WIMPS?

In article ,
Nicolaas Vroom writes:
When you remove all the stars above 0.4 mass the total mass left is
9,46 and the density is 1E-12. This are Red Dwarfs and Brown Dwarfs
and can be considered invisble baryonic matter.

Fair enough, except that at least the higher end of that mass range
would be detectable via lensing. One fly in the lensing ointment is
that measurements have been made only towards the LMC and the
Galactic center (unless I've missed other observations, which is
quite possible). If you can arrange for red dwarfs to occupy regions
other than these lines of sight and still explain the rotation
curves, you might get somewhere.

Regardless of lensing, if you want to explain rotation curves with
red dwarfs or similar objects, you have to postulate that they have a
different distribution than that of the visible stars. In
particular, you need more low-mass stars at large radii than expected
from the light distribution. That's possible, of course, but there's
no evidence for it. Galaxy colors don't change much with radius, for
example. Actually they become a little bluer at large radii because
of lower metallicity.

As I wrote earlier, I'm not sure all the parameter space is ruled
out, but it is shrinking.

Some comments on subsequent posts:

1) aside from rotation curves, there's a problem with spiral disk
instability. Putting mass in a halo rather than a disk stabilizes
the disk as well as solving the rotation curve problem. That doesn't
prove the halo explanation, but it makes it more attractive.

2) gravitational lensing can easily detect objects of half a solar
mass or larger. Smaller objects are more difficult, but observations
have improved. I'm not sure just where things stand now, but objects
more massive than some tenths of a solar mass cannot easily explain
the dark matter inferred for the Milky Way. (But see above about
"fly in the lensing ointment.")

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