Collisional versus Collisionless Dark Matter

We compare the structure and substructure of dark matter halos in modeluniverses dominated by collisional, strongly self interacting dark matter(SIDM) and collisionless, weakly interacting dark matter (CDM). While SIDMvirialised halos are more nearly spherical than CDM halos, they can berotationally flattened by as much as 20% in their inner regions. Substructurehalos suffer ram-pressure truncation and drag which are more rapid and severethan their gravitational counterparts tidal stripping and dynamical friction.Lensing constraints on the size of galactic halos in clusters are a factor oftwo smaller than predicted by gravitational stripping, and the recent detectionof tidal streams of stars escaping from the satellite galaxy Carina suggeststhat its tidal radius is close to its optical radius of a few hundred parsecs--- an order of magnitude smaller than predicted by CDM models but consistentwith SIDM. The orbits of SIDM satellites suffer significant velocity bias$\sigma_{SIDM}/\sigma_{CDM}=0.85$ and are more circular than CDM,$\beta_{SIDM}} \approx 0.5$, in agreement with the inferred orbits of theGalaxy's satellites. In the limit of a short mean free path, SIDM halos havesingular isothermal density profiles, thus in its simplest incarnation SIDM isinconsistent with galactic rotation curves.