Galaxies without dark matter halos?

[[Mod. note -- I have taken the liberty of removing a crosspost to
sci.physics.research, as (IMHO) this article falls reasonably within
s.a.r's subject area (astronomy/astrophysic) but is a bit off-topic
for s.p.r. -- jt]]

Joseph Lazio wrote in message ...

You're mixing up two different measurements. Within the optical disk,
I think gas and stars usually co-rotate. (There is a small number of
important exceptions, but the general rule is co-rotation.) Outside
the optical disk, no stars are detected so one has to rely on the gas
to trace the gravitational potential.

The fact remains that in most publications gas rotation curves are
used to support the hypothesis that stars are bound by dark matter.
Why ? Presumably because star rotation curves wouldn't be as
conclusive.



TS Anyway, I wonder how one can be so sure about the exact amount and
TS distribution of the 'visible mass' in a galaxy and therefore the
TS need for dark matter in the first place: I can't imagine that the
TS stars have been literally counted, so maybe there are actually
TS more stars than assumed (according to the mass-luminosity
TS relationship, a star with half the mass has only 1/10 of the
TS luminosity , so with 10 times as many stars of half the mass, you
TS would have the same overall brightness but 5 times the overall
TS mass, which might dispense with the need for dark matter).

So work out the numbers in more detail; I'd actually be curious to see
this. Take a couple of example late-type spirals for which good
rotation curves exist. Assume that all of the dark matter is in the
form of 0.1 solar mass stars. How many stars would be required and
would the integrated light from them still be undetectable?

Of course, outside the optical disk you would be hard pressed to
explain the rotation curves by low mass stars, but here, as you
admitted above, the curves do only represent the gas motion which
could be affected by a rotating magnetic field even in its neutral
form (as explained on my webpage
http://www.physicsmyths.org.uk/darkmatter.htm ).
However, even if observations of stars and globular clusters seem to
indicate the presence of dark matter, this is in my opinion not a
foregone conclusion, as the situation is by no means comparable for
instance to the discovery of Neptune (as claimed in the sci.astro
FAQs): in the latter case the conclusion could be reached solely by
position measurements, whereas for the galactic problem you have to
make additional assumptions regards the absolute brightness of the
objects. Any error in the mass-luminosity relationship could affect
the conclusion crucially. If you have a look for instance at
http://planetquest.jpl.nasa.gov/SIM/science_henry.pdf , you will find
that the luminosities for stars less than 1 solar mass are indeed
uncertain by about 2-3 magnitudes. I find it quite remarkable that the
mass luminosity relationship, which a) is quite uncertain for low mass
stars, b) obtained only in the solar neighbourhood and c) obtained
only from double stars, is applied to all stars in our or other
galaxies regardless. I don't think that all these points have been
sufficiently examined to justify the conclusion of dark matter from
the dynamics of galactic objects.
Besides, in the case of Uranus/Neptune, the 'dark matter' was
identified after a very short time, but decades of research have been
unable to clarify the physical nature of the galactic 'dark matter'.