lamdar said:
I’ve been reading about LCDM and MOND recently. And there have been reports of galaxies with little dark matter. I know the lack of dark matter in NGC1052-DF2 was shown not true afterwards. But if such a galaxy without dark matter is actually discovered, would this be sufficient to disprove MOND?
MOND predicts that galaxies will lack dark matter phenomena is they are either:
(1) in an area where there is a likely
external field effect from another nearby galaxy (which is the case in NGC1052-DF2
see Pieter van Dokkum, et al., "
The distance to NGC1042 in the context of its proposed association with the dark matter-deficient galaxies NGC1052-DF2 and NGC1052-DF4" (February 5, 2019) (To appear in RNAAS), and
here), or
(2) when it is sufficiently dense and compact that none of the stars are in the low gravitational acceleration MOND regime (inferred dark matter frequencies are lowest in compact, close to spherical, elliptical galaxies, and in the central regions of galaxies such as the central bulges of spiral galaxies).
Also, the usual way of detecting dark matter phenomena, in both LCDM and MOND assume gravitationally bound systems that are in equilibrium. If a galaxy is not in an equilibrium state (e.g., after a recent collision with another galaxy), the kinetic energy and atypical gravitational pulls arising from the collision make it much, much harder to determine what kind of dark matter distributions and/or MOND effects are at work from the observed dynamics. Essentially, something like a galaxy collision that puts a galaxy into a non-equilibrium state inserts so much noise into the system that the baseline gravitational signal you want to analyze becomes too difficult to separate out with current instruments and analysis tools.
To provide a serious challenge to MOND the "no dark matter" galaxy would need to be isolated from other external gravitational fields (e.g. in a "cosmic void"), diffuse, and in a near equilibrium state.
Even then, if the observation is a rare outlier even for those conditions, one might suspect an invisible mass (e.g. an isolated supermassive black hole) that creates an external field effect, or a systemic error in the observation (something that is particularly a concern at certain galaxy inclinations relative to our line of sight).
In the LCDM paradigm, no dark matter galaxies are explained by tidal stripping of dark matter from the no dark matter galaxy to a nearby galaxy, so a no dark matter galaxy that is diffuse, in isolation or a cosmic void, that is in equilibrium, also presents an equal problem for the LCDM paradigm.
One way of distinguishing MOND from the LCDM paradigm is to look at the distribution of inferred dark matter relative to ordinary matter in low surface brightness galaxies (which are diffuse).
MOND would predict a very strongly bimodal distribution with the vast majority of such galaxies either having extreme dark matter-like phenomena, or none at all (consistent with observations to date), with only a very small number of them in the intermediate transition range between systems with an external field effect and those without one.
In contrast, in LCDM, you would expect a far larger proportion of low surface brightness galaxies to have intermediate levels of inferred dark matter, since the tidal stripping of all dark matter from such galaxies could take a long time and many would be observed with that process only partially complete. There is not currently significant observational evidence of large numbers of galaxies that are partially dark matter deficient.
