Laurent Chemin — The orbital anisotropy of velocity in spiral and irregular galaxies

The ellipsoid of random motions is a fundamental ingredient of galaxy dynamics. In particular, it can be used to constrain the shape of orbits of stars or gas in galactic disks. First, I will present results from a recent study that constrained the structure of stellar orbits in nearby galactic disks. A new correlation between the stellar mass and the shape of the orbits has been evidenced: stellar orbits in more massive galaxies are radially biased, while less massive disks have more tangential to isotropic orbits. A discrepancy with expectations from the epicycle theory of orbits has also been evidenced. Then, results from ongoing studies of the velocity ellipsoid of HI and CO gas in other samples of galaxies will be shown. Contrary to the common assumption that random motions of gas are isotropic, it is shown a large diversity of orbits in nearby spiral galaxies. Disagreement with the epicyclic approximation are again observed in the atomic gas component.