The compression of dark matter halos by baryonic infall

The initial radial density profiles of dark matter halos are laid down by gravitational collapse in hierarchical structure formation scenarios and are subject to further compression as baryons cool and settle to the halo centers. Here we describe an explicit implementation of the algorithm, originally developed by Young, to calculate changes to the density profile as the result of adiabatic infall in a spherical halo model. Halos with random motion are more resistant to compression than are those in which random motions are neglected, which is a key weakness of the simple method widely employed. Young's algorithm results in density profiles in excellent agreement with those from N-body simulations. We show how the algorithm can be applied to determine the original uncompressed halos of real galaxies, a step that must be computed with care in order to enable a confrontation with theoretical predictions from theories such as Lambda CDM.