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NGC1052-DF2 Diffuse Galaxy without dark matter



 
 
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  #1  
Old March 30th 18, 10:46 PM posted to sci.astro.research
Martin Brown[_2_]
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Posts: 3
Default NGC1052-DF2 Diffuse Galaxy without dark matter

[Mod. note: this article arrived in my moderation mailbox with a number
of garbled non-ASCII characters. I have fixed things up as best as I can;
my apologies to the author if I've mis-inferred his intended meaning.
-- jt]]

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?

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

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.

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
Ads
  #2  
Old April 1st 18, 03:43 PM posted to sci.astro.research
jacob navia
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Posts: 332
Default NGC1052-DF2 Diffuse Galaxy without dark matter

Le 30/03/2018 eM- 23:46, Martin Brown a e(C)critM-BM- :
[Mod. note: this article arrived in my moderation mailbox with a number
of garbled non-ASCII characters. I have fixed things up as best as I can;
my apologies to the author if I've mis-inferred his intended meaning.
-- jt]]

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?

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

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.


It is already hot. Enormous efforts were (and are) being spent figuring
out why the stars misbehave and go their ways as if... something
invisible was pulling the strings.

Detectors over detectors were sent in that quest, and all came back empty.

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


One way of explaining what we perceive is to assume we understand what
is going on and suppose an unseen and undetectable mass hanging around.
Gravity is gravity and there is just an unseen mass (halo).

This galaxy then, is special in the sense that apparently globular
cluster are moving without any unseen influences and seem to obey the
laws of gravity as we understand them. The thing should be in the table
1, the physical properties of those clusters. No unseen dark matyter is
necessary to explain the movements of those clusters.

OK.

But why?

Mystery to me, sorry. The paper discusses a lot of stuff, and I surely
am missing something, but I do not find the calculations about the
velocity vectors of those clusters, and why gravity, in this case,
explains all those concrete movements. Yes, it is a weird galaxy, very
diffuse, just a blow of stars in the images (that the paper doesn't show).

I suppose that they measured the velocities and arrived at the
conclusion that there is no dark matter, but I did not see any
derivation of that in the paper. There are no velocity vectors shown to
me, maybe because I am not used to wade through an interminable sequence
of acronyms I do not understand and I missed them.

Question:

Where in that paper is the data about the connection between those GCs
(globular clusters) and gravity theories?

[[Mod. note --
1. The authors only have radial-velocity data, so they don't know the
3-D velocity vectors of the "compact objects".
2. They authors argue (page 2 of the arXiv preprint) that the "compact
objects" are *not* globular clusters. They don't say what they think
these objects are -- they just say that they'll discuss the properties
of these "enigmatic objects" in another (future) paper.
3. Despite not knowing what these objects are, they can still estimate
their radial velocity, and it's the dispersion (roughly speaking,
the variance) in these radial velocities that is the key measurement
for their argument that this galaxy has little or no dark matter.
They discuss their estimate of this dispersion on page 7 of the arXiv
preprint (section "Velocity dispersion"). The connection between
velocity dispersion and gravity theories is in the sections
"Dynamical Equliibrium", "Source of dynamical support", and
Dynamical mass measurement", on pages 8-9 of the arXiv preprint.
-- jt]]

The central sentence of that paper on page 5:

The second difference is that the galaxy has no (or very lit-
tle) dark matter (see vD18)

Yeah "vD18". What is that?

None of the figures has this label. It is not a citation since those are
in square brackets...

Is it "van Dokkum 2018" Could be, there is this citation at the end:
quote
van Dokkum, P., Danieli, S., Cohen, Y., Merritt, A., Romanowsky, A. J.,
Abraham, R., Brodie, J., Conroy, C., et al. 2018, Nature, XX
end quote

This refers to the nature paper that should contain (maybe) the data
they use to arrive at those conclusions. Maybe someone here has that
paper and can explain how those calculations were done?

I suppose that they measured the radial velocities of those clusters
around the central mass of the galaxy and arrived at the conclusion: we
have now not only unseen "dark" matter, we have also unseen unseen dark
matter, i.e. dark matter that disappears.

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?


Since nobody knows what dark matter is, you can put anything as answer
to that question.

Dark matter makes galaxies more rigid, as far as I understand all this.
Stars far away rotate at the same speed, like a rigid body, not like in
our solar system, for instance, where the farther you go, the longer an
orbit takes.

That simple relation doesn't apply to our galaxy and to many others.
Hence, to preserve the assumption that we understand gravity, we need
unseen matter to explain what we see.

Suppose that (being just an amateur) I missed something and the authors
of the paper are right: that galaxy is no longer rigid, just a diffuse
collection of stars.

We would have to admit then, that this rigidity can be absent, what
indirectly proves its existence.

That doesn't take us any further in the quest to find what that this
invisible rigidity is, of course.

Nobody discusses observations, and the unexpected movements of stars in
the periphery of galaxies is a fact. And it is not unexpected (the
universe is quite big and there are galaxies for all tastes) that one
galaxy is lacking this rigidity.

Modified gravity theories are not doomed since dark matter people
propose that what modifies gravity is an unseen mass. What would be new
here, is that this property is not tied to stars or visible matter, and
in some galaxies is just absent.


