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NGC1052-DF2 Diffuse Galaxy without dark matter



 
 
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  #11  
Old April 11th 18, 10:13 AM posted to sci.astro.research
brad
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Posts: 102
Default NGC1052-DF2 Diffuse Galaxy without dark matter

[[Mod. note -- I have added "" to mark quoted material (not written
by the author of this article. -- jt]]

Martin Brown

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?
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?

Thanks for any enlightenment.

--
Regards,
Martin Brown


Not a galaxy/galaxy interaction. But, perhaps another way.
When Voids merge they sometimes pinch outlying galaxies
out of a cluster. Those galaxies inside Voids are less luminous
than others. They also tend to be less structured. And finally,
because the Voids are the centers of expansion, and the expansion
overwhelms the gravity across the Universe; the gravitational
fields of Dark Matter inside Voids will also be more diffuse due
to its lower density.

BJ
  #12  
Old April 11th 18, 08:55 PM posted to sci.astro.research
Steve Willner
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Default NGC1052-DF2 Diffuse Galaxy without dark matter

In article ,
Nicolaas Vroom writes:
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)


The basic method is to measure the luminosity of the stars and
multiply by a derived mass-to-light ratio. There are complications
in deriving M/L, the largest being what initial mass function one
assumes. You can think about that as a calibration factor, but
otherwise, if one has observations in the rest near-infrared, the
derived mass won't be wrong by more than a factor of 2 and usually
less.

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.


The last two are more frequently called "baryonic dark matter" and
"non-baryonic dark matter." The phrase "dark matter" by itself can
mean the sum of these two or the non-baryonic component only,
depending on context. This isn't ideal, but the difference is only a
factor of 0.85 so not usually significant.

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.


There's not much dark matter, but the value doesn't have to be
identically zero.

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.


I doubt one could tell for systems with few stars because the stellar
masses aren't known well enough. A better case is globular clusters,
which have no detectable dark matter. That's in contrast to low-mass
galaxies, which overlap the mass range of globular clusters, and do
have dark matter.

In the original document...
What is missing (?) is the ratio: visible-baryonic/invisible-baryonic


The velocity dispersion measures _all_ matter, from which the stars
are subtracted. Therefore in this context, "dark matter" means the
sum of the two dark components. This should be obvious from the
explanation of the methods, but as noted above, it hardly matters.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

  #13  
Old April 13th 18, 11:24 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default Darker and darker

In article ,
jacobnavia writes:
An important "proof" for dark matter has vanished: Astronomers take back
the observation that a cluster was missing its dark matter, "left
behind".


That is a very strange way of stating the result. Previous
observations had suggested an offset between the dark matter and
visible stars for one of the four cluster galaxies. The new data
show no evidence of an offset. Either morphology is consistent with
existing theory; there is no "proof" for dark matter involved.

The derived dark matter masses associated with each galaxy are
uncertain but roughly 10 times the stellar masses.

Royal Astronomical Society. "Dark matter might not be interactive after
all." ScienceDaily. ScienceDaily, 5 April 2018.
www.sciencedaily.com/releases/2018/04/180405223407.htm.


The preprint is at
https://arxiv.org/abs/1708.04245
The refereed journal article will be published in MNRAS on Apr 15.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #14  
Old April 13th 18, 06:37 PM posted to sci.astro.research
Homo Lykos
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Posts: 14
Default NGC1052-DF2 Diffuse Galaxy without dark matter

Am 30.03.2018 um 23:46 schrieb Martin Brown:

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?


https://arxiv.org/pdf/1804.04167.pdf

[Moderator's note:

Title: MOND and the dynamics of NGC1052-DF2. Authors: B. Famaey, S.
McGaugh, M. Milgrom. Comments: 4 pages. Submitted to MNRAS. Subjects:
Astrophysics of Galaxies

I haven't read it yet. The authors are three of the main "MOND people".
Probably worth a read even if you don't agree with everything.

-P.H.]

 




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