In article ,
wrote:
No matter whether they chose beaming/shielding or a short
lifetime for each quasar (perhaps including low duty-cycles
of on/off radiation) they have a major problem: all these
explanations involve the actual number of quasars being very
much larger than is usually estimated.

We discussed this in the newsgroup last year, and I pointed out then
that there really isn't a problem with short duty cycles for quasars.
Here are some further thoughts:

1. The radio galaxy observations that you rely on don't put a very strong
*direct* constraint on the ages of the underlying AGN (must be 10^6
years in a few cases -- though the best estimates of the ages of those
systems are longer) and in any case there's a selection effect towards
long-lived systems in radio galaxies, which are selected on their
luminosity integrated over time.

2. Short duty cycles for quasars do *not* imply the `actual number of
quasars being very much larger than is usually estimated'. In fact,
they're entirely consistent with the consensus that all massive
galaxies host central massive black holes. In that scenario we might
well expect that every massive galaxy has a good chance of hosting a
number of AGN events in its lifetime (of varying duration and
luminosity -- these will depend on the availability of mass to fall
onto the central black hole and on the rate at which it can do so).

3. Even if point 2 were not correct, which it is, there is no way of
firing up `big bang theory' and predicting the `abundance of quasars'
as you imply. `Big bang theory' has *nothing to say* about the
abundance of quasars. Galaxy formation models need to produce the
required central supermassive black holes on a sensible timescale, and
that *is* an interesting problem, but we are a long way off having the
tools to go directly from the state of the universe on the largest
scales to the dynamics of the matter around a central AGN on scales
comparable to that of the solar system, and that's what we'd need to
do in order to use the properties of AGN to overthrow the entire
theory.

Martin
--
Martin Hardcastle
School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK
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