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Old April 3rd 18, 10:04 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default NGC1052-DF2 Diffuse Galaxy without dark matter

In article ,
Martin Brown writes:
http://nature.com/articles/doi:10.1038/nature25767
[[Mod. note -- Open-access preprint
https://arxiv.org/abs/1803.10237
-- jt]]


That's a remarkable result. The authors have covered all the
possible objections that occur to me, though I'm not 100% convinced
that the galaxy can't be rotationally supported and face-on. The
authors make a good case against, though.

If their result is confirmed then it would presumably put the nail in
the coffin of all modified gravity theories


A few more cases such as this one should do it. For those not
following, the argument is that in most galaxies and galaxy clusters,
the mass of detectable stars exerting Newtonian gravity account for
the observed motions.
[[
Mod. note -- I suspect that the author has inadvertently omitted
"does NOT" or some similar wording in the previous sentence, and meant
to write something like this:
For those not
following, the argument is that in most galaxies and galaxy clusters,
the mass of detectable stars exerting Newtonian gravity does not
account for the observed motions.
My apologies if I've misunderstood the author's intent here!
-- jt
]]
Therefore, _either_ there is more mass than
that of the visible stars ("dark matter"), _or_ the Newtonian gravity
law is wrong. However, we know Newtonian gravity is right in our
solar system (except for tiny GR corrections) and in objects as large
as globular clusters, so if the gravity law is the problem,
modification ("MOND") is required only in objects as large as
galaxies. Here we have a large galaxy where no modified gravity law
is needed. If MOND is right, how can it fail to apply to this
galaxy?

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


Yes. In particular, observed gravitational force (based on standard
Newtonian gravity) is sometimes that expected from stars and
sometimes (nearly always for galaxies) much more, and the two cases
are not distinguished simply by size scale. That suggests dark
matter in most galaxies but not all. In other words, if you want a
modified gravity law, it has to be modified by something more
complicated than size scale.

Dynamically can anyone see how a bunch of stars could be peeled off by a
galaxy galaxy interaction without also taking dark matter with it?


The authors discuss several possibilities in their penultimate
paragraph, but I don't find any of them compelling. The authors
interpret the galaxy's blue color as low metallicity, but it could
also be young age. The large peculiar velocity suggests some kind of
tidal ejection, perhaps of a TDG. In this scenario, the TDG need not
have been associated with much dark matter. A high-speed ejection of
a gas cloud (that later formed stars) is another possibility.

If the velocity in the plane of the sky is the same as the radial
velocity offset from NGC 1052 (293 km/s), the ejection (if it was
from NGC 1052) took place about 300 Myr ago. All this is _quite_
speculative; maybe the full spectra of the luminous objects will tell
us more. I'm sure clever theorists can come up with many
possibilities. :-)

[p.s. I expect to be away from Internet access for a couple of
weeks. Don't expect further messages for awhile.]

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