We investigate the effect of dark energy on the density profiles of dark matter haloes with a suite of cosmological N-body simulations and use our results to test analytic models. We consider constant equation of state models, and allow both w>-1 and w<-1. Using five simulations with w ranging from -1.5 to -0.5, and with more than ~1600 well-resolved haloes each, we show that the halo concentration model of Bullock et al. (2001) accurately predicts the median concentrations of haloes over the range of w, halo masses, and redshifts that we are capable of probing. We find that the Bullock et al. (2001) model works best when halo masses and concentrations are defined relative to an outer radius set by a cosmology-dependent virial overdensity. For a fixed power spectrum normalization and fixed-mass haloes, larger values of w lead to higher concentrations and higher halo central densities, both because collapse occurs earlier and because haloes have higher virial densities. While precise predictions of halo densities are quite sensitive to various uncertainties, we make broad comparisons to galaxy rotation curve data. At fixed power spectrum normalization (fixed sigma_8), w>-1 quintessence models seem to exacerbate the central density problem relative to the standard w=-1 model. Meanwhile w<-1 models help to reduce the apparent discrepancy. We confirm that the Jenkins et al. (2001) halo mass function provides an excellent approximation to the abundance of haloes in our simulations and extend its region of validity to include models with w<-1.
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