The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey

Galaxy formation entails the hierarchical assembly of mass, along with the condensation of baryons and the ensuing, self-regulating star formation 1,2 . The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy’s formation history 3 . The orbits dominated by ordered rotation, with near-maximum circularity λ z  ≈ 1, are called kinematically cold, and the orbits dominated by random motion, with low circularity λ z  ≈ 0, are kinematically hot. The fraction of stars on ‘cold’ orbits, compared with the fraction on ‘hot’ orbits, speaks directly to the quiescence or violence of the galaxies’ formation histories 4,5 . Here we present such orbit distributions, derived from stellar kinematic maps through orbit-based modelling for a well-defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey 6 , includes the main morphological galaxy types and spans a total stellar mass range from 108.7 to 1011.9 solar masses. Our analysis derives the orbit-circularity distribution as a function of galaxy mass and its volume-averaged total distribution. We find that across most of the considered mass range and across morphological types, there are more stars on ‘warm’ orbits defined as 0.25 ≤ λ z  ≤ 0.8 than on either ‘cold’ or ‘hot’ orbits. This orbit-based ‘Hubble diagram’ provides a benchmark for galaxy formation simulations in a cosmological context.