Specific Angular Momentum of Disc Merger Remnants and the $λ_R$-Parameter

Abstract

We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the $λ_R$-parameter, which is a luminosity weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early type galaxies in so called fast $λ_R > 0.1$ and slow rotators $λ_R < 0.1$. Tests on merger remnants reveal that $λ_R$ is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of $λ_R$ values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in creating a slow or a fast rotator in a single binary merger, the former being created mostly in an equal mass merger. Slow rotators have a $λ_R$ which does not vary with projection. The confusion rate with face-on fast rotators is very small. Merger with low gas fractions form slow rotators with smaller ellipticities and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of $λ_R$ as high as our progenitor disc galaxies.