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NGC1052-DF2 Diffuse Galaxy without dark matter
[[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 |
#12
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NGC1052-DF2 Diffuse Galaxy without dark matter
In article ,
Nicolaas Vroom 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 Cambridge, MA 02138 USA |
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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". 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 Cambridge, MA 02138 USA |
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NGC1052-DF2 Diffuse Galaxy without dark matter
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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