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 lambda(z) approximate to 1, are called kinematically cold, and the orbits dominated by random motion, with low circularity lambda(z) approximate to 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 10(8.7) to 10(11.9) 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 <= lambda(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.