First a hearty congratulations to all the theorists who predicted that
accretion would become inefficient below L ~ 0.01
LEdd. Apparently the initial argument to this effect
is that of
Rees et al 1982.
Parts of the theory were anticipated by
Shapiro et al 1976
and
Ichimaru 1977.
More recently,
Esin et al 1997
quotes 0.4 
2LEdd as the cutoff luminosity
in the prescription,
which they find fits well with black hole binary state changes.
The first radio galaxy discovered and interferometrically mapped was Cygnus A, which has a whopping apparent brightness of 8700 Jansky at 178 MHz. At low frequencies, you can actually point the VLA 90 degrees away from Cygnus A and map it in a sidelobe! (F. Owen, pc 2010.) Also interesting: considering the volume enclosed by the Cyg A distance and the very strong cosmological evolution of FR II radio sources, folklore says only one universe in 10,000 should be so lucky as to have such a prize!
The first map of Cygnus A is very impressive for the time, and the map is very charming (Fig. 11 here, from Jennison and Das Gupta 1953). There is no question that it has a hidden Quasar of moderate luminosity (Antonucci et al 1994; Ogle et al 1997; infrared fluxes from NED). See Barthel and Arnaud 1996 to find out how such a modest Quasar can be so incredibly powerful in the radio.
 |
Figure 11. Approximate intensity distribution of the extra-terrestrial radio source in Cygnus. (Jennison & Das Gupta 1953). This very early interferometric observation first revealed the double-lobed nature of most radio galaxies. |
We know now that some powerful radio galaxies, and many weaker ones,
lack detectable visible or hidden thermal AGN. It is however very important
to remember that in all but a couple of contentious cases, Type 2
radio quiet Quasars have hidden AGN,
until you get to the LINER (very low luminosity) regime, which then
shows ADAF behavior according to most investigators
(Elitzur and Shlossman
2006;
Chiaberge et al 2000;
Ho 2009;
many others), along with weak broad wings
to H in many cases.
The time has come to stop proving this! We have altogether quite settled the question. It's worthwhile exploring more parameter space, e.g., weaker radio/IR/optical/X-ray sources, and to follow up on individual interesting cases, but the overall pattern is clear now for all types of bright radio source.
So what should we do instead? The result of all this work is that we can now
hold many things constant while we vary just one thing, the tremendous
thermal emission
(Big Blue Bump). The two types must differ drastically in their structure
on relativistic scales. We are limited by our imaginations in how to take
advantage of this situation. My group is trying to determine how the
thermal/nonthermal states correlate with VLBI properties, i.e. jet
launching, collimation, and proper motion, to the extent that's possible
with current VLBI angular resolution, all while holding the large-scale
structure constant as far as we can discern. Another obvious
observation, which really requires next-generation X-ray telescopes to
do well, is to compare the reflection signatures of the two types. The
putative accretion disk
K- fluorescence line
isn't expected to be so broad or strong if there is no opaque accretion
disk at small radii. Next, AGN feedback in galaxy evolution might be
mediated by radiation pressure on dust in many cases, or else by PdV
work or particles from jets and lobes, or other mechanisms. Here we can
keep everything the same (?) as far as the latter go, but turn off the
radiation. Does it make a difference? These new data might actually help
with the physics of AGN and of galaxies, and not just the astronomy of
AGN. Enjoy!
Acknowledgements
Several astronomers have provided advice and unpublished data for this paper. These include S. Baum, M. Begelman, P. Best, O. Blaes, S. Buttliglione, M. Chiaberge, C.M. Gaskell, M. Gu, M. Haas, L. Ho, S. Hoenig, W. Keel, P. Kharb, M. Kishimoto, M. Kunert-Bajraszewska, C. Leipski, R. Maiolino, C. O'Dea, F. Owen, S. Phinney, D.J. Saikia, A. Singal, D. Whysong and B. Wills. C. Leipski, S. Hoenig, D. Whysong and P. Ogle provided unpublished figures. P. Ogle, S. Willner, and R. Barvainis suffered the most (after the author), locating many errors and ambiguities. To the extent that the text sounds literate, my editor/typist Debbie L. Ceder deserves credit.