57. A splash too far: “On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework”

The isolated but nearby galaxy NGC 3109 has a very high radial velocity compared to ΛCDM expectations, that is, it is moving away from the Local Group rapidly, as shown by Peebles (2017) and Banik & Zhao (2018). One of the few possible explanations within this framework is that NGC 3109 was once located within the virial radius of the Milky Way or Andromeda, before being flung out at high velocity in a three-body interaction with e.g. a massive satellite. In the new research paper “On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework”, which was led by Dr. Indranil Banik, it is shown that such a backsplash galaxy is extremely unlikely within the ΛCDM framework. Basically, such galaxies cannot occur in ΛCDM because they ought to be slowed-down due to Chandrasekhar dynamical friction exerted on NGC 3109 and its own dark matter halo by the massive and extended dark matter halo of the Milky way. Making it worse, NGC 3109 is in a thin plane of five associated galaxies (the “NGC 3109 association”, rms height 53 kpc; diameter 1.2 Mpc), all of which are moving away from the Local Group (Pawlowski & McGaugh 2014), whereby the dynamical friction ought to slow down the galaxies in dependence of their dark matter halo masses. This makes its thin planar structure today unexplainable in ΛCDM.

Interestingly, the backsplash scenario is favoured by the authors (Banik et al. 2021), but in the context of MOND. In this theory, much more powerful backsplash events are possible for dwarf galaxies near the spacetime location of the past Milky Way-Andromeda flyby because in MOND galaxies do not have dark matter halos made of particles. A galaxy thus orbits through the potential of another galaxy unhindered and ballistically. The envisioned flyby could also explain the otherwise mysterious satellite galaxy planes which are found around the Milky Way and Andromeda. It now seems that the flyby may well be the only way to explain the properties of NGC 3109, since a less powerful three-body interaction is just not strong enough to affect its velocity as much as would be required. But a Milky Way-Andromeda flyby is not possible in ΛCDM as their overlapping dark matter halos would merge.

In a series of Tweets, the co-author Dr. Marcel Pawlowski briefly explains on his Twitter account @8minutesold the main results of this recent publication:

Source: https://twitter.com/8minutesold/status/1392430171240677376

Source: https://twitter.com/8minutesold/status/1392430171240677376

In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.

4 thoughts on “57. A splash too far: “On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework”

    • Apologies for the late posting of your comment, I have not been checking the comments recently. The paper you mention is an interesting study and it implicitly claims to find evidence against MOND. Unfortunately the authors do not quantify with which significance the claims are made. The distance of DF2 is obtained via the TRGB method, which depends on the IMF and its variation with metallicity and SFR, none of which are known well. The galaxy DF2 may be near a host galaxy which produces the EFE. The authors argue that their distance measurement excludes this (but fail to give the confidence). The text of the paper appears to be written with a bias against MOND (why is the level of confidence not stated explicitly, given the uncertainties?). Thus, the original Nature paper by van Dokkum et al. “A galaxy lacking dark matter” (https://ui.adsabs.harvard.edu/abs/2018Natur.555..629V/abstract ) has many authors, three referees, and they all got it wrong (concerning their conclusion on MOND and probably also the distance). In fact, the error on the EFE was elementary, and would have been avoided if a MOND expert had been consulted (e.g. by the editors of Nature). Instead, Nature prefers, in this case at least, to publish an entirely wrong statement as long as it is against MOND it seems. This is duly unscientific and is damaging science. The journal Nature is playing a distinctly negative role here. Turning the coin around, the here-relevant galaxies DF2 and DF4 rule out LCDM at >5.8 sigma confidence if both lack dark matter and are at a distance of 20Mpc as claimed by the paper you mention: Haslbauer et al. 2019: “The ultra-diffuse dwarf galaxies NGC 1052-DF2 and 1052-DF4 are in conflict with standard cosmology” (https://ui.adsabs.harvard.edu/abs/2019MNRAS.489.2634H/abstract
      ). Put in other words: one dwarf galaxy lacking dark matter is exceedingly unlikely in LCDM, having two at the same time is next to impossible. Why do the authors of the paper you mention not discuss this important result which rigorously rules out LCDM? We need to avoid beliefs and personal biases and if the authors of the paper you mention had quantified the confidence of their claims and provided a balanced discussion of Haslbauer et al.’s results then we would be on more robust scientific footing. It seems that the same bias as drove that infamous Nature paper continues to drive the same authors: is it “politically correct” and generally accepted by the community to publish claims against MOND without even the need to state significance levels? We are working on an analysis of this problem and hopefully will be finishing this in due time.


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