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Using the Illustris simulation, we follow thousands of elliptical galaxiesback in time to identify how the dichotomy between fast and slow rotatingellipticals (FRs and SRs) develops. Comparing to the$\textrm{ATLAS}^\textrm{3D}$ survey, we show that Illustris reproduces similarelliptical galaxy rotation properties, quantified by the degree of orderedrotation, $\lambda_\textrm{R}$. There is a clear segregation between low-mass($M_{\rm *} < 10^{11} M_{\rm \odot}$) ellipticals, which form a smoothdistribution of FRs, and high-mass galaxies ($M_{\rm *} > 10^{11.5} M_{\rm  \odot}$), which are mostly SRs, in agreement with observations. We find thatSRs are very gas poor, metal rich and red in colour, while FRs are generallymore gas rich and still star forming. We suggest that ellipticals beginnaturally 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 majormergers tend to disrupt galaxy spin, though in rare cases can lead to aspin-up. No major difference is found between the effects of gas-rich andgas-poor mergers and the amount of minor mergers seem to have littlecorrelation with galaxy spin. In between major mergers, lower-mass ellipticals,which are mostly gas-rich, tend to recover their spin by accreting gas andstars. For galaxies with $M_{\rm *}$ above $\sim 10^{11} M_{\rm \odot}$, thistrend reverses; galaxies only retain or steadily lose their spin. More frequentmergers, accompanied by an inability to regain spin, lead massive ellipticalsto lose most of ordered rotation and transition from FRs to SRs.

The Origin and Evolution of Fast and Slow Rotators in the Illustris Simulation