Galaxy-induced transformation of dark matter haloes

We use N-body/gasdynamical cosmological simulations to examine the effect of the assembly of a central galaxy on the shape and mass profile of its surrounding dark matter halo. Two series of simulations are compared; one that follows only the evolution of the dark matter component of individual haloes in the proper Lambda cold dark matter (Lambda CDM) cosmological context, and a second series where a baryonic component is added and followed hydrodynamically. The simulations with baryons include radiative cooling but neglect the formation of stars and their feedback. The efficient, unimpeded cooling that results leads most baryons to collect at the halo centre in a centrifugally supported disc which, due to angular momentum losses, is too small and too massive when compared with typical spiral galaxies. This admittedly unrealistic model allows us, nevertheless, to gauge the maximum effect that galaxies may have in transforming their surrounding dark haloes. We find, in agreement with earlier work, that the shape of the halo becomes more axisymmetric: post galaxy assembly, haloes are transformed from triaxial into essentially oblate systems, with well-aligned isopotential contours of roughly constant flattening (<< c/a >> similar to 0.85). Haloes always contract as a result of galaxy assembly, but the effect is substantially less pronounced than predicted by the traditional 'adiabatic-contraction' hypothesis. The reduced contraction helps to reconcile Lambda CDM haloes with constraints on the dark matter content inside the solar circle and should alleviate the longstanding difficulty of matching simultaneously the scaling properties of galaxy discs and the galaxy luminosity function. The halo contraction we report is also less pronounced than found in earlier simulations, a disagreement which suggests that halo contraction is not solely a function of the initial and final distribution of baryons. Not only how much baryonic mass has been deposited at the centre of a halo matters, but also the mode of its deposition. Although simple formulae might work in particular cases where galaxies form nearly adiabatically, in general it might prove impossible to predict the halo response to galaxy formation without a detailed understanding of a galaxy's detailed assembly history.