Contents List of Figures

6.2 Jets as Super(magneto)sonic Collimated Plasma Flows 275
hundreds of kiloparsecs across, several times bigger than their host galaxies. 1
DRAGNs are almost invariably associated with elliptical galaxies, rather than
with spirals. There appears to be a strong connection, still to be explained in detail,
between the processes that determine the bulge-to-disk ratio of the host galaxy and
the ability to form (and maintain) the active nuclear engine for the time it takes to
build up a large-scale DRAGN.
Ages of DRAGNs: Powerful DRAGNs have estimated lifetimes of order 20 million
years (estimated from synchrotron ages); 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. 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.
6.2 Jets as Super(magneto)sonic Collimated Plasma Flows
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 galactic 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.
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 (Fig. 129). 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
1 Patrick Leahy and Richard Strom have collected an Atlas of DRAGNs that includes images,
vital statistics and brief descriptions of each object. The atlas is served from Patrick’s web site at
Jodrell Bank http://www.jb.man.ac.uk/atlas/. The Atlas is a complete sample of all the DRAGNs
from the Third Cambridge Catalogue of Radio Sources (Second Revision, known as 3CRR), out
to a redshift of z=0.5.