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.
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