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

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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 mₓ, 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.