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Galactic Bar Formation?



 
 
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  #1  
Old February 22nd 17, 09:03 PM posted to sci.astro.research
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Posts: n/a
Default Galactic Bar Formation?

It's been a while since I've explored this topic. Years ago no one
had posited this model and I was told it was wrong. I thought I'd
check if anything has changed since.

Has anyone posited the following model for creating a galactic bar?

Two galaxies merge. A pair of massive black holes (and stars)
become trapped in orbit at some kpc separation (the ends of the
bar).

A bar of stars forms between them, probably having orbits that
are co rotating and longitudinal. In a frame rotating with the
forming bar, it would appear the stars are moving essentially
along a line within the bar.

At some point enough stars are within the bar that the gravity of
the stars between the BH's causes the massive black holes to slide
down the bar to the center, where they orbit and merge.

We see the bar and we see a line of dust down each bar, that curves
into what appears to be a circular trajectory near the center.

See: https://en.wikipedia.org/wiki/Barred_spiral_galaxy

to see the dust lanes.

Any aspects matching (or refuting) this model been observed?

Thanks,

rt

[[Mod. note -- One big problem with this model is that massive black
holes aren't massive enough to dominate the stellar dynamics at kiloparsec
(= galactic-bar-size) distances. That is, on the size scale of a galactic
bar, even a "supermassive" black hole is still only a tiny fraction of
the mass in stars, interstellar gas/dust, and maybe dark matter.
-- jt]]
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  #2  
Old February 23rd 17, 04:47 PM posted to sci.astro.research
Martin Brown
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Posts: 1,707
Default Galactic Bar Formation?

On 22/02/2017 20:03, wrote:
It's been a while since I've explored this topic. Years ago no one
had posited this model and I was told it was wrong. I thought I'd
check if anything has changed since.


No. It's still wrong. The BH influence is confined to the small region
nearest the centre of the galaxy where they are in mutual orbit out to a
modest multiple of their separation. Fireworks will ensue when/if they
spiral into each other but apart from that once you are some distance
from their barycentre the total mass of everything else is the main
influence on the forces that a distant star encounters.

I suppose there is an outside chance that at certain distances from the
galactic centre resonance effects might cause stars to avoid certain
regions or orbital periods but it would be very difficult to spot.

Has anyone posited the following model for creating a galactic bar?

Two galaxies merge. A pair of massive black holes (and stars)
become trapped in orbit at some kpc separation (the ends of the
bar).

A bar of stars forms between them, probably having orbits that
are co rotating and longitudinal. In a frame rotating with the
forming bar, it would appear the stars are moving essentially
along a line within the bar.

At some point enough stars are within the bar that the gravity of
the stars between the BH's causes the massive black holes to slide
down the bar to the center, where they orbit and merge.

We see the bar and we see a line of dust down each bar, that curves
into what appears to be a circular trajectory near the center.

See:
https://en.wikipedia.org/wiki/Barred_spiral_galaxy

to see the dust lanes.

Any aspects matching (or refuting) this model been observed?


None that I can think of.

Moreover numerical simulations of the evolution spirals into barred
spiral galaxies tends to suggest that they are an inevitable consequence
of small pertubations of the whole ensemble as it tries to find a lowest
energy state after being shaken up by an interaction. eg.

https://arxiv.org/abs/1211.6754

[[Mod. note -- One big problem with this model is that massive black
holes aren't massive enough to dominate the stellar dynamics at kiloparsec
(= galactic-bar-size) distances. That is, on the size scale of a galactic
bar, even a "supermassive" black hole is still only a tiny fraction of
the mass in stars, interstellar gas/dust, and maybe dark matter.
-- jt]]

Indeed.

--
Regards,
Martin Brown
  #3  
Old February 26th 17, 05:41 AM posted to sci.astro.research
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Posts: n/a
Default Galactic Bar Formation?

On Thursday, February 23, 2017 at 7:47:38 AM UTC-8, Martin Brown wrote:
On 22/02/2017 20:03, wrote:
It's been a while since I've explored this topic. Years ago no one
had posited this model and I was told it was wrong. I thought I'd
check if anything has changed since.


No. It's still wrong. The BH influence is confined to the small region
nearest the centre of the galaxy where they are in mutual orbit out to a=


modest multiple of their separation.


Yes agreed / understood.


Moreover numerical simulations of the evolution spirals into barred
spiral galaxies tends to suggest that they are an inevitable consequence=


of small pertubations of the whole ensemble as it tries to find a lowest=


energy state after being shaken up by an interaction. eg.

https://arxiv.org/abs/1211.6754

Thanks for the reference, I'll check it out..........but,

we have Dark Matter and Dark Energy that are NOT accurately accounted
for in our theoretical models. It's possible they fit the ideas
currently used within such models, and, it's possible that the
models are wrong and new equations need to be discovered.

If the latter, then the simulations are wrong, period. So we need
to be open minded in our quest for what's really going on.



[[Mod. note -- One big problem with this model is that massive black
holes aren't massive enough to dominate the stellar dynamics at kilopar=

sec
(=3D galactic-bar-size) distances. That is, on the size scale of a gal=

actic
bar, even a "supermassive" black hole is still only a tiny fraction of
the mass in stars, interstellar gas/dust, and maybe dark matter.
-- jt]]

Indeed.


Yes.........Indeed, I fully agree. But that's not the point I am
going after.

Let's slightly change the idea to make more clear what I'm wondering
if anyone has seen.

Suppose in some way, two elliptical galaxies were in an orbit with
separation 100kpc so that they just have their outermost stars
coming into contact. Their orbital velocity is such that they are
locked in orbit and we'll ignore 3 body issues (maybe we suppose
the ejection of a 3rd galaxy allowed them to come into a close
stable orbit).

Anyway, we begin with two elliptical galaxies of 100kpc diameter
each with 100kpc separation distance. What would happen?

As these two galaxies orbit, some stars from either can fall toward
the other. This (highly idealized model) would naturally evolve
with a lot of time, into a bar. A real merger is messy, but, the
core of two galaxies might undergo this sort of thing and that's
what I'm looking for to test whether this might be so.

The stars within the bar wouldn't really be moving along a line,
because the bar would be rotating. so the stars moving along the
bar would be moving in ellipses....can you see this?

Eventually, after a lot of stars had fallen down into the bar orbital
path, (which by the way becomes much like stellar orbits within a
globular cluster where stars are moving along approximately radial
lines, and not in circular orbits) then the mutual galactic
gravitational potential would favor objects falling down along the
bar.....including the BH's.

So the BH's could just fall down along the bar that the stars had
formed...or more likely if this is so, the bar the stars were
beginning to form.

Make better sense? Anyone seen anything that looks at all like
stages in the above model?

rt

[[Mod. note -- Models of this sort have a long history. See, e.g.,
the classic 1972 paper by Toomre & Toomre,
http://adsabs.harvard.edu/abs/1972ApJ...178..623T
I've never heard of such a model producing a *bar* (they tend to
produce "bridges" and "tails", as in the Toomre & Toomre paper),
but I am not an expert on this area of research.
-- jt]]
 




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