Global Probes of the Impact of Baryons on Dark Matter Halos

The halo mass function, dn/dM, predicted by hierarchical clustering modelscan be measured indirectly using dynamical probes like the distribution ofgravitational lens image separations, dn/dDelta-theta, or halo circularvelocities, dn/d v_c. These dynamical variables depend on the halo structure aswell as the halo mass. Since baryonic physics, particularly cooling,significantly modifies the central density structure of dark matter halos, bothobservational distributions show a feature corresponding to the mass scalebelow which the baryons in the halo can cool (i.e. galaxies versus clusters).We use simplified but self-consistent models to show that the structuralchanges to the halos produced by the cooling baryons explain bothdistributions. Given a fixed halo mass function, matching the observed imageseparation distribution or local velocity function depends largely on Omega_bthrough its effects on the cooling time scales. These baryonic effects on thehalo structure also affect the evolution of the velocity function of galaxieswith redshift.Comment: 30 pages, 12 figure