SHAO Astrophysics Colloquium

Title: A study of stellar orbital fractions: the CALIFA observed and the IllustrisTNG simulated galaxies

Speaker: Dandan Xu (Institute for Advanced Studies, Centre for Astrophysics, Tsinghua University.)

Time: 3:00 pm, May 16 (Thursday)

Location: Lecture Hall, 3rd floor

Abstract: Motivated by the recently discovered kinematic “Hubble sequence” appeared in the stellar orbit circularity distribution of 260 CALIFA galaxies, we make use of a comparable galaxy sample selected from the IllustrisTNG simulation and study their stellar orbit compositions in relations to a number of other basic galaxy properties. We find that the TNG-100 simulation broadly reproduces the observed fractions of different kinematic components and their stellar mass dependence. In particular, the mean mass dependencies of the luminosity fractions for the kinematically warm and hot orbits are well reproduced within model uncertainties of observed galaxies. The simulation also largely reproduces the observed peak and sink features  in the mean distributions of the cold- and hot-orbit fractions, respectively, in relation with galaxy stellar mass. Marginal disagreements are seen for the kinematically cold and counter-rotating orbits such that the mean orbital fractions at a given mass scale below and above some intermediate mass scale, respectively, are systematically higher than that of the observational data by less than 10%. The simulation also seems to produce more scatters for the cold-orbit fraction and less so for the non-cold orbits at any given galaxy mass, in comparison to observations. Possible causes are discussed. We point out that as galaxy mass increases, the observed galaxy type transitions (from late-type galaxies to early types), are encoded by changes in the stellar orbit compositions such that the more massive galaxies are, the less cooler orbits and more hotter orbits they possess. Such changes in the kinematic compositions are of a natural consequence of the hierarchical structure assembly process under the framework of the cold dark matter cosmology and are seen for both the CALIFA and the simulated galaxies.