A plane of high-velocity galaxies across the Local Group

Abstract

We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than predicted by a 3D dynamical model of the standard cosmological paradigm. Here, we show that 6 of these 7 galaxies define a thin plane with root mean square thickness of only 101 kpc despite a widest extent of nearly 3 Mpc, much larger than the conventional virial radius of the Milky Way (MW) or M31. This plane passes within ∼ 70 kpc of the MW-M31 barycentre and is oriented so the MW-M31 line is inclined by 16• to it. We develop a toy model to constrain the scenario whereby a past MW-M31 flyby in Modified Newtonian Dynamics (MOND) forms tidal dwarf galaxies that settle into the recently discovered planes of satellites around the MW and M31. The scenario is viable only for a particular MW-M31 orbital plane. This roughly coincides with the plane of LG dwarfs with anomalously high RVs.Using a restricted N -body simulation of the LG in MOND, we show how the once fast-moving MW and M31 gravitationally slingshot test particles outwards at high speeds. The most distant such particles preferentially lie within the MW-M31 orbital plane, probably because the particles ending up with the highest RVs are those flung out almost parallel to the motion of the perturber. This suggests a dynamical reason for our finding of a similar trend in the real LG, something not easily explained as a chance alignment of galaxies with an isotropic or mildly flattened distribution (probability = 0.0015).

Origin of the Local Group satellite planes

“…For M31, the equivalent quantity is 6.5 × 10 −6 . As discussed in Section 4.4, we expect the MW-M31 orbital plane to align with the plane of high-velocity galaxies discovered by Banik & Zhao (2018b). sense in which the HVGs rotate within their preferred plane, we thus add an extra contribution to the model χ 2 of…”

“…For M31, we use 21.2 kpc radially and 15.9 kpc vertically. Our plane-fitting procedure is similar to that used in section 5.1 of Banik & Zhao (2018b) but also considers the unequal statistical weights of the particles and applies outlier rejection with a threshold of 2.58σ, corresponding to the 99% confidence interval of a Gaussian distribution. As rejecting outliers reduces the root mean square (rms) plane thickness, we repeat the procedure iteratively until the algorithm no longer finds any outliers.…”

“…We recently revisited this analysis by performing a much more thorough search for the best-fitting model (Banik & Zhao 2018b, section 4.1). As our best-fitting model still yielded several HVGs that it could not account for, we considered possible ΛCDM-based explanations for them at some length (see their section 7.2).…”

Testing gravity with interstellar precursor missions

“…However, this has led to several problems (Kroupa 2012(Kroupa , 2015. In the Local Group, the most serious are the existence of satellite galaxy planes (Pawlowski 2018) and high-velocity dwarfs (Banik & Zhao 2018c). Both problems are rather immune to baryonic physics because they involve length scales of several hundred kpc and relate to the motion of galaxies as a whole.…”

Directly testing gravity with Proxima Centauri

“…However, this approach causes tension with observational data, in particular the apparent absence of dynamical friction expected to arise between the postulated massive and extended dark matter halos (Angus et al 2011;Kroupa 2012Kroupa , 2015Oehm et al 2017). Another problem is that several Local Group dwarf galaxies have anomalously high radial velocities compared to a detailed three-dimensional timing argument calculation (Peebles 2017; Banik & Zhao 2018b). The latter authors showed that these dwarfs are more naturally explained in MOND due to its implication that there was a past close flyby between the Milky Way and Andromeda galaxies ≈ 8 Gyr ago (Zhao et al 2013;Bílek et al 2018;Banik et al 2018).…”