Neutrinos as galactic dark matter in the Ursa Major galaxy group?

TL;DR

This study analyzes galaxy rotation curves in the Ursa Major group within the MOND framework, exploring the potential presence of a neutrino halo as an additional dark matter component, and finds marginal evidence for such a halo.

Contribution

It introduces a speculative neutrino halo component into MOND fits for galaxy rotation curves and constrains its density using observational data.

Findings

Best-fit neutrino density is non-zero at 87% confidence.

Upper limit for neutrino density is 9.6 x 10^{-27} g/cm^3 at 95% confidence.

Results are consistent with sterile neutrino models of 5 eV mass.

Abstract

We present the analysis of 23 published rotation curves of disk galaxies belonging to the Ursa Major group of galaxies, with kinematics free of irregularities. The rotation curves are analysed in the context of MOND (Modified Newtonian Dynamics). We add an extra component to the rotation curve fits, in addition to the stellar and gaseous disks: a speculative halo of constant density made of, e.g., neutrinos, which would solve the bulk of the problem currently faced by MOND on rich galaxy clusters scales. We find that this additional unseen mass density is poorly constrained (as expected a priori, given that a neutrino halo never dominates the kinematics), but we also find that the best-fit value is non-zero: rho = 3.8 x 10^{-27} g/cm^3, and that a zero-density is marginally excluded with 87% confidence; also, the 95% confidence upper limit for the density is rho = 9.6 x 10^{-27} g/cm^3. These limits are slightly above the expectations from the Tremaine-Gunn phase space constraints on ordinary 2 eV neutrinos, but in accordance with the maximum density expected for one or two species of 5 eV sterile neutrinos.