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 $\lambda_z \simeq1$ and the
random-motion dominated orbits with low circularity $\lambda_z \simeq0$ 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,
$p(\lambda_z~|~M_\star)$, 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.
