Inferred Dark Matter Distributions In Galaxy Clusters Also Track Baryonic Matter

We study the total and dark matter (DM) density profiles as well as their correlations for a sample of 15 high-mass galaxy clusters by extending our previous work on several clusters from Newman et al. Our analysis focuses on 15 CLASH X-ray-selected clusters that have high-quality weak- and strong-lensing measurements from combined Subaru and Hubble Space Telescope observations. The total density profiles derived from lensing are interpreted based on the two-phase scenario of cluster formation. 

In this context, the brightest cluster galaxy (BCG) forms in the first dissipative phase, followed by a dissipationless phase where baryonic physics flattens the inner DM distribution. This results in the formation of clusters with modified DM distribution and several correlations between characteristic quantities of the clusters. 

We find that the central DM density profiles of the clusters are strongly influenced by baryonic physics as found in our earlier work. The inner slope of the DM density for the CLASH clusters is found to be flatter than the Navarro--Frenk--White profile, ranging from α=0.30 to 0.79. We examine correlations of the DM density slope α with the effective radius Re and stellar mass Me of the BCG, finding that these quantities are anti-correlated with a Spearman correlation coefficient of ∼−0.6. We also study the correlation between Re and the cluster halo mass M500, and the correlation between the total masses inside 5 kpc and 100 kpc. We find that these quantities are correlated with Spearman coefficients of 0.68 and 0.64, respectively. These observed correlations are in support of the physical picture proposed by Newman et al.

Antonino Del Popolo et al., "Correlations between the Dark Matter and Baryonic Properties of CLASH Galaxy Clusters" (August 6, 2018).

This is not a smoking gun for any theory, but it reaffirmed the lesson of inferred dark matter distributions in galaxies that inferred dark matter distributions closely track the distribution of ordinary baryonic matter there. This means that the an appropriate modified gravity theory can probably also explain the dark matter phenomena of clusters, even though the simply toy model formula of MOND is not adequate to do so.

The failure to the NFW distribution of dark matter in clusters as well as has been previously shown, in galaxies, also strongly supports the conclusion that if there are indeed dark matter particles, that they interact with ordinary matter more strongly than by gravity alone, and hence, dark matter cannot be truly collisionless.

As an aside, some of these authors were also authors of a recent paper claiming that the data do not disclose a fundamental acceleration scale, which is simply shoddy work (see critiques spelling out the flaws in their analysis here and here). So a grain of salt may be required when looking at their data analysis and conclusions.

On the other hand, a 2017 paper by many of the same authors confirms as other have found that the NFW inferred dark matter halo shape is a poor fit to more than 75% of galaxies and an indifferent fit to the remainder, even in large galaxies where the usual justifications for deviations from the NFW shaped halo expected for collisionless dark matter are weaker.  Its abstract is as follows, emphasis added:

We develop and apply new techniques in order to uncover galaxy rotation curves (RC) systematics. Considering that an ideal dark matter (DM) profile should yield RCs that have no bias towards any particular radius, we find that the Burkert DM profile satisfies the test, while the Navarro-Frenk-While (NFW) profile has a tendency of better fitting the region between one and two disc scale lengths than the inner disc scale length region. Our sample indicates that this behaviour happens to more than 75% of the galaxies fitted with an NFW halo. Also, this tendency does not weaken by considering "large" galaxies, for instance those with M∗≳1010M⊙. Besides the tests on the homogeneity of the fits, we also use a sample of 62 galaxies of diverse types to perform tests on the quality of the overall fit of each galaxy, and to search for correlations with stellar mass, gas mass and the disc scale length. In particular, we find that only 13 galaxies are better fitted by the NFW halo; and that even for the galaxies with M∗≳1010M⊙ the Burkert profile either fits as good as, or better than, the NFW profile. This result is relevant since different baryonic effects important for the smaller galaxies, like supernova feedback and dynamical friction from baryonic clumps, indicate that at such large stellar masses the NFW profile should be preferred over the Burkert profile. Hence, our results either suggest a new baryonic effect or a change of the dark matter physics.

A 2016 paper by one of the authors and a co-author summarizes the small scale issue of LambadCDM:

The ΛCDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem , the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. ΛCDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the ΛDCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the ΛCDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe.