Abstract
Elliptical galaxies, and more generally early-type galaxies, are spheroidal galaxies whose stellar dynamics are based on velocity dispersion. They are sometimes flattened, but not by rotation as in the case of spirals. Detailed studies of their kinematics showed in the 1980s that elliptical galaxies rotate little or not at all. These galaxies are formed by mergers of spiral galaxies, which gradually cancel out angular momentum. As the first galaxies are very gaseous, they first form a disk as the gas dissipates. Then the fraction of gas decreases considerably, and the merging of purely stellar systems leads to spheroids, supported by an anisotropic velocity dispersion. The final merging direction dominates and determines the direction of greatest elongation of the final galaxy.
Of course, the more realistic scenario is not quite so simple, as an elliptical field galaxy can continue to accrete gas from cosmic filaments, and reform a disk. Recent studies of these red galaxies have shown that they all possess a disk, even of small size, and that they often have a residual rotation. A distinction is made between fast rotators and slow rotators, which are the result of several mergers.
The 3D shape of spheroidal galaxies can be either oblate (the two equal axes are the major axes), or prolate (the two equal axes are the minor axes). Sometimes the shape is that of a triaxis. Proof that galaxies merged to form ellipticals can be found in the existence of shells of stars around them, either in an equatorial plane (the oblate case), or aligned on the major axis of a prolate galaxy. In this case, the shells alternate, with the youngest forming in the center. The number of shells allows us to date the merger event, and the radial distribution of shells provides information on the distribution of dark matter, through knowledge of past dynamical friction.
Scaling relationships for elliptical galaxies are more complex than for spirals: in addition to the Faber-Jackson relationship, analogous to the Tully-Fisher relationship, a fundamental plane can be defined.
