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274 6 MACRO– AND MICRO–JETS DRIVEN BY BLACK HOLES Megaparsecs from end to end. The lobes of the radio source shown below were discovered by a low-resolution all-sky survey made with the VLA. They were later shown to be a single DRAGN by observations that revealed the radio core and the jets connecting it to these lobes (left panel). The core coincides (central panel) with a galaxy (right panel) whose redshift is z=0.154. This reveals the double radio source to be over 4 Mpc in extent, and the second largest DRAGN known. The largest of them all, 3C236, is about 6 Mpc across. Typical powerful DRAGNs are Figure 128: Difference in morphology between FR I jet of 3C 31 and a typical quasar jet (3C 175 at redshift 0.768). Quasars show the most luminous jets with double sided lobe structure, but only one–sided jets.
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.
Page 1 and 2: CONTENTS 265 Contents 6 Macro- and
Page 3 and 4: 6 Macro- and Micro-Jets Driven by B
Page 5 and 6: 6.1 DRAG(o)Ns - FR I and FR II 269
Page 7 and 8: 6.1 DRAG(o)Ns - FR I and FR II 271
Page 9: 6.1 DRAG(o)Ns - FR I and FR II 273
Page 13 and 14: 6.2 Jets as Super(magneto)sonic Col
Page 15 and 16: 6.3 Fundamental Parameters for Jets
Page 17 and 18: 6.3 Fundamental Parameters for Jets
Page 19 and 20: 6.4 A 3-Phase Model for Kiloparsec-
Page 25 and 26: 6.5 Relativistic Jet Propagation 28
Page 27 and 28: 6.6 Structure and Emission of Micro
Page 37 and 38: 6.7 Formation of Micro-Jets 301 6.7
Page 39 and 40: 6.7 Formation of Micro-Jets 303 Fig
Page 41 and 42: 6.7 Formation of Micro-Jets 305 Thi
Page 43 and 44: 6.7 Formation of Micro-Jets 307 VLB
Page 45 and 46: 6.7 Formation of Micro-Jets 309 as
Page 53 and 54: 6.7 Formation of Micro-Jets 317 the
Page 55 and 56: 7.1 The Dark Age 319 Figure 152: Da
Page 57 and 58: 7.3 The First Quasars 321 Figure 15
Page 59 and 60: 7.3 The First Quasars 323 Bromm & L
Page 61:
REFERENCES 325 [17] Leahy, P. 1993,
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