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. The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy’s formation history. The ordered-rotation dominated orbits with near maximum circularity ${\lambda }_z\simeq 1$ and the random-motion dominated orbits with low circularity ${\lambda }_z\simeq 0$ are called kinematically cold and kinematically hot, respectively. The fraction of stars on cold' orbits, compared to the fraction of stars on hot’ orbits, speaks directly to the quiescence or violence of the galaxies’ formation histories. Here we present such orbit distributions, derived from stellar kinematic maps via orbit-based modelling for a well defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey, includes the main morphological galaxy types and spans the 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, $p({\lambda }_z\mathrm{\mid }{M}_{\star })$, and its volume-averaged total distribution, $p({\lambda }_z)$. We find that across most of the considered mass range and across morphological types, there are more stars on warm' orbits defined as $0.25\le \lambda_z \le 0.8$ than on either cold’ or `hot’ orbits. This orbit-based “Hubble diagram” provides a benchmark for galaxy formation simulations in a cosmological context.