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?

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.

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?

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.

Is this logic in keeping with consensus thinking?

Thanks,

Ross

[[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.)
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]]