GMRT observations of IC 711 -- The longest head-tail radio galaxy known

TL;DR

This study presents low-frequency GMRT observations of IC 711, revealing it as the longest known head-tail radio galaxy with complex morphology and spectral steepening along its extensive tail, challenging previous models of its motion.

Contribution

It provides the first detailed low-frequency imaging of IC 711, showing its unprecedented length and complex tail structure, and questions existing theories about its motion and tail formation.

Findings

IC 711 is the longest head-tail radio galaxy known, with a 900 kpc extent.

The tail exhibits sharp bends inconsistent with simple galaxy motion models.

Spectral steepening along the tail suggests aging of relativistic electrons.

Abstract

We present low-frequency, GMRT observations at 240, 610 and 1300 MHz of IC~711, a narrow angle tail (NAT) radio galaxy. The total angular extent of the radio emission, $\sim 22$ arcmin, corresponds to a projected linear size of $\sim 900$ kpc, making it the longest among the known head-tail radio galaxies. The objectives of the GMRT observations were to investigate the radio morphology, especially of the long tail structure, at low frequencies. The radio structure, especially initial $\sim 10$ arcmin of tail being a long straight feature, does not seem to be consistent with a simple circular motion around the cluster centre, as previously suggested in the literature. Two sharp bends after the straight section of the tail cast doubt on the prevailing idea in the literature that the long narrow tails represent trails left behind by the fast moving parent optical galaxy with respect to the cluster medium, as the optical galaxy could not have undergone such sharp bends in its path, under any conceivable gravitational influence of some individual galaxy or of the overall cluster gravitational potential. In fact the tail does not seem to have been influenced by the gravitational field of any of the cluster-member galaxies. The radio spectrum of the head, coinciding with the optical galaxy, is flat ($\alpha \stackrel{<}{_{\sim}}0.4$ for $S \propto \nu ^{-\alpha}$), but steadily steepens along the radio tail, with the end part of the tail showing the steepest spectrum ($\alpha \, {\sim}$ 4 -- 5) ever seen in any diffuse radio emission region.