Steve Willner wrote in message
...
In article ,
Joseph Lazio writes:
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.

I've not been following this discussion, but doesn't the existence of
even one galaxy with a dark halo prove that dark matter exists?

The existence of a dark halo is an ad hoc speculation invoked when the
measured motion of gas in a galaxy does not match the motion predicted by
Newtonian/Einsteinian gravitation applied to the visible density of stars
(matter) in the galaxy.

Dark matter (being an ad hoc addition) cannot be invoked to explain *some*
rotation curves and ignored to explain others.

As
Joe says later, galaxy evolution is complex, so the relative numbers
of galaxies with and without haloes could be anything (though
eventually a good theory of galaxy evolution should predict the
correct value).

There is no part of the dark matter speculation that allows only *some*
galaxies to contain dark matter. Dark matter is required by all -- if it
exists at all. Dark matter doesn't 'evolve' with the galaxy. It preexists,
and -- supposedly -- drives galactic formation.

Of course in a larger sense, we know dark matter exists in many
forms. What is at issue is its amount, nature, and distribution.

In no sense whatsoever do we have any indication that 'dark matter' exists.
'Dark matter' does not mean simply normal matter that is not radiating.
'Dark matter' is -- by definition -- a 'new' kind of matter that does not
interact by EM or strong forces at all. That is purely gravitational. The
'non-radiating' 'normal matter' is already included in those galactic
dynamics and evolution theories.

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

Are we talking about velocity dispersions or rotation curves? I
thought the evidence for dark matter was the latter.

It *is* the latter. Mr. Lazio screwed up. But it's a minor point, because
it was his search criteria that he fouled up. The essence of his argument
is that so long as even 'one' galaxy is found that 'needs' dark matter, then
dark matter is proved. Which is the reverse of the scientific method.

There is also the theoretical issue that spiral arms, as in the Milky
Way, are unstable unless there is a dark halo.

That's either a claim I haven't heard, or a muddle of the fact that spiral
arms are dynamic structures -- not staying the same shape or containing the
same stars. Do you have a reference?

Also, the velocity
dispersion of halo stars is consistent with dark matter.

That's because dark matter is added *after* motions are measured. And dark
matter is added (ad hoc) in just the amounts necessary to 'explain' the
rotation curve.

[another poster, re the Galactic center]
g Reference, please. I think you'll find that there are either NO
g stars or O and B stars.

I'm probably going to be sorry I asked, but why should O or B stars
have different velocities than any other stars?

O and B stars are fantastically luminous -- 4 to 5 orders of magnitude more
than later type stars (which is why they show up at a distance, and dominate
the integrated spectra of galaxies). In order to maintain this tremendous
energy output, they burn through their fuel in around a million years (as
opposed to billions of years for later types). Therefore, O and B stars
cannot be very old.

Since we believe stars condense from gas, a 'newly-born' star will begin
life with the motion of the gas cloud from which it condensed. Once any
star has condensed, it will eventually (over tens of millions to hundreds of
millions of years) be accelerated to the average motion of the other *stars*
around it. But an 'O' or a 'B' star won't live long enough to match the
general rotation of the later-type stars.

In any case, there
are plenty of late-type stars in the Galactic center region.

But we can't measure them -- because they aren't luminous enough.

greywolf42
ubi dubium ibi libertas