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 \times 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 \times 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 .