"g" == greywolf42 writes:

g Joseph Lazio wrote in message
g ...

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."

g ?? Exceptions *never* prove the rule. Especially in science, one
g exception disproves the rule.

g The point is simple. You can't have a valid scientific theory that
g allows you to arbitrarily add and remove 'dark matter' from
g galaxies as needed -- just to match observation and 'save' a
g theory. Such an approach is not disprovable.

You seem to misunderstand dark matter. The expectation that dark
matter exists is not on par with various theories, such as special
relativity or electromagnetism.

The velocities of various components of many galaxies cannot be
reproduced with our best theory of gravity if we assume that the
luminous matter represents all of the matter. Thus, we have two
choices: Either general relativity is wrong or we're not seeing
some of the matter. (I suppose one might say three choices, both
are wrong.) General relativity has been well tested, so we're
reluctant to dump it. It's easy to think that we have missed some
matter, particularly because we know of many objects that would be too
faint to be seen at the distances of other galaxies. Thus, the more
simple explanation is that dark matter exists.

However, galaxies are complicated things. Their formation probably
involves dissipation and turbulence. Moreover, they can be affected
by interactions with other galaxies. Should we expect all galaxies to
be nearly identical? No. Should they all contain the same amount or
same proportion of dark matter? No. Hence, a few exceptions
(galaxies without dark matter) may prove a rule (most galaxies have
dark matter and galaxies are complicated things). This is kind of
equivalent to trying to predict the number of planets in a solar
system. It should be simple, right? It's just gravity and
electromagnetism, both well understood forces. Yet the actual number
of planets is determined by so many aspects of planetary formation
that an actual a priori prediction is well-nigh impossible.


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.

g I disagree with your characterization of your discussion with Smid.
g It's true that you made several similar assertions. But those
g assertions were not backed up by Smid -- or by the work of
g Vega-Beltran.

Umm, care to be more specific? I commented on several papers by
Vega-Beltran. Unless you can point out where I've misread his papers,
I'm going to have to chalk this up to continuing to try to prove by
assertion.


Moreover, one would think that ionized gas would be more likely to
be affected by magnetic fields than neutral gas.

g Neutral gas is affected by magnetic fields in the same manner as
g ionized gas. It just accelerates more slowly. [...]

More proof by assertion? Ionized gas should respond strongly to
magnetic fields. If ionized gas and stars have the same velocities,
why would neutral gas behave differently than ionized gas?

There's also the issue I've raised a couple of times in this newsgroup
that the energy contained in the magnetic field does not seem to be
sufficient to explain the velocities.


Admittedly, this is not my area, but I'm astounded. Nobody's ever
attempted to measure the stellar velocity dispersion of a spiral
disk?

g That's a pretty silly strawman. Of course people have attempted to
g measure stellar velocity distribution. But it's very difficult
g (...) to do this on a star-by-star basis.
[...]
This is done all the time for bulges and elliptical galaxies.

g Those use GAS when they can get it. It's only when gas is not
g available that one resorts to counting individual stars.

Nobody counts individual stars in galaxies. Resolving a galaxy into
stars is only possible for the nearest galaxies. What's measured is
the velocity dispersion from the integrated light of the stars. This
is done all the time for elliptical galaxies and the bulges of spiral
galaxies (both of which often contain *little* gas).


Moreover, as a counter-argument I'll point out the case of the
Galactic center (i.e., center of the Milky Way Galaxy). [...]
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. [...]

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.

g I'm not asking for general stratgies, but for specific references.
g I've done the usual searches. The point is, I believe that there
g aren't any stellar motion studies that don't either measure gas
g motions alone, or measure gas plus a few O and B stars.

That's why I referred specifically to Sellgren. She and her
collaborators have been measuring the stellar velocity dispersion from
late-type giants.

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.

g You're going to have to start reading abstracts a bit better (or
g perhaps you missed the fact that 'likely' early-type stars of 15-20
g solar masses are 'O' and 'B' stars). Here is the abstract (the
g paper itself is not available on the website, [...]):

The paper is indeed available on the Web site. See the links at the
top that say, "Full Refereed Journal Article" or "Full Refereed
Scanned Article."

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html