Relics of Double Radio Sources

TL;DR

This paper identifies a new sample of dead double radio sources with steep spectra, lacking active nuclei, which are remnants of past AGN activity, providing insights into the lifecycle of radio galaxies.

Contribution

The study introduces a new sample of extended, inactive double radio sources with steep spectra, revealing their properties and implications for AGN lifecycle understanding.

Findings

Most sources are dead double radio sources with no active cores.

Spectral steepness indicates cessation of AGN activity 15-100 million years ago.

These sources were once much brighter, comparable to active radio galaxies.

Abstract

We have formed a new sample which consists of extended extragalactic radio sources without obvious active galactic nuclei (AGN) in them. Most of these sources appear to be dead double radio sources. These sources with steep spectra ($\alpha < $ -1.8; S $\propto \nu^{\alpha}$) were identified using the 74 (VLSS) and the 1400 MHz (NVSS) surveys and further imaged using the Very Large Array (VLA) and the Giant Meterwave RadioTelescope (GMRT). The radio morphologies of these sources are rather unusual in the sense that no obvious cores and jets are detected in these sources, but, two extended lobes are detected in most. The mean redshift of 4 of the 10 sources reported here is $\sim$ 0.2. At a redshift of 0.2, the linear extents of the sources in the current sample are $\sim$ 250 kpc with their spectral luminosities at 1.4 GHz in the range 2-25 x 10$^{23}$ W Hz$^{-1}$. The steep spectra of these sources is a result of the cessation of AGN activities in them about 15 -- 100 million years ago. Before the cessation of AGN activity, the radio luminosities of these galaxies were $\sim$ 1000 times brighter than their current luminosities and would have been comparable to those of the brightest active radio galaxies detected in the local universe (L$_{1.4} \sim$ 10$^{27}$ W Hz$^{-1}$). The dead radio galaxies reported here represent the $'$tip of the iceberg$'$ and quantifying the abundance of such a population has important implications to the life cycle of the AGN.