Constant surface gravity and density profile of dark matter

Abstract

Cumulative observational evidence confirm that the surface gravity of dark matter (DM) halo mu_{0 D} = r_0 rho_0 where r_0 and rho_0 are the halo core radius and central density, respectively, is nearly constant and independent of galaxy luminosity for a high number of galactic systems (spirals, dwarf irregular and spheroidals, elliptics) spanning over 14 magnitudes in luminosity and of different Hubble types. Remarkably, its numerical value mu_{0 D} = 140 M_{sun}/pc^2 = (18.6 Mev)^3 is approximately the same (up to a factor of two) in all these systems. First, we present the physical consequences of the independence of mu_{0 D} on r_0: the energy scales as the volume sim r_0^3 while the mass and the entropy scale as the surface ~ r_0^2 and the surface times log r_0, respectively. Namely, the entropy scales similarly to the black-hole entropy but with a much smaller coefficient. Second, we compute the surface gravity and the density profile for small scales from first principles and the evolution of primordial density fluctuations since the end of inflation till today using the linearized Boltzmann-Vlasov equation. The density profile rho_{lin}(r) obtained in this way decreases as r^{-1-n_s/2} for intermediate scales where n_s = 0.964 is the primordial spectral index. This scaling is in remarkable agreement with the empirical behaviour found observationally and in N-body simulations: r^{-1.6\pm 0.4}. The observed value of mu_{0 D} indicates that the DM particle mass m is in the keV scale. The theoretically derived density profiles rho_{lin}(r) turn to be cored for m in the keV scale and they look as cusped for m in the GeV scale or beyond. We consider both fermions and bosons as DM particles decoupling either ultrarelativistically or non-relativistically. Our results do not use any particle physics model and vary slightly with the statistics of the DM particle.