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 $z \simeq 1$. We follow the spin evolution of 10,037 color-selected ETGs
more massive than 10$^{10} \rm \, M_{\odot}$ 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,
$f_{\rm SR}$, and the mean spin of massive ETGs. Although we do not find a
clear environmental dependence of $f_{\rm SR}$, a weak trend is seen in the
mean value of 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 central ETGs of halos with
$M_{vir} > 10^{12.5}\rm M_{\odot}$, but only 22% of satellite and field ETGs.
We find that non-merger induced tidal perturbations better correlate with the
galaxy spin-down in satellite ETGs than 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.
