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

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Penoyre, Z 
Moster, BP 
Sijacki, D 
Genel, S 


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 ATLAS3D survey, we show that Illustris reproduces similar elliptical galaxy rotation properties, quantified by the degree of ordered rotation, λR. There is a clear segregation between low-mass (M<1011M) ellipticals, which form a smooth distribution of FRs, and high-mass galaxies (M>1011.5M), 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 above ∼1011M, 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.



methods: numerical, galaxies: elliptical and lenticular, cD, galaxies: evolution, galaxies: kinematics and dynamics, galaxies: structure

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Monthly Notices of the Royal Astronomical Society

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Oxford University Press
Science and Technology Facilities Council (ST/N000927/1)
European Research Council (638707)
Science and Technology Facilities Council (ST/L000725/1)
BPM acknowledges support from the Kavli Foundation and the German Science Foundation (DFG) for an Emmy Noether grant. The Flatiron Institute is supported by the Simons Foundation. DS acknowledges support by the STFC and the ERC Starting Grant 638707 ‘Black holes and their host galaxies: co-evolution across cosmic time’.