"g" == greywolf42 writes:

g Joseph Lazio wrote in message
g ...

"TS" == Thomas Smid writes:

TS It has never been the case that all galaxies show evidence for
TS dark matter halos (...).

While true, I think it is still the case that the number of
galaxies requiring dark matter to explain their rotation curves
vastly outnumbers those that do not.

g The fact of existence of a significant number of galaxies without
g dark matter halos (...) would add another layer of 'ad hoc' to the
g dark matter 'ad hoc' postulate.

It would be nice to have some numbers associated with this. One might
expect a few "exceptions to prove the rule." I'm not aware that a
"significant" number of galaxies do not require a dark halo to explain
their rotation curve. References? You may also wish to review the
discussion between Thomas Smid and myself. Smid pointed to the work
of Vega-Beltran. I then commented that much of Vega-Beltran's work
concerns comparisons between stellar and *ionized* gas motions. He
shows reasonable agreement between the two, at least in the outer
portions of galaxies, where a dark halo requirement is most
stringent. Moreover, one would think that ionized gas would be more
likely to be affected by magnetic fields than neutral gas.


In the outer reaches of galaxies, rotation curves are based on gas
velocities because there are no (or so few) stars from which to
obtain stellar velocities. However, I think it is the case that
within the optical disk, gas and stellar velocities match fairly
well.

g You have no basis for this claim. There is not a single paper
g (...) for galactic rotation curves that does not depend on gas
g velocities or O and B stars. The O and B stars by necessity retain
g the initial velocities of the gas clouds which created them (...).

Admittedly, this is not my area, but I'm astounded. Nobody's ever
attempted to measure the stellar velocity dispersion of a spiral disk?
This is done all the time for bulges and elliptical galaxies. Are
spiral disks really that faint that nobody's tried it even with nearby
galaxies?

Moreover, as a counter-argument I'll point out the case of the
Galactic center (i.e., center of the Milky Way Galaxy). For many
years, people measured gas velocities that seemed to indicate a
large dark mass (i.e., a supermassive black hole). A key
uncertainty was whether the gas velocities were being affected by
non-gravitational forces. There is no shortage of
non-gravitational forces in the Galactic center, either, strong
magnetic fields, stellar winds, etc.

When stellar velocities became available, they matched the gas
velocities.

g Reference, please. I think you'll find that there are either NO
g stars or O and B stars.

g I'd be happy to be disproved. I've been asking for any paper to
g the contrary for years.

Where to start? There's a huge literature on the Galactic center.

I suggest looking at the ADS. Authors to search on include
K. Sellgren, R. Genzel, A. Ghez, J. Zhao, and P. Ho. The last two
have been more involved in determining the gas motions, while the
first three have been more involved in determining stellar motions.

A good place to start might be Genzel et al. (1997, URL:
http://adsabs.harvard.edu/cgi-bin/np...NRAS.291..219G )
as they show a plot of the mass distribution in the Galactic center as
inferred from both stellar and gas motions (Figure 8). They also have
a summary of a number of observations in Section 1. Also note that
much of the stellar work they summarize or reference concerns
late-type stars.

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