The connection between mass, environment and slow rotation in simulated
  galaxies
  Recent observations from integral field spectroscopy (IFS) indicate that the
fraction of galaxies that are slow rotators, $F_{\rm SR}$, depends primarily on
stellar mass, with no significant dependence on environment. We investigate
these trends and the formation paths of slow rotators (SRs) using the EAGLE and
Hydrangea hydro-dynamical simulations. EAGLE consists of several cosmological
boxes of volumes up to $(100\,\rm Mpc)^3$, while Hydrangea consists of $24$
cosmological simulations of galaxy clusters and their environment. Together
they provide a statistically significant sample in the stellar mass range
$10^{9.5}\,\rm M_{\odot}-10^{12.3}\,\rm M_{\odot}$, of $16,358$ galaxies. We
construct IFS-like cubes and measure stellar spin parameters, $\lambda_{\rm
R}$, and ellipticities, allowing us to classify galaxies into slow/fast
rotators as in observations. The simulations display a primary dependence of
$F_{\rm SR}$ on stellar mass, with a weak dependence on environment. At fixed
stellar mass, satellite galaxies are more likely to be SRs than centrals.
$F_{\rm SR}$ shows a dependence on halo mass at fixed stellar mass for central
galaxies, while no such trend is seen for satellites. We find that $\approx
70$% of SRs at $z=0$ have experienced at least one merger with mass ratio $\ge
0.1$, with dry mergers being at least twice more common than wet mergers.
Individual dry mergers tend to decrease $\lambda_{\rm R}$, while wet mergers
mostly increase it. However, $30$% of SRs at $z=0$ have not experienced
mergers, and those inhabit halos with median spins twice smaller than the halos
hosting the rest of the SRs. Thus, although the formation paths of SRs can be
varied, dry mergers and/or halos with small spins dominate.
