On 06/02/2017 05:56, wrote:
On Tuesday, January 24, 2017 at 9:06:13 AM UTC-8, Martin Brown wrote:
On 23/01/2017 22:45, wrote:
On Friday, January 20, 2017 at 8:44:02 AM UTC-8, Martin Brown wrote:

Thanks for the links and comments.

Question: Is there evidence that material (gas and or stars) is
"raining" back down onto galaxies where the radio jets appear to
be old and fading?

I shouldn't expect so. The momentum of the jet carries the shocked
material along with it away from the galaxy probably forever. There will
be some backwash but it won't have anything like the velocity to carry
it back to the core. Rough indications from the shapes of radio lobes in
powerful FR II sources are perhaps 1/3 the way back at most.

In other words, suppose FR I galaxies with intense jets observed
near the central engine are "young" and FR II galaxies with wider
opening angles are "older", meaning, it's been longer since the
initial launch of the radio jets.

There is a significant difference between the power of the central
engine in FR I and FR II galaxies with the latter being pretty much the
biggest and most powerful things in the universe. Whilst I would think
it possible that an FR I might sometimes evolve into an FR II by either
excessive feeding or BH merger. I don't think any would go the other
way. A discussion of their characteristics and modern interpretation is
online he

https://ned.ipac.caltech.edu/level5/..._fanaroff.html

then, objects with fading radio lobes, and no central jets, are
taken to be older still.

(does this classification fit what people think?.....and)

Is there any evidence for material (gas and or stars) raining back
down onto the galaxies? And if so, does it fit that the above
sequence holds, with the matter raining back down being most
associated with the "oldest" systems, ie, longest time since initial
jet launch?

I get the feeling you are thinking about the jets as fountains rather
than as astrophysical things. The backwash does fill up some of the void
punched out by the jet but I can't see if has much scope for raining
down onto the galaxy. It is way too tenuous for that.

One of the early incorrect models for double radio galaxies were that
the lobes were effectively plumes of less dense hot plasma rising
buoyantly in the intergalactic medium. Like hot air does on Earth.

In other words, if excess angular momentum is what gives rise to
the jets, then, shutting down that angular momentum would kill the
jets. If matter is ejected along the polar axis, and then that
material later rains down onto the galaxy, the angular momentum of
the "rain" would be orthogonal to the angular momentum of the jets,
thus, shutting down the jets.

It isn't excess angular momentum causing the jets it is the very fact
that the BH is spinning and magnetised. This provides a way to extract a
huge amount of rest mass energy out of infalling matter some of which
gets converted to jets escaping at the *poles* like aurora in reverse
but carrying away very little angular momentum compared with what is
being the input by the accretion disk. Arguably it may well be spinning
up a central BH by accretion of gas and stars at the centre of the
galaxy that spins them up to the point where the FRII relativistic jets
kick in. Prior to that FRI's only have jets with a roughly comparable
energy to the IGM and so get blown around more like smoke on Earth.

Is this logic in keeping with consensus thinking?

Not really. You are guessing and still haven't provided us with some
idea of where to pitch explanations.

[[Mod. note -- I am not an expert in this area, but (following the
standard model of jet production from accretion onto a supermassive
central black hole):
1. The angular momentum which gives rise to the jet would be that of
the accretion disk very close to the central supermassive BH (say
within no more than 100 Schwarzschild radia). The jets would come
out just about radially (along the BH's rotation axis, perpendicular
to the accretion disk), and so carry little net angular momentum.
(Recall that purely radial motion has zero angular momentum.)

One of the classic papers on jets is from 1979 Blandford & Konigl
https://ned.ipac.caltech.edu/level5/..._fanaroff.html
(Blandford and Rees is behind a paywall for non-academic use)

2. I think jets usually start out going much faster than a galaxy's
(or even a galaxy cluster's) escape velocity. The jet then plows
into the interstellar medium in the galaxy, but I personally have
no idea of how much mass or linear/angular momentum ever returns
to the vicinity of the central BH. Is there enough to be a significant
influence on the accretion disk, magnetic field, or the BH's mass/spin?
Hopefully someone who knows the relevant modelling can provide
better-informed answers than my educated-guesses.
-- jt]]

I don't know for certain but my instinct is that once the radiation from
the central source really gets going its radiation pressure will expel
any gas and dust that isn't falling towards it in big chunks and quite
fast. This isn't a bad review for the OP with fewer equations:

https://arxiv.org/pdf/astro-ph/0301309v1.pdf

--
Regards,
Martin Brown