• NGC1052-DF2 Diffuse Galaxy without dark matter

    From Martin Brown@21:1/5 to All on Fri Mar 30 23:46:48 2018
    [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

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  • From jacob navia@21:1/5 to All on Sun Apr 1 16:43:27 2018
    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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  • From Steve Willner@21:1/5 to Martin Brown on Tue Apr 3 11:04:04 2018
    In article <p9kvng$nti$1@gioia.aioe.org>,
    Martin Brown <newspam@nezumi.demon.co.uk> 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 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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  • From Gary Harnagel@21:1/5 to Steve Willner on Wed Apr 4 11:54:22 2018
    On Tuesday, April 3, 2018 at 3:04:06 AM UTC-6, Steve Willner wrote:

    In article <p9kvng$nti$1@gioia.aioe.org>,
    Martin Brown <newspam@nezumi.demon.co.uk> 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]]

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  • From John Heath@21:1/5 to All on Wed Apr 4 13:51:38 2018
    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]]

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  • From Phillip Helbig (undress to reply)@21:1/5 to willner@cfa.harvard.edu on Thu Apr 5 13:29:18 2018
    In article <p9tub3$qrp$1@dont-email.me>, Steve Willner <willner@cfa.harvard.edu> 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.

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  • From Nicolaas Vroom@21:1/5 to Martin Brown on Sat Apr 7 23:15:36 2018
    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_matter#Baryonic_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_matter#Galaxy_rotation_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

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  • From jacobnavia@21:1/5 to All on Sat Apr 7 23:21:45 2018
    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?

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  • From Phillip Helbig (undress to reply)@21:1/5 to Nicolaas Vroom on Sun Apr 8 22:17:08 2018
    In article <c88a8629-cf1c-404b-af46-46b2d74987f5@googlegroups.com>,
    Nicolaas Vroom <nicolaas.vroom@pandora.be> 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_matter#Galaxy_rotation_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.

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  • From Phillip Helbig (undress to reply)@21:1/5 to jacob@jacob.remcomp.fr on Sun Apr 8 22:17:23 2018
    In article <pa9ttf$6ed$1@dont-email.me>, jacobnavia
    <jacob@jacob.remcomp.fr> 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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  • From brad@21:1/5 to All on Wed Apr 11 11:13:40 2018
    [[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

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  • From Steve Willner@21:1/5 to Nicolaas Vroom on Wed Apr 11 20:55:19 2018
    In article <c88a8629-cf1c-404b-af46-46b2d74987f5@googlegroups.com>,
    Nicolaas Vroom <nicolaas.vroom@pandora.be> 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 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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  • From Steve Willner@21:1/5 to jacobnavia on Fri Apr 13 12:24:59 2018
    In article <pa9ttf$6ed$1@dont-email.me>,
    jacobnavia <jacob@jacob.remcomp.fr> 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 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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  • From Homo Lykos@21:1/5 to All on Fri Apr 13 18:37:27 2018
    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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