Thanks for any enlightenment.


I think in this dark matter stuff, we are all in the dark. Light is nowhere.
  #3  
Old April 3rd 18, 10:04 AM posted to sci.astro.research
Steve Willner
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Posts: 1,141
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.]

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #4  
Old April 4th 18, 10:54 AM posted to sci.astro.research
Gary Harnagel
external usenet poster
 
Posts: 645
Default NGC1052-DF2 Diffuse Galaxy without dark matter

On Tuesday, April 3, 2018 at 3:04:06 AM UTC-6, Steve Willner wrote:

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.


Doesn't the Bullet cluster count?

https://en.wikipedia.org/wiki/Bullet_Cluster

[[Mod. note -- A few researchers claim that the bullet cluster can be
explained by modified gravity without dark matter, e.g.,
Brownstein & Moffat
"The Bullet Cluster 1E0657-558 evidence shows modified gravity
in the absence of dark matter"
Monthly Notices of the Royal Astronomical Society, Volume 382, Issue 1,
21 November 2007, Pages 29--47,
https://doi.org/10.1111/j.1365-2966.2007.12275.x
or
Dai, Matsuo, & Starkman
"Gravitational Lenses in Generalized Einstein-Aether theory:
the Bullet Cluster"
https://arxiv.org/abs/0806.4319
Physical Review D 78, 104004 (November 2008)

I think modified gravity is very much the minority opinion among
researchers who study this topic, but it does continue to have a
few supporters.
-- jt]]
  #5  
Old April 4th 18, 12:51 PM posted to sci.astro.research
John Heath
external usenet poster
 
Posts: 13
Default NGC1052-DF2 Diffuse Galaxy without dark matter

Yes indeed. both dark matter and dark energy have one thing in
common , they are both dark as in can not be seem. Human nature
stays consistent though history as dark demons has been used in the
past to explain what is not understood. In short I second Jacob's
position. Where is the beef for dark matter. Why all the smoke and
mirrors when it comes to the details for dark matter justification.
In astro physics they know what they are doing but when it is put
into print it is hard to put a finger on where the beef is as the
description is not detailed.

A moderator added a comment

Quote

and it's the dispersion (roughly speaking,
the variance) in these radial velocities that is the key measurement
for their argument that this galaxy has little or no dark matter.

End quote

To said moderator. Not sure you have the time but could you expand
on this = in a way that a layman can understand. If it takes two
pages then so be it.

[[Mod. note -- ###
Sorry, no time. But you could start with
https://en.wikipedia.org/wiki/Dark_matter
https://en.wikipedia.org/wiki/Galaxy_rotation_curve
At least the beginnings of both of these articles are non-technical.
-- jt]]
  #6  
Old April 5th 18, 12:29 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 240
Default NGC1052-DF2 Diffuse Galaxy without dark matter

In article , Steve Willner
writes:

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?


Just a quick note since I am travelling with limited access and haven't
yet read the paper. In MOND, it is not the size which matters, but
rather the acceleration. MOND effects are supposed to kick in when the
acceleration is below some fiducial value. Accelerations are high in
the Solar System and low in the outskirts of galaxies.
  #7  
Old April 7th 18, 10:15 PM posted to sci.astro.research
Nicolaas Vroom
external usenet poster
 
Posts: 212
Default NGC1052-DF2 Diffuse Galaxy without dark matter

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
  #8  
Old April 7th 18, 10:21 PM posted to sci.astro.research
jacobnavia
external usenet poster
 
Posts: 100
Default Darker and darker

An important "proof" for dark matter has vanished: Astronomers take back
the observation that a cluster was missing its dark matter, "left
behind". See:
Royal Astronomical Society. "Dark matter might not be interactive after
all." ScienceDaily. ScienceDaily, 5 April 2018.
www.sciencedaily.com/releases/2018/04/180405223407.htm.

This happened with improved observations after several years.

Will this observation of a galaxy without dark matter hold?
  #9  
Old April 8th 18, 09:17 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 240
Default NGC1052-DF2 Diffuse Galaxy without dark matter

In article ,
Nicolaas Vroom writes:

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".


I think the OP was referring to "missing baryons" or "dark baryons". If
we believe the BBN values for baryons, then not all are accounted for.
They COULD be visible but they are not detectable by us now.

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.


Depending on what the dark matter is and how it is distributed, there
could be dark matter in the solar system. Its density wouldn't be high
enough to affect the dynamics. Some people think that dark matter might
actually accumulate inside stars.

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]]


By the time the flat-rotation-curve region is reached, there is so
little baryonic matter that one can assume that all such matter is
interior to the orbit. Assuming it is spherically distributed, the
effect is the same as that of a point mass at the centre.
  #10  
Old April 8th 18, 09:17 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
external usenet poster
 
Posts: 240
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". See:
Royal Astronomical Society. "Dark matter might not be interactive after
all." ScienceDaily. ScienceDaily, 5 April 2018.
www.sciencedaily.com/releases/2018/04/180405223407.htm.

This happened with improved observations after several years.

Will this observation of a galaxy without dark matter hold?


Keep in mind that this is ONE cluster. Modern cosmology is based on
several facts, which support each other. Even if one turns out to be
not quite correct, the whole edifice doesn't come toppling down.
 




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