Tailed Radio Galaxies: Cometary-Shaped Radio Sources in Clusters of Galaxies (Part 1)
Today’s post is written by Heinz Andernach (Univ. of Guanajuato, Mexico), a member of the Radio Galaxy Zoo science team and an expert on radio galaxies. This is the first half of a detailed science post explaining what we know — and what we don’t know — about tailed radio galaxies, along with how Radio Galaxy Zoo volunteers are helping us understand them. There’s a lot of information here, so if you have questions please ask them in the comments.
In 1968, Ryle and Windram found the first examples of a type of radio galaxies, whose radio emission extends from the optical galaxy in one direction in the form of a radio “tail” or “trail”. These examples were NGC 1265 and IC 310 in the Perseus cluster. Soon other examples were found in the Coma cluster of galaxies (NGC 4869, alias 5C4.81), as well as the radio galaxy 3C 129. The latter lies right in the plane of our Galaxy, and its membership in a cluster of galaxies was only confirmed much later, hampered by the dust obscuration of our own Milky Way. All three of these tailed radio galaxy were located close to another radio source, with their tails pointing more or less away from their radio neighbor, leading these authors to suspect that the tails were blown by winds of relativistic particles ejected from the radio neighbor. Soon thereafter higher-resolution observations of these sources revealed that the host galaxies showed the same two opposite radio jets as known from other, less bent, radio galaxies like Cygnus A, but close to the outskirts of the optical galaxies the jets would both bend in some direction, thought to be the direction opposite to the motion of the host galaxy through the so-called “intergalactic” or “intracluster” medium, which had been discovered from X-ray observations at about the same time. Later on, doubts were cast on this scenario, as it was found that the optical hosts of tailed radio galaxies in clusters did not move about within their clusters with high enough velocities to explain the bends in the jet.
A detailed study of the prototypical WAT source 3C 465 (image above) at the center of the rich Abell cluster of galaxies A2634 by these authors did not result in any plausible explanation for the bending of their radio trails. Later on, with more detailed X-ray images of clusters of galaxies, it was found that tailed radio galaxies occur preferentially in high-density regions of the intracluster medium (i.e. where the X-ray intensity is high; for more information, see this 1994 study) and even cluster mergers were made responsible for the formation of radio tails in this 1998 study. More recently authors seem to converge on the compromise idea that the combination of high ambient density and modest speeds of the host galaxy with respect to the ambient medium are able to produce the bends, but in this blog I would rather like to concentrate on the variety of morphologies shown by these objects in order to help RGZ users to classify them.
Above all, the term “tailed radio galaxies” should never be separated from the word “radio”. Some authors talk about “tailed” or “head-tail (HT) galaxies”, but the tail always occurs in their radio emission, usually far beyond the optical extent of their host galaxies. So, let us reserve the term “head-tail galaxies” for a future when optical tails may be detected in certain galaxies. Also, please note that while galaxies sometimes show optical tails due to tidal interactions, these are of totally different origin than the radio tails we discuss here.
Tailed radio galaxies are often subdivided into wide-angle (WAT) and narrow-angle tailed (NAT) radio galaxies, referring to the opening angle between the two opposite jets emanating from the nucleus of the optical galaxy, where we expect the supermassive black hole doing its job of spewing out the jets. However, the distinction between WATs and NATs depends strongly on the angular resolution and/or the distance to the radio source. E.g., the first HT radio galaxy to be discovered (NGC 1265; images below) may be called a WAT at high resolution, but appears as a (much larger) NAT at lower resolution, shown by this sequence of high, medium and low resolution radio images: