5 ms·

Yeah, well. That is an observation that is pretty bloody difficult to explain with MOND type theories. Any MOND proponents want to have a try?

cmarschner 5y ago
(What is meant here is Modified Newton Dynamics, see https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics )
dogma1138 5y ago
Not a MOND proponent but why does it exactly contradict it? It’s a UDG with very little to no star forming gas and very low overall mass and density, looking up the galaxy it’s about the same size Milky Way but has less than 0.5% of its stars and much less than that when it comes to overall mass. It seems that there are some papers that attempt to reconcile the observations with MOND and on the face of it the velocities expected if MOND holds true and are not far off from the current observations. https://arxiv.org/abs/1804.04167 https://arxiv.org/abs/1804.04167 Considering the distance of the galaxy (72m light years) and the difficulty of establishing the actual mass of UDGs and even globular clusters at that distance it doesn’t look like a MOND killer not nearly as much as gravitational waves and the overall challenges to generalize MOND and make it relativistic.
- fghorow 5y ago
  Let me preface this by saying that I am by NO means an expert in any of this. My thought process was simply along the lines of: 1) If MOND is true, it should be evident in the rotational structure of every galaxy observed. 2) Here is a galaxy that doesn't (at first glance, at least -- I need to read the paper you pointed out) show such rotational structure. 3) Hence, this is an "outlier" observation in the panoply of the MOND vs dark matter debate -- one in which MOND seems unsupportable. Nothing deeper than that, and again this is ABSOLUTELY NOT my field!
  - fghorow 5y ago
    FWIW, NGC-1052-DF2 has it's own Wikipedia page, that discusses (and more importantly cites some literature about) the relation between it and MOND. https://en.wikipedia.org/wiki/NGC_1052-DF2 https://en.wikipedia.org/wiki/NGC_1052-DF2
  - dnautics 5y ago
    I think one should run simulations of MOND first; MOND does not directly make a statement about rotational structures; it makes a statement about the gravity law. This galaxy clearly has unique structural considerations that could yield results that you don't expect from a trivial glance at higher order factors. Edit, after digging: someone has, apparently. https://www.researchgate.net/publication/336982682_A_new_formulation_of_the_external_field_effect_in_MOND_and_numerical_simulations_of_ultra-diffuse_dwarf_galaxies_-_application_to_NGC_1052-DF2_and_NGC_1052-DF4 https://www.researchgate.net/publication/336982682_A_new_for... I can't speak to whether or not the paper is any good, but my quick understanding is that: the galaxy is consistent with MOND if the distance to the galaxy has been misestimated; at least one published estimate (that disagrees with most estimations) of how far it is to 1052 falls inside the range where it's explainable by mond. My (limited; I last took astro a very long time ago) understanding is that how we measure distances to foreign galaxies (slam-dunk methods like cepheids don't work if there are no cepheids) could be very sketchy for galaxies of different structures. Here is part of the ongoing debate about distance. Linked is a paper from the perspective that would disfavor MOND (the Trujillo estimate is what needs to be true for 1052 to be consistent with MOND according to the first paper): https://www.pietervandokkum.com/ngc1052-df2 https://www.pietervandokkum.com/ngc1052-df2
    - stan_rogers 5y ago
      MOND has very little to say about gravity. It claims that f=ma breaks down at sufficiently low values. That, in turn, implies that the rules should still be consistent with these modifications, and that galaxies with apparently different proportions of dark matter (as it's understood) shouldn't exist - all galaxies with similar visible sizes and structures should be essentially identical in gravitational behaviour, and that simply hasn't been demonstrated to be the case.
- DebtDeflation 5y ago
  >the galaxy it’s about the same size Milky Way but has less than 0.5% of its stars and much less than that when it comes to overall mass. What's the mechanism behind the formation of such a sparsely populated galaxy? Was there just way less gas in that region of space for stars to form in the first place? How is there even enough gravity to bind those stars into a galaxy rather than just a forming a series of rogue stars that move independently of one another?
  - ncmncm 5y ago
    Nobody knows. Galaxies like that were not supposed to be possible.
gizmondo 5y ago
https://tritonstation.com/2018/04/04/the-dwarf-galaxy-ngc1052-df2/ https://tritonstation.com/2018/04/04/the-dwarf-galaxy-ngc105... A post by MOND proponent. Not sure what if anything changed in the data since then.

cmarschner 5y ago

(What is meant here is Modified Newton Dynamics, see https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics )

dogma1138 5y ago

Not a MOND proponent but why does it exactly contradict it? It’s a UDG with very little to no star forming gas and very low overall mass and density, looking up the galaxy it’s about the same size Milky Way but has less than 0.5% of its stars and much less than that when it comes to overall mass. It seems that there are some papers that attempt to reconcile the observations with MOND and on the face of it the velocities expected if MOND holds true and are not far off from the current observations. https://arxiv.org/abs/1804.04167 https://arxiv.org/abs/1804.04167 Considering the distance of the galaxy (72m light years) and the difficulty of establishing the actual mass of UDGs and even globular clusters at that distance it doesn’t look like a MOND killer not nearly as much as gravitational waves and the overall challenges to generalize MOND and make it relativistic.

fghorow 5y ago
Let me preface this by saying that I am by NO means an expert in any of this. My thought process was simply along the lines of: 1) If MOND is true, it should be evident in the rotational structure of every galaxy observed. 2) Here is a galaxy that doesn't (at first glance, at least -- I need to read the paper you pointed out) show such rotational structure. 3) Hence, this is an "outlier" observation in the panoply of the MOND vs dark matter debate -- one in which MOND seems unsupportable. Nothing deeper than that, and again this is ABSOLUTELY NOT my field!
- fghorow 5y ago
  FWIW, NGC-1052-DF2 has it's own Wikipedia page, that discusses (and more importantly cites some literature about) the relation between it and MOND. https://en.wikipedia.org/wiki/NGC_1052-DF2 https://en.wikipedia.org/wiki/NGC_1052-DF2
- dnautics 5y ago
  I think one should run simulations of MOND first; MOND does not directly make a statement about rotational structures; it makes a statement about the gravity law. This galaxy clearly has unique structural considerations that could yield results that you don't expect from a trivial glance at higher order factors. Edit, after digging: someone has, apparently. https://www.researchgate.net/publication/336982682_A_new_formulation_of_the_external_field_effect_in_MOND_and_numerical_simulations_of_ultra-diffuse_dwarf_galaxies_-_application_to_NGC_1052-DF2_and_NGC_1052-DF4 https://www.researchgate.net/publication/336982682_A_new_for... I can't speak to whether or not the paper is any good, but my quick understanding is that: the galaxy is consistent with MOND if the distance to the galaxy has been misestimated; at least one published estimate (that disagrees with most estimations) of how far it is to 1052 falls inside the range where it's explainable by mond. My (limited; I last took astro a very long time ago) understanding is that how we measure distances to foreign galaxies (slam-dunk methods like cepheids don't work if there are no cepheids) could be very sketchy for galaxies of different structures. Here is part of the ongoing debate about distance. Linked is a paper from the perspective that would disfavor MOND (the Trujillo estimate is what needs to be true for 1052 to be consistent with MOND according to the first paper): https://www.pietervandokkum.com/ngc1052-df2 https://www.pietervandokkum.com/ngc1052-df2
  - stan_rogers 5y ago
    MOND has very little to say about gravity. It claims that f=ma breaks down at sufficiently low values. That, in turn, implies that the rules should still be consistent with these modifications, and that galaxies with apparently different proportions of dark matter (as it's understood) shouldn't exist - all galaxies with similar visible sizes and structures should be essentially identical in gravitational behaviour, and that simply hasn't been demonstrated to be the case.
DebtDeflation 5y ago
>the galaxy it’s about the same size Milky Way but has less than 0.5% of its stars and much less than that when it comes to overall mass. What's the mechanism behind the formation of such a sparsely populated galaxy? Was there just way less gas in that region of space for stars to form in the first place? How is there even enough gravity to bind those stars into a galaxy rather than just a forming a series of rogue stars that move independently of one another?
- ncmncm 5y ago
  Nobody knows. Galaxies like that were not supposed to be possible.

gizmondo 5y ago

https://tritonstation.com/2018/04/04/the-dwarf-galaxy-ngc1052-df2/ https://tritonstation.com/2018/04/04/the-dwarf-galaxy-ngc105... A post by MOND proponent. Not sure what if anything changed in the data since then.