View Single Post
  #3  
Old February 22nd 09, 02:40 PM posted to sci.astro,sci.physics,sci.physics.relativity
Robert Karl Stonjek
external usenet poster
 
Posts: 196
Default Questions on the enigmatic rotational curve of spiral galaxies


"Ian Parker" wrote in message
...
On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote:
Snip

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

Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. For instance in The method of Galactic Rotation
http://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.

My quick skim of several results leads me to conclude that the Kepler model
is the first approximation and this fits well with the inner region of a
galaxy eg 1pc for the Milky Way (the supermassive black hole accounts for
the motion of the inner objects). But the bar model may be a better
approximation for the outer region ie starting with evenly spaced massive
objects across, say 20kpc (comparable to the Milky Way) and then consider
the initial conditions of a static and rotating bar and see what happens.

The simplified model is the bread and butter of physics modelling, but the
sphere or disc may not be the best simplified model in this case. The bar
may be more instructive for the outer region.

Here is my objection clearly outlined in the abstract of another paper:
"Galaxies as rotating systems involve a balance of gravitational attraction
and centripetal force. When the central mass is dominant the dynamics,
referred to as Keplerian, are that the orbital velocities are proportional
to the inverse square root of the path radius. A plot of velocity vs. path
radius is a Rotation Curve. For galaxies viewed as the disk edge we see one
end moving toward us relative to the center and the other moving away. The
rotational velocities are measured from the variation in redshift along the
galactic diameter. Rotation curves so obtained are not the expected
Keplerian inverse square root; rather, [beyond the galactic core] they are
flat. In a solid sphere all parts move at rotational velocities directly
proportional to radius. Since the observed flat curves lie between the
Keplerian inverse square root of radius and the solid's direct proportion to
radius, a mass discrepancy is inferred -- that unseen matter is dispersed
throughout the galaxy, a halo of "dark matter" that causes the curve form
exhibited -- thus the "dark matter" hypothesis. The mass discrepancy only
appears where the acceleration is less than 10-8 cm/sec2. Modeling gives an
alternative hypothesis: Modification of Newtonian Dynamics or MOND, that
gravity and or inertia are modified when g or a is less than 10-8 cm/sec2.
No justification has been developed except correlation with the mass
discrepancies. An alternative explanation is presented -- the general
exponential decay of the overall universe, which has been analyzed and
developed in several papers. The universal decay accounts for the mass
discrepancies because the effect of the decay is to make the rotation curves
appear to deviate from the form expected in a Keplerian galactic disk
although the actual rotational behavior does not."
http://www.citeulike.org/group/48/article/70704

Referring back to my illustration at the root of this thread we note that
force '3' has been ignored.
I can't tell if the models on the page you point to consider those forces or
not.

Robert