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Barred galaxies mass distribution



 
 
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Old April 4th 07, 10:40 PM posted to sci.astro.research
Nicolaas Vroom
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Default Barred galaxies mass distribution

schreef in bericht
...
Nicolaas Vroom wrote:


Yes, it is a problem with MOND. It is just slightly inaccurate.


What do you mean by that ?
Do you mean that a0 could have a different value ?


No. Just that MOND the equation is not completely accurate.
The world is not newtonian so any newtonian formulation will
not be entirely accurate, and you will have effects that don't
make sense. Just read the n body document I gave you.
The reason it doesn't make sense is because of the newtonian
formulation of MOND. It explains things better as TeVeS, which
will not have such wierd aspects.

So you cannot apply it simply. You must think of where you
are applying it and then derive a form which will apply in that
case. eg. In a galaxy you must assume a disk with simple non-
uniformity and then derive a form that can be used. That is what
the accompanying papers do.

If you have to use individual stars for your application then you
must use TeVeS.


you wrote:
There are dozens of papers that
show you how to fit the data in galaxies.

I expect one of the documents is this one:
http://www.astro.umd.edu/~ssm/mond/mdlg.gif
which shows 12 rotation curves with MOND.

I want to understand those curves.

IMO generally speaking those curves should be based
on individual stars.
The same with Newton.
The problem is that that is not very practical.
So you have to combine stars in groups.
That is easy with Newton but more difficult with MOND

Outside a0 the acceleration of one star at distance r
is equal to a = sqr(M*g*a0) / r
If there is a group of n stars at distance r we get:
a = n * sqr(M*g*a0) / r

The problem is we can not replace this group
of n stars with total mass n * M
(With Newton you can)
with one star of mass n * M.
You have to use n * n * M
If you do that than
a = sqr(n * n * M*g*a0)/ r = n * sqr(M*g*a0) / r
which is the "correct" value for n stars of mass M.

That is what the scientists who made those
12 curves should have done (using MOND).

The problem is when you that the speeds
are becoming gargantuan and
and the speeds do not decrease at large
distances.

(Horrible solution: change a0)

There are dozens of papers that
show you how to fit the data in galaxies.


I have also studied those curves.
As I have mentioned I have two major problem.

First if the rotation curve is not flat but decreases
at larger distances than you can not simulate those
with MOND.
(MOND can only be used if the speed increases.)


Don't talk about distances. MOND does not work
with distances, it works with acceleration scale a0.


of course MOND works with distances.
See above.
a is a function of distance.

Are there any galaxies where rotation curves decrease
at distances where accelarations are less than a0?


Yes
At ranges where MOND applies
Yes.
Look at NGC 2903 NGC 4100
The speed decreases at larger distances.
You can not simulate that using MOND.

If you have found one, please publish your results,
Scientists have been searching for such results for
the last 25 years.


Secondly starting point is a certain visible (baryonic) mass
distribution.
With Newton you can calculate the rotation curve
but this curve does not match the measured curve.
Solution add an halo of darkmatter.

With MOND you should start with this same mass
distribution.
The problem is that the speeds (rotation curves) calculated
based on that assumption with MOND
(assuming that MOND is applicable at scales
larger than 0.1 ly See below) are far too much.


Again you are talking about distances. Distances don't
matter in MOND. MOND regime does not start for
100s of Lightyears within our own galaxy, at the center.


See below.

For our SUN it would start at much below 0.1ly. Actually
at a distance of a couple of days.


There is no distance scale in MOND. There is only an
acceleration scale, ie a0. Above a0 it is Newtonian,
below a0 it is MONDian.


For the Sun the distance for a to get smaller than a0
(1.21E-10m/s^2) is roughly 0.1 ly.


I would think it would be more like 0.01 ly.

The distance between then sun and the nearest star
is 1.3 pc or 4.3 ly.
That means the behaviour between almost all the stars
in our Galaxy is described by MOND.
Only when two stars become very close the description
becomes Newton.

Is that the correct application of MOND ?


That is correct. But it will not apply near the center of
the galaxy.


Please explain.

Nicolaas Vroom
http://users.pandora.be/nicvroom/

 




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