A study of stellar orbit fractions: simulated IllustrisTNG galaxies
  compared to CALIFA observations
  Motivated by the recently discovered kinematic "Hubble sequence" shown by the
stellar orbit-circularity distribution of 260 CALIFA galaxies, we make use of a
comparable galaxy sample at z = 0 with a stellar mass range from 5E9 to 5E11
solar masses, selected from the IllustrisTNG simulation and study their stellar
orbit compositions in relation to a number of other fundamental galaxy
properties.We find that the TNG100 simulation broadly reproduces the observed
fractions of different orbital components and their stellar mass dependencies.
In particular, the mean mass dependencies of the luminosity fractions for the
kinematically warm and hot orbits are well reproduced within model
uncertainties of the observed galaxies. The simulation also largely reproduces
the observed peak and trough features at a stellar mass of 1-2E10 solar masses,
in the mean distributions of the cold- and hot-orbit fractions, respectively,
indicating fewer cooler orbits and more hotter orbits in both more- and
less-massive galaxies beyond such a mass range. Several marginal disagreements
are seen between the simulation and observations: the average cold-orbit
(counter-rotating) fractions of the simulated galaxies below (above) a stellar
mass of 6E10 solar masses, are systematically higher than the observational
data by < 10% (absolute orbital fraction); the simulation also seems to produce
more scatter for the cold-orbit fraction and less so for the non-cold orbits at
any given galaxy mass. Possible causes that stem from the adopted heating
mechanisms are discussed.
