The Origin and Evolution of Fast and Slow Rotators in the Illustris
  Simulation
  Using the Illustris simulation, we follow thousands of elliptical galaxies
back in time to identify how the dichotomy between fast and slow rotating
ellipticals (FRs and SRs) develops. Comparing to the
$\textrm{ATLAS}^\textrm{3D}$ survey, we show that Illustris reproduces similar
elliptical galaxy rotation properties, quantified by the degree of ordered
rotation, $\lambda_\textrm{R}$. There is a clear segregation between low-mass
($M_{\rm *} < 10^{11} M_{\rm \odot}$) ellipticals, which form a smooth
distribution of FRs, and high-mass galaxies ($M_{\rm *} > 10^{11.5} M_{\rm
  \odot}$), which are mostly SRs, in agreement with observations. We find that
SRs are very gas poor, metal rich and red in colour, while FRs are generally
more gas rich and still star forming. We suggest that ellipticals begin
naturally as FRs and, as they grow in mass, lose their spin and become SRs.
While at $z = 1$, the progenitors of SRs and FRs are nearly indistinguishable,
their merger and star formation histories differ thereafter. We find that major
mergers tend to disrupt galaxy spin, though in rare cases can lead to a
spin-up. No major difference is found between the effects of gas-rich and
gas-poor mergers and the amount of minor mergers seem to have little
correlation with galaxy spin. In between major mergers, lower-mass ellipticals,
which are mostly gas-rich, tend to recover their spin by accreting gas and
stars. For galaxies with $M_{\rm *}$ above $\sim 10^{11} M_{\rm \odot}$, this
trend reverses; galaxies only retain or steadily lose their spin. More frequent
mergers, accompanied by an inability to regain spin, lead massive ellipticals
to lose most of ordered rotation and transition from FRs to SRs.
