NIHAO - IV: core creation and destruction in dark matter density profiles across cosmic time

We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects) cosmological simulations to investigate the effects of baryonic physics on the time evolution of dark matter central density profiles. The sample is made of approximate to 70 independent high-resolution hydrodynamical simulations of galaxy formation and covers a wide mass range: 10(10) less than or similar to M-halo/M-circle dot less than or similar to 10(12), i.e. from dwarfs to L-star. We confirm previous results on the dependence of the inner dark matter density slope, alpha, on the ratio between stellar-to-halo mass, M-star/M-halo. We show that this relation holds approximately at all redshifts (with an intrinsic scatter of similar to 0.18 in alpha measured between 1 and 2 per cent of the virial radius). This implies that in practically all haloes the shape of their inner density profile changes quite substantially over cosmic time, as they grow in stellar and total mass. Thus, depending on their final M-star/M-halo ratio, haloes can either form and keep a substantial density core (R-core similar to 1 kpc), or form and then destroy the core and recontract the halo, going back to a cuspy profile, which is even steeper than cold-dark-matter predictions for massive galaxies (10(12) M-circle dot). We show that results from the NIHAO suite are in good agreement with recent observational measurements of a in dwarf galaxies. Overall our results suggest that the notion of a universal density profile for dark matter haloes is no longer valid in the presence of galaxy formation.