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Old February 22nd 09, 01:28 PM posted to sci.astro,sci.physics,sci.physics.relativity
Ian Parker
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Posts: 2,554
Default Questions on the enigmatic rotational curve of spiral galaxies

On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote:
I just want to add a thought experiment to help us visualise what is being
suggested/asked.

Consider a number of stars arrayed in a line across space, say a few light
years in length.

Assuming the initial condition is one of a stationary motion of each star
relative to the other, what would we expect to occur next?

I assume that the mutual attraction of the stars will cause the row of stars
to shorten until they end up clumped together.

So to make my 'barred' galaxy stable I rotate it fast enough so that the
stars on the two ends don't proceed toward or away from the rotational
centre.

Will this work? *I assume it will not work close to the rotational centre
but will work further out.

There is a greater gravitational pull on objects closer to the two ends
because there is more mass between those objects and the centre, but the
rotational speed is greater as well - nicely balanced

Now we add more mass to the centre. *Objects toward the ends of the arms are
going to be drawn inwardly unless the arm describes an arc. *Now the pull
directly from the central mass can be added to the less effective pull of
the curved arm and the galaxy is again stable.

Why do I think this has been missed?
Models consist of known and unknown quantities, such as the numbers of and
masses of the stars that can be observed. *But unobserved is the central
mass which is estimated. *The central mass, I assume, is estimated at far
higher than it actually is. *I don't think modellers have considered
relatively tiny central masses, as in my barred galaxy model above. *Thus a
higher central mass then requires a greater mass beyond the galaxy for it to
rotate as observed.

I do not have the skills to test these ideas but I assume that either:
It has already been considered and shown to be flawed/viable or
members of this list can estimate the viability of the idea using a
simplified model (as I have suggested above).

http://www.google.co.uk/search?hl=en...tation&me ta=

All galaxies seem to contain a supermassive black hole. This seems to
be required from the point of view of stability. A number of
simulations have been performed.

http://www.mpa-garching.mpg.de/galform/

This is a website for one of the foremost groups.


- Ian Parker