Herschel Telescope are reported. The spectrum of J1420-0545 is typical of elliptical galaxies in which continuum emission with the characteristic 4000A discontinuity and the H and K absorption lines are dominated by evolved stars. The dynamical age of the source, its jets' power, the energy density, and the equipartition magnetic field are calculated and compared with the corresponding parameters of other giant and normal-sized radio galaxies from a comparison sample. The source is characterized by the exceptionally low density of the surrounding IGM and an unexpectedly high expansion speed of the source along the jet axis. All of these may suggest a large inhomogeneity of the IGM.