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We use numerical simulations to investigate how the statistical properties ofdark matter (DM) haloes are affected by the baryonic processes associated withgalaxy formation. We focus on how these processes influence the spin and shapeof a large number of DM haloes covering a wide range of mass scales, fromgalaxies to clusters at redshifts zero and one, extending to dwarf galaxies atredshift two. The haloes are extracted from the OverWhelmingly LargeSimulations, a suite of state-of-the-art high-resolution cosmologicalsimulations run with a range of feedback prescriptions. We find that the medianspin parameter in DM-only simulations is independent of mass, redshift andcosmology. At z=0 baryons increase the spin of the DM in the central region(<=0.25 r_200) by up to 30 per cent when feedback is weak or absent. Thisincrease can be attributed to the transfer of angular momentum from baryons tothe DM, but is no longer present at z=2. We also present fits to the massdependence of the DM halo shape at both low and high redshift. At z=0 thesphericity (triaxiality) is negatively (positively) correlated with halo massand both results are independent of cosmology. Interestingly, thesemass-dependent trends are markedly weaker at z=2. While the cooling of baryonsacts to make the overall DM halo more spherical, stronger feedbackprescriptions tend to reduce the impact of baryons by reducing the central halomass concentration. More generally, we demonstrate a strongly positive(negative) correlation between halo sphericity (triaxiality) and galaxyformation efficiency, with the latter measured using the central halo baryonfraction. In conclusion, our results suggest that the effects of baryons on theDM halo spin and shape are minor when the effects of cooling are mitigated, asrequired by realistic models of galaxy formation, although they remainsignificant for the inner halo.

The impact of baryons on the spins and shapes of dark matter haloes