Formation and Evolution of Self-Interacting Dark Matter Halos

In the standard cold dark matter (CDM) theory of cosmic structure formation, CDM particles are assumed to be collisionless. The actual identity and microscopic nature of CDM is still unknown, however. It is important, therefore, to explore the consequences of varying this underlying assumption about CDM in the hope that astronomical observations can be used to place meaningful constraints. It has been suggested, for example, that CDM particles might interact with each other non-gravitationally – the self-interacting dark matter (SIDM) hypothesis – motivated, in part, by the apparent discrepancy between the observed density profiles of dark-matter dominated dwarf and low surface brightness galaxies (LSB) and those predicted by N-body simulations of collisionless CDM. We have derived the first, fully-cosmological, similarity solutions for CDM halo formation in the presence of nongravitational collisionality (i.e. elastic scattering), which provides an analytical theory of the effect of the SIDM hypothesis on halo density profiles. Collisions transport heat inward which flattens the central cusp of the CDM density profile to produce a constant-density core, while continuous infall pumps energy into the halo to stabilize the core against gravothermal catastrophe. This is contrary to previous analyses based upon isolated halos, which predict core collapse within a Hubble time. These solutions improve upon earlier attempts to model the formation and evolution of SIDM halos, offer deeper insight than existing N-body experiments, and yield a more precise determination of the dependence of halo density profile on the value of the CDM self-interaction cross section. Different solutions arise for different values of the dimensionless collisionality parameter Q ≡ σρbrvir ∝ rvir/λmfp, where σ is the scattering cross section per unit mass, ρb is the cosmic mean matter density, rvir is halo virial radius and λmfp is the collision mean free path. The maximum flattening of central density occurs for an intermediate value of Q, Qth, at which the halo is maximally relaxed to isothermality. The density profiles with constant-density cores preferred by dwarf and LSB rotation curves are best fit by the maximally-flattened (Q = Qth) solution. If we assume that dwarfs and LSB galaxies formed at their typical collapse epoch in ΛCDM, then the value of σ which makes Q = Qth is σ ≃ 200 cm 2 g, much higher than previous estimates, σ ≃ [0.5−5] cm g, based on N-body experiments. If σ is independent of collision velocity, then the same value σ ≃ 200 cm g would make Q > Qth for clusters, which typically formed only recently, resulting in relatively less flattening of their central density profile and a smaller core.