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Thread: Can dark matter be pulled out of a small galaxy by a larger galaxy?

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    Question Can dark matter be pulled out of a small galaxy by a larger galaxy?

    Hope to not embarrass myself. This paper, as I read it, implies that a dwarf galaxy's dark matter can be pulled from it by the gravitational tides of a major galaxy. Am I reading this correctly? If not, where is the misunderstanding?

    https://arxiv.org/abs/1904.10461

    The effect of tides on the Sculptor dwarf spheroidal galaxy

    G. Iorio, C. Nipoti, G. Battaglia, A. Sollima (Submitted on 23 Apr 2019)

    Dwarf spheroidal galaxies (dSphs) appear to be some of the most dark matter dominated objects in the Universe. Their dynamical masses are commonly derived using the kinematics of stars under the assumption of equilibrium. However, these objects are satellites of massive galaxies (e.g.\ the Milky Way) and thus can be influenced by their tidal fields. We investigate the implication of the assumption of equilibrium focusing on the Sculptor dSph by means of ad-hoc N -body simulations tuned to reproduce the observed properties of Sculptor following the evolution along some observationally motivated orbits in the Milky Way gravitational field. For this purpose, we used state-of-the-art spectroscopic and photometric samples of Sculptor's stars. We found that the stellar component of the simulated object is not directly influenced by the tidal field, while ≈30%−60% the mass of the more diffuse DM halo is stripped. We conclude that, considering the most recent estimate of the Sculptor proper motion, the system is not affected by the tides and the stellar kinematics represents a robust tracer of the internal dynamics. In the simulations that match the observed properties of Sculptor, the present-day dark-to-luminous mass ratio is ≈6 within the stellar half-light radius (≈0.3 kpc) and >50 within the maximum radius of the analysed dataset (≈1.5 ∘ ≈2 kpc).
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    The basic idea is that under certain very specific conditions, when the motion of the companion galaxy just happens to have an orbital period that syncs up in the right way with the orbital motions of stars in the about-to-be-stripped galaxy, the companion can separate dark matter from stars.

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    I've skimmed the paper, and what they seem to be saying is that they cannot offer any explanation as to why dwarf spherical galaxies tend to have high dark matter content, but the presence of a large neighboring galaxy does not create any significant problems in interpreting the dark matter content. That is, using the stellar dynamics to understand the dark matter situation is valid for this system, despite the tidal effects of the neighboring galaxy, because the stars are not strongly affected by the tidal field.

    That would seem to support previous conclusions that this system has a large dark matter contingent, but they don't actually say what the previous estimates were, they only say that their result for the dark-to-luminous ratio varies from 6 to 50 as you go out into the halo. That seems on the high side using the Milky Way as a comparison, but it's not clear if it agrees with previous estimates or if it represents a modification of that ratio. Also, they say that tidal forces can strip 30-60% of the diffuse dark matter halo (which isn't terribly surprising since tidal forces will be more important for the diffuse gas than the tightly bound stellar component), so this dwarf galaxy had an even higher dark matter component in its distant past. That would seem to argue that dwarf galaxies in orbit around bigger galaxies can lose a significant fraction of their dark matter to tidal stripping, yet still end up being higher in dark matter fraction than larger galaxies.

    But in answer to the question in the OP, yes, some of the dark matter in an extended halo can be stripped from a dwarf galaxy by a neighboring galaxy. The reason this is possible is that dark matter tends to be much more spread out, and hence more weakly bound and vulnerable to being stripped off by external gravity-- almost like how the gravity of a binary companion can strip the envelope off of a star.

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    Thank you both, that helped.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Second question occurred to me: could gravitational interaction with a large mass (e.g., another galaxy) pull most or all of the dark matter out of a smaller galaxy? Wondering if this related to recent reports of galaxies lacking dark matter.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Sounds like the same question, but yes, that is a thought for how it might be done. Ironically, some people feel that it is the galaxies that don't have dark matter that provide the best evidence for dark matter in the ones that do, because it creates a possibility for contrast. But there is always some MOND type theory that can explain any given situation you can think of, the problem is it takes combinations of multiple MOND effects to explain all the situations.

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    Quote Originally Posted by Ken G View Post
    Sounds like the same question, but yes, that is a thought for how it might be done. Ironically, some people feel that it is the galaxies that don't have dark matter that provide the best evidence for dark matter in the ones that do, because it creates a possibility for contrast. But there is always some MOND type theory that can explain any given situation you can think of, the problem is it takes combinations of multiple MOND effects to explain all the situations.
    You are right, I did not phrase that well. I meant, could this account for a lack of DM in some galaxies. My apologies. Thank you for the answer.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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