The Phantom Dark Matter Halos of the Local Volume in the Context of Modified Newtonian Dynamics

Abstract

We explore the predictions of Milgromian gravity (MOND) in the local universe by considering the distribution of the “phantom” dark matter (PDM) that would source the MOND gravitational field in Newtonian gravity, allowing an easy comparison with the dark matter framework. For this, we specifically deal with the quasi-linear version of MOND (QUMOND). We compute the “stellar-to-(phantom)halo mass relation” (SHMR), a monotonically increasing power law resembling the SHMR observationally deduced from spiral galaxy rotation curves in the Newtonian context. We show that the gas-to-(phantom)halo mass relation is flat. We generate a map of the Local Volume in QUMOND, highlighting the important influence of distant galaxy clusters, in particular Virgo. This allows us to explore the scatter of the SHMR and the average density of PDM around galaxies in the Local Volume, ΩPDM ≈ 0.1, below the average cold dark matter density in a ΛCDM universe. We provide a model of the Milky Way in its external field in the MOND context, which we compare to an observational estimate of the escape velocity curve. Finally, we highlight the peculiar features related to the external field effect in the form of negative PDM density zones in the outskirts of each galaxy, and test a new analytic formula for computing galaxy rotation curves in the presence of an external field in QUMOND. While we show that the negative PDM density zones would be difficult to detect dynamically, we quantify the weak-lensing signal they could produce for lenses at z ∼ 0.3.

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity

“…We can think of this as an effective density −4πGρ eff because the Newtonian gravity sourced by ρ eff is the same as the QUMOND gravity g due to ρ s alone. Note that ρ eff is not a real density but a mathematical quantity which can be negative over rather large regions [65,66]. Thinking of QUMOND in this way can help, not least because it allows the use of standard codes where ρ eff is fed in as the density distribution in a second step after first obtaining g N from ρ alone.…”

“…However, there could be an extended halo of hot gas, which we might expect would be flattened along the disc spin axis. Moreover, the EFE from large-scale structure can also induce anisotropy in the potential, even for a point mass lens (Equation (34); see also [66]). Since the orientation of the central disc is likely to correlate with large-scale structure due to the EFE and other effects, it is possible that the PDM halo of a galaxy is typically flattened along the same axis as its disc.…”

“…Since there is no direct evidence for the DM halo of the MW, it is possible to choose a near-spherical halo consistent with the Sagittarius tidal stream. This would make for a similar halo to the phantom halo in MOND, which would be almost spherical at distances exceeding a few disc scale lengths [102], though the EFE from other galaxies becomes relevant at distances of several hundred kpc and causes the potential to become flattened in a well-defined manner (as discussed in Section 2.4; see also [66]). Consequently, it is difficult to distinguish between ΛCDM and MOND using the Sagittarius tidal stream [490].…”

“…Note that this equation is only an approximation even in spherical symmetry-one should instead use a smooth interpolating function between the different asymptotic limits (see Section 2.3), yielding g = νg N . The phantom density ρ p is generally positive, but it can be negative in more complicated geometries [64,65], potentially leading to a clear smoking gun signal for a breakdown of GR [66].…”

3D hydrodynamic simulations for the formation of the Local Group satellite planes

“…This is a critical next step for MOND, though care is required when comparing with observations as these could have a rather different interpretation to what is usually assumed. One possible smoking gun signature of MOND in weak lensing convergence maps would be the discovery that the convergence parameter is negative in some regions (Oria et al 2021). This cannot arise in GR and is possible only in gravitational theories that are non-linear in the weak-field regime.…”