Re: Dark matter hides, physicists seek (Forwarded)
- From: Richard Saam <rdsaam@xxxxxxx>
- Date: Tue, 13 Feb 2007 14:58:02 GMT
Nicolaas Vroom wrote:
"Steve Willner" <willner@xxxxxxxxxxxxxxx> schreef in berichtA flat rotation curve requires mass proportional to radius
news:45a57583$1@xxxxxxxxxxxxxxxxxxxxxxxxxx
Let's back up a little. Galaxy disks are described by an exponential
law, where density d = d0*exp(-r/l). In this formula, d0 is the
central density of the disk, r is distance from the galaxy center,
and l is a constant called the "scale length." The scale length for
M31, for example, is about 6 kpc; other galaxies have their own
values. In order to find the total stellar light, observers measure
the surface brightness out as far as they can, then fit the
observations to the above functional form. (There's a complication
from the galaxy's bulge, but it is fit and subtracted away.) Then
one calculates the _total_ light at all radii (in other words,
integrate the function from zero to infinity). This is accurate
enough even if the measurements only extend to a few scale lengths;
the light outside 3 scale lengths is only 20% of the total, and the
light outside 5 scale lengths is 4% of the total.
SNIP
Here you are asking about the stellar mass function: the relative
numbers of stars of different masses. As you say, the least massive
stars are not measured directly, and one has to assume a mass
function and thus a mass to light ratio. There are a number of
constraints on M/L (not least local observations in the Sun's
neighborhood), but it could be systematically wrong or different in
different galaxies. People work on this question in various ways,
for example by observing at many different wavelengths and by using
spectroscopy, but there is still some uncertainty. However, none of
the observations you have cited changes our knowledge of the mass
function, nor will any _simple_ change in the mass function by itself
get rid of the need for dark matter. Of course you can always "fix
up" a mass function that changes with radius in just such a way as to
reproduce the rotation curve, but there's no separate evidence for
such a change.
One document which gives many examples of galaxy rotation curves
is: http://www.arxiv.org/abs/astro-ph/0506370
"Galaxy Rotation Curves Without Non-Baryonic Dark Matter"
Authors: J. R. Brownstein, J. W. Moffat
The interesting part of this document is, as the title suggests, that
you can simulate all those rotation curves without darkmatter.
I'am not surprised by that result.
As part of ongoing interest in galaxy simulation I have also
written 3 programs in excel to do the same.
The three documents to describe each is available at:
1 http://users.pandora.be/nicvroom/circ11.xls.htm
2 http://users.pandora.be/nicvroom/grotc.xls.htm
3 http://users.pandora.be/nicvroom/grotcexp.xls.htm
(At those url's you can retrieve the excel programs in zip format)
in line with Keplerian requirement
as a check on your numbers indicates.
Richard
.
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