"TS" == Thomas Smid writes:

TS Joseph Lazio wrote in message
TS ...
Vega-Beltran et al. (1999, Ap&SS) find, "In the outer regions of
all of the sample of 20 galaxies the stellar rotation is comparable
to that of the ionized gas [...], while in the inner regions the
following kinematic features are noteworthy."

TS I had a closer look at some of Vega-Beltran's papers now as well
TS and noticed that these studies regards the difference between star
TS and gas velocities are confined to some 'early-type' spirals, and
TS it is quite possible that for these the magnetic dynamo may not
TS have fully developed yet, hence explaining the co-rotation of the
TS gas and the stars in the outer regions, [...]

Notice that Vega-Beltran's work concerns *ionized* gas. Ionized gas
should be quite receptive to any magnetic influences. Indeed, most
people assume that ionized gas is essential in producing a magnetic
dynamo.

TS look at any textbook or webpage about galaxies and you will only
TS find rotation curves based on gas (usually HI 21cm measurements)
TS and the co-rotation of stars assumed implicitly without any
TS justification [...]

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.

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, you also have the related problem of, How would those stars
get there? Star formation in spiral galaxies falls off quite quickly
at large distances from the center of the galaxies, and the gas in the
outer parts of spiral galaxies is fairly diffuse. How would stars
form out there?

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