Contents List of Figures

280 6 MACRO– AND MICRO–JETS DRIVEN BY BLACK HOLES
Cyg A
L_R = 10^45 erg/s
L_J = 10^46 erg/s
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3C295
___________ FR II / FR I Transition ___________
Figure 131: Luminosity of radio galaxies as a function of redshift. Open circles:
galaxies in the 3C catalog. The radio power varies over many orders of magnitude.
At high redshifts, only the most luminous sources with power comparable to Cyg A
can be detected.
On the FR I – FR II Transition: Many explanations have been given for the
explanation of the FR I / FR II transition. The most widely known explanation is
that, while the jets in both cases start out moving at very high (relativistic speeds),
those in FR II sources remain that way out to multi–kpc distances, while those in
FR I’s decelerate to much slower speeds within a few kpc of the galaxy core. Bicknell
(1995) developed detailed models for decelerating relativistic jets.
An alternative approach assumes that the FR I and FR II sources differ primarily
in the importance of the beam thrust relative to the basic parameters of the ambient
medium (Gopal–Krishna & Wiita 1988; 2001). In this version, the emphasis is on
the slowing down of the advance of the hotspot at the end of the jet, rather than
the slowing down of the bulk flow of the beam. In this scenario, the hotspots of FR
II’s have supersonic advance speed with respect to the ambient medium. When this
advance speed becomes transonic relative to the ambient medium, its Mach disk
weakens due to the fall in ram pressure, and the jet becomes decollimated.