Measurement of the Dark Matter Velocity Dispersion with Galaxy Stellar Masses, UV Luminosities, and Reionization

Abstract

The root-mean-square of non-relativistic warm dark matter particle velocities scales as $v_{h\textrm{rms}}(a) = v_{h\textrm{rms}}(1)/a$, where $a$ is the expansion parameter of the universe. This velocity dispersion results in a cut-off of the power spectrum of density fluctuations due to dark matter free-streaming. Let $k_\textrm{fs}(t_\textrm{eq})$ be the free-streaming comoving cut-off wavenumber at the time of equal densities of radiation and matter. We obtain $v_{h\textrm{rms}}(1) = 0.41^{+0.14}_{-0.12} \textrm{ km/s}$, and $k_\textrm{fs}(t_\textrm{eq}) = 2.0^{+0.8}_{-0.5} \textrm{ Mpc}^{-1}$, at 68\% confidence, from the observed distributions of galaxy stellar masses and rest frame ultra-violet luminosities. This result is consistent with reionization. From the velocity dispersion cut-off we obtain the limits $v_{h\textrm{rms}}(1) < 0.54 \textrm{ km/s}$, and $k_\textrm{fs}(t_\textrm{eq}) > 1.5 \textrm{ Mpc}^{-1}$. These results are in agreement with previous measurements based on spiral galaxy rotation curves, and on the formation of first galaxies and reionization. These measured parameters determine the temperature-to-mass ratio of warm dark matter. This ratio happens to be in agreement with the no freeze-in and no freeze-out warm dark matter scenario of spin 0 dark matter particles decoupling early on from the standard model sector. Spin 1/2 and spin 1 dark matter are disfavored if nature has chosen the no freeze-in and no freeze-out scenario. An extension of the standard model of quarks and leptons, with scalar dark matter that couples to the Higgs boson, that is in agreement with all current measurements, is briefly reviewed. Discrepancies with limits on dark matter particle mass that can be found in the literature, are addressed.