What are DRAGNs?

On the nature of DRAGNs

It would be nice to be able to give a neat definition of DRAGNs; for instance:
DRAGNs are clouds of radio-emitting plasma which have been shot out of active galactic nuclei (AGN) via narrow jets.

Unfortunately this says more than we really know. It would be more honest to say the the clouds look as if they have been shot out of the nuclei via jets... at least they do once you know the conventional theory. We'll give a more rigorous operational definition later, but the one above will do for now.

DRAGNs are one of several phenomena associated with AGN (see the glossary entry on AGN for a brief overview). For our purposes, we can think of AGN as "black boxes" which produce twin jets, pointing in opposite directions.

DRAGNs are made, so it seems, of the stuff that flows down the jets, the so-called "synchrotron plasma". We do not know exactly what this is. It radiates in the radio band (and sometimes at higher frequencies) via the synchrotron process, from which we can tell that it contains magnetic fields and cosmic-ray electrons. Since cosmic plasmas must be neutral, there must be also be protons or positrons in the jet. Near the AGN the jets are supersonic, in the sense that the flow speed is faster than the speed of sound in the jet plasma. As a result, shockwaves form easily, giving rise to small regions with high pressure, which radiate intensely. Further away from the AGN, the weaker jets become subsonic and turbulent. Both shocks and turbulence may help to give the cosmic rays their high energies, via the Fermi mechanism.

Radio image of a typical DRAGN, showing the main features.
Image by C. L. Carilli.
Cyg A with labels

Most of the radio emission from typical DRAGNs comes not from the jets themselves but from twin lobes, which are much broader clouds around and near the inferred path of the two jets. The synchrotron plasma in the lobes is believed to have been supplied over a long period through the jets. The jets themselves can sometimes be seen as narrow features threading the lobes. In powerful DRAGNs, small, bright hotspots are found near the end of each lobe; these are thought to mark the ends of the jets.

The standard interpretation of DRAGNs was proposed independently by Scheuer (1974) and Blandford & Rees (1974), and the underlying physical mechanism was broadly confirmed by numerical fluid dynamics in the 1980's. To understand DRAGNs, it is crucial to bear in mind that space is not a perfect vacuum. The jets travel out first through the atmosphere of the active nucleus, then through the interstellar medium of the host galaxy, then (if they get that far) through the successively lower densities and pressures of the outer halo of the galaxy,the intra-cluster medium of any surrounding group or cluster of galaxies, and out into the inter-galactic medium. --X-ray picture of cluster gas-- Although much of this gas is pretty tenuous even by astronomical standards, it is always denser than the jet plasma. This means the jets cannot flow freely away from the AGN, but must push their way through the external medium. As a result, the ends of the jets move outwards much more slowly than material flows up the jet. As envisaged by Scheuer and Blandford & Rees, the plasma arriving at the end of the jet is deflected back to form the lobe, which can be thought of as a large bubble surrounding the jet. The "end of the jet" is just the point where the jet collides with the surface of the lobe. If the jet is still supersonic at this point, this collision will take place through a system of strong shockwaves, and the resulting high-pressure region will be seen as the hotspot.

In practice jets do not get an easy ride through the lobe, and in some cases seem to disrupt before they reach the lobe surface (e.g. 3C 401). Jets are liable to buckle under their own internal stresses and are also buffeted by the lobe plasma, which is constantly churning around. This "heavy weather" is thought to be particularly violent near the hotspots, and the structure of these regions is probably changing all the time, although at a rate too slow to be apparent in a human lifetime.
Right:Numerical Simulation of a lobe: the image shows a cross-section of the gas density, using pseudo-colour. The high-density external medium appears red. The bowshock in the external medium is visible as a dark red arc in front of the lobe. The jet is a similar density to the lobe, and disrupts about half-way through it. You can look at a movie of this simulation (240 kbytes) and also a movie showing simulated radio emission (680 kbytes). All simulations by David A. Clarke. Picture of simulation

Jets in powerful DRAGNs are initially relativistic, with Lorentz factors of order 10. It may be that all jets in DRAGNs are this fast. The Lorentz factors are not constant along the jets, and the passage of weak jets from supersonic to subsonic flow is also a passage to essentially non-relativistic motion (since the speed of sound cannot exceed about half the speed of light). Because of their high speeds, jets in DRAGNs can have extremely low density and still carry massive amounts of kinetic energy: rough lower limits on the kinetic luminosity of jets from radio-loud quasars are comparable to the quasar's electromagnetic luminosity, equivalent to the conversion of several solar masses into energy each year. Such hyper-powerful DRAGNs are very rare, (although grossly over-represented in this Atlas due to Malmquist bias); the kinetic and electromagnetic luminosities of the common weak DRAGNs are a million or more times lower. The lobes of DRAGNs, where all this energy is eventually stored, are perhaps the emptiest places in the universe: an object like 3C 236 is a cavity in even the inter-galactic medium, with densities estimated as low as 1 particle per ?? cubic metres.

