Cold Dark Matter Still Doesn't Work In Galaxies

Replication of observational claims is good in science because it makes the results more robust to systemic errors that are not obvious from prior observations. 

The hypothesis that dark matter phenomena are due to collisionless matter that interacts only gravitationally has been repeatedly proven wrong by observational evidence. Once again, a new study replicates the conclusion that cold dark matter is observationally inconsistent with the dark matter halos inferred from the distribution and dynamics of ordinary matter in galaxies. 

This latest study, in particular, rules out baryonic feedback in galaxies as a possible explanation for the discrepancy between theory and observation in cold dark matter theory as a resolution of the so called core-cusp problem with cold dark matter.

According to the current concordance cosmological model, the dark matter (DM) particles are collision-less and produce self-gravitating structures with a central cusp which, generally, is not observed. 

The observed density tends to a central plateau or core, explained within the cosmological model through the gravitational feedback of baryons on DM. This mechanism becomes inefficient when decreasing the galaxy stellar mass so that in the low-mass regime (Mstar << 10**6 Msun) the energy provided by the baryons is insufficient to modify cusps into cores. Thus, if cores exist in these galaxies they have to reflect departures from the collision-less nature of DM. 

Measuring the DM mass distribution in these faint galaxies is extremely challenging, however, their stellar mass distribution can be characterized through deep photometry. Here we provide a way of using only the stellar mass distribution to constrain the underlying DM distribution. The so-called Eddington inversion method allows us to discard pairs of stellar distributions and DM potentials requiring (unphysical) negative distribution functions in the phase space. In particular, cored stellar density profiles are incompatible with the Navarro, Frenk, and White (NFW) potential expected from collision-less DM if the velocity distribution is isotropic and the system spherically symmetric. Through a case-by-case analysis, we are able to relax these assumptions to consider anisotropic velocity distributions and systems which do not have exact cores. In general, stellar distributions with radially biased orbits are difficult to reconcile with NFW-like potentials, and cores in the baryon distribution tend to require cores in the DM distribution.

Jorge Sanchez Almeida, Angel R. Plastino, Ignacio Trujillo, "Can cuspy dark matter dominated halos hold cored stellar mass distributions?" arXiv:2307.01256 (July 3, 2023) (Accepted for publication in ApJ).