Early-type Galaxy Spin Evolution in the Horizon-AGN Simulation

Using the Horizon-AGN simulation data, we study the relative role of mergers and environmental effects in shaping the spin of early-type galaxies (ETGs) after z similar or equal to 1. We follow the spin evolution of 10,037 color-selected ETGs more massive than 10(10) M-circle dot that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals (5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, f(SR), and the mean spin of massive ETGs. Although we do not find a clear environmental dependence of f(SR), a weak trend is seen in the mean value of the spin parameter driven by the satellite ETGs as they gradually lose their spin as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of the central ETGs of halos with M-vir > 10(12.5) M-circle dot, but only 22% of satellite and field ETGs. We find that non-mergerinduced tidal perturbations better correlate with the galaxy spin down in satellite ETGs than in mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local (z < 1) universe.