On Friday, 30 March 2018 23:46:51 UTC+2, Martin Brown wrote:

What do people think of the recent claim in Nature that one of the new
wide field instruments has found a candidate diffuse galaxy NGC1052-DF2
which appears to have little or no dark matter in it?

Open-access preprint https://arxiv.org/abs/1803.10237

This article starts with the sentence:
" Their average ratio Mhalo/Mstars has a minimum of about 30 for galaxies
with stellar masses near that of the Milky Way"
That means in simple language that the Halo of the Milky way is 30 times
more massive than the mass of the bulge and the disc of the Milky Way.
The Mhalo is calculated by performing simulations.
Please do a Google search with: "How is halo mass calculated"
IMO (?) this number 30 seems very high.
To see how the steller masses are calculated see:
https://arxiv.org/abs/1607.04678
My impression is that this is not simple (Figure 9)

If their result is confirmed then it would presumably put the nail in
the coffin of all modified gravity theories and the search for the
mysterious cold dark matter that only interacts via gravity will hot up.

Finding a diffuse galaxy with a velocity dispersion that shows there is
only baryonic matter in suggests that dark matter really does exist.

The main problem around dark matter related to galaxies is in the name,
which is is confusing.
In simple language there are three types of matter: visible baryonic
invisible baryonic and non-baryonic.

In the case of the solar system it consists of two:
visible baryonic: the Sun. Invisible: the planets and the kuiper belt.
You can consider the Oort Cloud as the Halo of the solar system
There is (almost) no non-baryonic matter.

[[Mod. note -- In the context of dark matter, "visible" means
"interacts with electromagnetic radiation (which includes light,
radio, X-rays, etc)". So anything baryonic (including stars,
planets, the Kuiper belt, neutron stars, interstellar dust, and
the interstellar and intergalactic (gaseous) mediums) are all
"visible". Electrons and positrons are also "visible".

Neutrinos are not "visible".

Technically speaking black holes can scatter electromagnetic
radiation, but this is a pretty small effect, so we usually call
black holes "not visible" in this context.
-- jt]]

If we assume that all the stars in the disc of our Galaxy are equal
than it means that there is no non-baryonic directly outside each
star. i.e. all non-baryonic can only be in interstellar space (or
in halo)

The reason why there is no non-baryonic matter in the solar system
is because all the planets (the movements) are accordingly (almost)
to Newton's law. See also below with the point marked (*) The
question is if the same can be said for binary stars in our galaxy,
(or for clusters of three or 4 stars) of which the masses accurately
can be observed. If that is the case than, within such clusters,
there is no extra non-baryonic matter required.

Please visit:
https://en.wikipedia.org/wiki/Dark_m...aryonic_matter
Here you can read:
"However multiple lines of evidence suggest the majority of dark
matter is not made of baryons:"
IMO all what follows has more to do with the Universe at large than
with individual galaxies.
(*)
In the paragraph:
https://en.wikipedia.org/wiki/Dark_m...otation_curves
Keppler's law is mentioned. IMO in relation to galaxy rotation
curves Keppler's law should not be mentioned. Even if you want to
study the movement of the planets you should use Newton's law. If
curve B is what is observed than the true size of the galaxy is
much larger than what is shown. It is easy possible that the full
curve also starts to level off. (MOND does not support such an
behaviour. It will always be flat)

[[Mod. note -- Kepler's law (strictly speaking, Kepler's 3rd law)
is a mathematical consequence of Newton's law (strictly speaking,
Newton's 2nd law & his law of universal gravitation), and vice versa,
so "using Newton's law" is the same thing as using Kepler's law.
-- jt]]

In the original document we read:
"Here we report the radial velocities of ten luminous globular-cluster-like
objects in the ultra-diffuse galaxy NGC1052=E2=80=93DF2, which has
a stellar mass of approximately 2*10^8 solar masses." and:
" We conclude that NGC1052-DF2 is extremely deficient in
dark matter, and a good candidate for a "baryonic galaxy" with no
dark matter."
What is missing (?) is the ratio: visible-baryonic/invisible-baryonic

Nicolaas Vroom