From that paper:MOND and the dynamics of NGC1052-DF2
B. Famaey, S. McGaugh, M. Milgrom
(Submitted on 11 Apr 2018)
The dwarf galaxy NGC1052-DF2 has recently been identified as potentially lacking dark matter. If correct, this could be a challenge for MOND, which predicts that low surface brightness galaxies should evince large mass discrepancies. However, the correct prediction of MOND depends on both the internal field of the dwarf and the external field caused by its proximity to the giant elliptical NGC1052. Taking both into consideration under plausible assumptions, we find σMOND=13.4+4.8−3.7kms−1. This is only marginally higher than the claimed 90\% upper limit on the velocity dispersion (σ<10.5kms−1), and compares well with the observed root mean square velocity dispersion (σ=14.3kms−1). We also discuss a few caveats on both the observational and theoretical side. On the theory side, the internal virialization time in this dwarf may be longer that the time scale of variation of the external field. On the observational side, the paucity of data and their large uncertainties call for further analysis of the velocity dispersion of NGC1052-DF2, to check whether it poses a challenge to MOND or is a success thereof.
In contrast, the paper notes, this is a big problem for the Standard Model of Cosmology: "With the important exception of tidal dwarfs (Bournaud et al. 2007; Gentile et al. 2007; Lelli et al. 2015), it is thought that a gravitationally-dominant dark matter halo is the sine qua non for the formation of a galaxy. If galaxies such as NGC1052-DF2 are fairly common we may have to revise our concept of what a galaxy is, and come up with alternative pathways for creating galaxy-mass stellar systems."MOND also has a good track record of predictive success for pressure supported systems like NGC1052-DF2 (McGaugh & Milgrom 2013a,b; Pawlowski & McGaugh 2014; McGaugh 2016). The analysis in such cases is complicated by the same uncertainties as in Newtonian analyses, such as that in the stellar mass-to-light ratio and the unknown anisotropy in the velocity tracers. Unique to MOND is the external field effect (EFE, Milgrom 1983; Bekenstein & Milgrom 1984; Famaey & McGaugh 2012; Haghi et al. 2016; Hees et al. 2016). Because of the nonlinearity of MOND, the internal dynamics of a system can be affected by the external gravitational field in which it is immersed. When the external field dominates over the internal one, the amplitude of the MOND effect, and the corresponding amount of dark matter inferred, is reduced. Some interesting effects unique to MOND are related to the EFE, such as the prediction of asymmetric tidal streams of globular clusters (Thomas et al. 2018). An essential consequence of the EFE in MOND is that the predicted velocity dispersion of a dwarf galaxy depends on its environment. An object in isolation is expected to have a higher velocity dispersion than the same object in orbit around a massive host. This difference is perceptible in pairs of photometrically indistinguishable dwarf satellites of Andromeda (McGaugh & Milgrom 2013b). Indeed, the EFE was essential to the correct a priori prediction (McGaugh & Milgrom 2013a) of the velocity dispersions of the dwarfs And XIX, And XXI, and And XXV. These cases are notable for their large scale lengths and low velocity dispersions (Collins et al. 2013) — properties that were surprising in the context of dark matter but are natural in MOND. A further example is provided by the recently discovered Milky Way satellite Crater 2 (Torrealba et al. 2016). McGaugh (2016) predicted that this object would have a velocity dispersion of 2.1 +0.9 −0.6 km s−1 , much lower than the nominal expectation in the context of dark matter. Caldwell et al. (2017) subsequently observed 2.7±0.3 km s−1 . Here we take the external field of the host galaxy NGC1052 into account to predict the expected velocity dispersion of NGC1052-DF2 in MOND.
A tweet from van Dokkum (the lead author) states:A second galaxy missing dark matter in the NGC1052 group
Pieter van Dokkum, Shany Danieli, Roberto Abraham, Charlie Conroy, Aaron J. Romanowsky
(Submitted on 17 Jan 2019)
The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of D=19.9±2.8 Mpc; and has a similar population of luminous globular clusters extending out to 7 kpc from the center of the galaxy. Accurate radial velocities of seven clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. Their median velocity is ⟨v⟩=1445 km/s, close to the central velocity of 22 galaxies in the NGC1052 group. The rms spread of the observed velocities is very small at σobs=5.8 km/s. Taking observational uncertainties into account we determine an intrinsic velocity dispersion of σintr=4.2+4.4−2.2 km/s, consistent with the expected value from the stars alone (σstars≈7 km/s) and lower than expected from a standard NFW halo (σhalo∼30 km/s). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.
This image was distributed by van Dokkum:
The fact that the discoverers of this galaxy's interesting properties state that the "origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood," doesn't necessarily imply that nobody else in the astronomy and physics community has an explanation.If the astronomers who observe this don't know, can you expect an answer here?
Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio Mdm/Mstar<1) in the Illustris simulation of structure formation in the flat-ΛCDM cosmogony, we find $M_rm star approximately 2 times 10^8, M_sun galaxies that have properties similar to those ascribed by (vanDokkumetal 2018a) to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with M_rm star approximately 9 times 10^10 M_sun and Mdm/Mstar ≈ 0.1 and a half-stellar-mass radius of ≈ 2 kpc. If the Illustris simulation and the ΛCDM model are accurate, there are a significant number of dark matter deficient galaxies, including massive luminous compact ones. It will be interesting to observationally discover these galaxies, and to also more clearly understand how they formed, as they are likely to provide new insight into and constraints on models of structure formation and the nature of dark matter.
Hai Yu, Bharat Ratra, Fa-Yin Wang, "Dark matter deficient galaxies in the Illustris flat-ΛCDM model structure formation simulation" (September 16, 2018).