Baryon Dynamics, Dark Matter Substructure, and Galaxies

By comparing a collisionless cosmological N-body simulation (DM) to an SPHsimulation with the same initial conditions, we investigate the correspondencebetween the dark matter subhalos produced by collisionless dynamics and thegalaxies produced by dissipative gas dynamics in a dark matter background. Whengalaxies in the SPH simulation become satellites in larger groups, they retainlocal dark matter concentrations (SPH subhalos) whose mass is typically fivetimes their baryonic mass. The more massive subhalos of the SPH simulation havecorresponding subhalos of similar mass and position in the DM simulation; atlower masses, there is fairly good correspondence, but some DM subhalos are indifferent spatial positions and some suffer tidal stripping or disruption. Thehalo occupation statistics of DM subhalos -- the mean number of subhalos,pairs, and triples as a function of host halo mass -- are very similar to thoseof SPH subhalos and SPH galaxies. Gravity of the dissipative baryon componentamplifies the density contrast of subhalos in the SPH simulation, making themmore resistant to tidal disruption. Relative to SPH galaxies and SPH subhalos,the DM subhalo population is depleted in the densest regions of the mostmassive halos. The good agreement of halo occupation statistics between the DMsubhalo and SPH galaxy populations leads to good agreement of their two-pointcorrelation functions and higher order moments on large scales. The depletionof DM subhalos in dense regions depresses their clustering at R<1 Mpc/h. Inthese simulations, the "conversation" between dark matter and baryons is mostlyone-way, with dark matter dynamics telling galaxies where to form and how tocluster, but the "back talk" of the baryons influences small scale clusteringby enhancing the survival of substructure in the densest environments.