By their nature, DRAGNs grow in size as they get older. The smallest DRAGNs known are only a few tens of parsecs across and hence entirely contained in the active nucleus (there are none of these "compact symmetric objects" in this Atlas); at the other extreme, giant DRAGNs are the largest known objects; the largest of all, 3C 236, is 4 Mpc (13 million light-years) from end to end, dwarfing most clusters of galaxies. Typical powerful DRAGNs are around ten times smaller; still several times bigger than their host galaxies. Powerful DRAGNs have estimated lifetimes of order 20 million years; this makes them a brief outburst in the life of a galaxy (compare the 100 million years a star may take to orbit the galaxy's hub). On the other hand, weak DRAGNs are so common in the largest elliptical galaxies that the jets must be "on" essentially all the time.

Operational Definition

Because we are still piecing together the true story, a definition like the one given earlier runs the risk that if the theory is wrong, all the things we currently call DRAGNs will have to be reclassified. We prefer a more complicated definition in terms of directly observable features:
A DRAGN is a radio source containing at least one of the following types of extended, synchrotron-emitting structures: jet, lobe, and hotspot complex.

If a DRAGN is identified as such purely on the basis of containing hotspot complexes, there should be no more than two of them.

[The last condition is necessary to distinguish DRAGNs from starburst and normal galaxies, since the latter contain many compact sources (supernova remnants etc.) which meet our simple definition of "hotspot"].

Although some DRAGNs contain only one kind of extended structure, the vast majority contain lobes and either hotspots or jets (if not both).

In normal usage, hotspots are regarded as part of the lobes, but we have phrased the definition as we have because sometimes the hotspot is the only part of the lobe that is visible, in which case our definition of "lobe" cannot be checked.

There are generally two lobes in each DRAGN, although they often run into each other, at least in projection. Occasionally there is only one.

The name DRAGN refers to the double structure and implicitly to the active nucleus, seen in the radio as the compact core. These are very common features of DRAGNs but are deliberately excluded from the definition, so that some DRAGNs may be neither double nor associated with an active nucleus! This is because apparently one-sided objects exist (e.g. the quasar 3C 273) which are clearly DRAGNs in every other sense; and because classification as a DRAGN should not need to wait for identification of the active nucleus, which may be much more difficult than discovering the radio source in the first place. A corollary is that objects consisting of just a compact core (which are quite common) are not DRAGNs. This seems wise as in these objects there is too little information to be sure that we are dealing with the same phenomenon.

On the naming of DRAGNs

Ostensibly, DRAGN is an acronym for Double Radiosource Associated with Galactic Nucleus (Leahy 1993); however this expansion in no way constitutes a definition (see above).

In the past, DRAGNs have usually been called extragalactic radio sources. Unfortunately, this term also has a second, more literal meaning, namely an extragalactic object which is found in a radio survey of some region of sky (the absolutely literal meaning is hardly ever used, as just about every type of extragalactic object emits radio waves at some level). The confusion arose because in early radio surveys essentially all extragalactic sources were DRAGNs. However in modern deep surveys (e.g. Condon 1989), the majority of sources are believed to be starbursts, not DRAGNs.

There have been a number of previous attempts to separate DRAGNs from other extragalactic radio sources, which we have not adopted for various reasons. A common strategy is to define a class of "powerful radio sources", with a minimum power high enough to exclude starbursts. However, there is no sudden change in structure around the cutoff power typically used, and these low-power DRAGNs are spatially far more common than their powerful cousins. Thus this strategy sidesteps rather than solves the problem of isolating DRAGNs, and does so at the price of focussing on on the tip of the iceberg of the phenomenon as a whole. Condon calls our DRAGNs "monsters" (as opposed to starbursts), referring to the ultimate energy source, the "monster" at the heart of the AGN. In effect this identifies DRAGNs with AGN, whereas we would like to keep the two concepts distinct; partly because an object that is several hundred kpc across can hardly be described as part of a galactic nucleus, and partly because most AGNs (or monsters) do not produce detectable DRAGNs. Muxlow & Garrington (1991) call the two populations "A" and "B", but nobody can remember which is which. Nilsson et al. (1993) use "Double Radio Source", which is close to DRAGN except that it yields an unpronounceable acronym. We have also seen the term "Classical Double" pressed into service to represent DRAGNs in general, but this normally has a much more restricted meaning (see the description of classical doubles in our section on classification).


typical i.e. the very best example we could find. The image shows Cygnus A, the brightest DRAGN in the sky. It is not in the Atlas sample, but we had to put a picture in somewhere.
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Last modified: 1997 February 3
J. P. Leahy