THE MYSTERY OF THE σ-BUMP-A NEW SIGNATURE FOR MAJOR MERGERS IN EARLY-TYPE GALAXIES?

The stellar velocity dispersion as a function of the galactocentric radius of an early-type galaxy can generally be well approximated by a power law sigma proportional to r(beta). However, some observed dispersion profiles show a deviation from this fit at intermediate radii, usually between one and three R-eff, where the velocity dispersion remains constant with radius, showing a bump-like behavior, which we term the sigma-bump. To understand the origin of this sigma-bump, we study a set of simulated early-type galaxies formed in major mergers. We find the sigma-bump in all of our simulated early-type galaxies, with the size and position of the bump slightly varying from galaxy to galaxy, suggesting that the bump is a characteristic of the major merger formation scenario. The feature can be seen both in the intrinsic and projected stellar velocity dispersions. In contrast to shells that form during the merger event but evolve with time and finally disappear, the sigma-bump stays nearly constant with radius and is a permanent feature that is preserved until the end of the simulation. The sigma-bump is not seen in the dark matter and gas components and we therefore conclude that it is a purely stellar feature of merger remnants.