Dark matter halo response to the disc growth

Abstract

We consider the sensitivity of the circular-orbit adiabatic contraction approximation to the baryon condensation rate and the orbital structure of dark matter halos in theCDM paradigm. Using one-dimensional hydrodynamic simulations including the dark matter halo mass accretion history and gas cooling, we demonstrate that the adia- batic approximation is approximately valid even though halos and disks may assemble simultaneously. We further demonstrate the validity of the simple approximation for �CDM halos with isotropic velocity distributions using three-dimensional N-body sim- ulations. This result is easily understood: an isotropic velocity distribution in a cuspy halo requires more circular orbits than radial orbits. Conversely, the approximation is poor in the extreme case of a radial orbit halo. It overestimates the response a core dark matter halo, where radial orbit fraction is larger. Because no astronomically relevant models are dominated by low-angular momentum orbits in the vicinity of the disk and the growth time scale is never shorter than a dynamical time, we conclude that the adiabatic contraction approximation is useful in modeling the response of dark matter halos to the growth of a disk.