On the separation between baryonic and dark matter: evidence for phantom dark matter?

TL;DR

This paper investigates whether phantom dark matter, based on MONDian gravity, can explain observed offsets in galaxy clusters, concluding it is unlikely and supporting the need for true non-collisional matter.

Contribution

The study assesses the potential of phantom dark matter to account for cluster offsets within a MOND framework, finding it insufficient and reinforcing the role of actual non-collisional matter.

Findings

Phantom dark matter can produce offsets in idealized models with external fields.

In a cosmological context, the effect size is too small to explain observations.

Neutrino-like matter may be responsible for observed offsets.

Abstract

The recent years have seen combined measurements of X-ray and (weak) lensing contours for colliding galaxy clusters such as, for instance, the famous "Bullet" cluster. These observations have revealed offsets in the peaks of the baryonic and (dominant) gravitational matter component of order ~(100-200) kpc. Such discrepancies are difficult to explain using modified theories for gravity other than dark matter. Or are they not? Here we use the concept of "phantom dark matter" that is based upon a Newtonian interpretation of the MONDian gravitational potential. We show that this idea is in fact capable of producing substantial offsets in idealistic density configurations, involving a uniform external field. However, when analysed in a MONDian cosmological framework we deduce that the size (and probablity) of the effect is too small to explain the observed offsets found in the most recent observations, at least in the simplest incarnation of phantom dark matter as discussed here. The lensing centers in merging galaxy clusters are likely very close to the centers of true mass even in a MONDian cosmology. This gives the support to the idea that neutrino-like non-collisional matter might be responsible for the observed offsets of lensing and X-ray peaks.