Quantum WDM fermions and gravitation determine the observed galaxy structures

Quantum mechanics is necessary to compute galaxy structures at kpc scales and below. This is so because near the galaxy center, at scales below 10–100 pc, warm dark matter (WDM) quantum effects are important: observations show that the interparticle distance is of the order of, or smaller than the de Broglie wavelength for WDM. This explains why all classical (non-quantum) WDM N-body simulations fail to explain galactic cores and their sizes. We describe fermionic WDM galaxies in an analytic semiclassical framework based on the Thomas–Fermi approach, we resolve it numerically and find the main physical galaxy magnitudes: mass, halo radius, phase-space density, velocity dispersion, fully consistent with observations, including compact dwarf galaxies. Namely, fermionic WDM treated quantum mechanically, as it must be, reproduces the observed galaxy DM cores and their sizes. [In addition, as is known, WDM simulations produce the right DM structures in agreement with observations for scales ≳ kpc]. We show that compact dwarf galaxies are natural quantum macroscopic objects supported against gravity by the fermionic WDM quantum pressure (quantum degenerate fermions) with a minimal galaxy mass and minimal velocity dispersion. Interestingly enough, the minimal galaxy mass implies a minimal mass m min for the WDM particle. The lightest known dwarf galaxy (Willman I) implies m > m min = 1.91 keV. These results and the observed halo radius and mass of the compact galaxies provide further indication that the WDM particle mass m is approximately around 2 keV.