Warm Dark Matter: Clues to Primordial Phase Density from the Structure of Galaxy Halos

The Cold Dark Matter paradigm successfully explains many phenomena on scales larger than galaxies, but seems to predict galaxy halos which are more centrally concentrated and have a lumpier substructure than observed. Endowing cosmic dark matter with a small primordial velocity dispersion preserves the successful predictions of the Cold Dark Matter scenario on large scales and improves the agreement with halo structure. A phase density'' $Q$, proportional to the inverse entropy for nonrelativistic matter, is estimated for relativistically decoupled thermal or degenerate relic particles of mass $m_X$, with a numerical factor depending on the particle type but no cosmological parameters. Since $Q$ cannot increase for dissipationless, collisionless matter, at a given velocity dispersion there is a maximum space density; this phase packing’’ constraint eliminates the singular density predicted by CDM. The core radius and halo circular velocity scale analogously to degenerate dwarf stars. Particle velocities also filter primordial perturbations on a scale depending on $Q$ and on details of particle distributions. Particle candidates for warm matter are briefly discussed; for warm thermal relics to have the observed mass density requires decoupling prior to the QCD epoch and therefore a superweak interaction with thermal Standard Model particles.