Structure of merger remnants. I - Bulgeless progenitors

Abstract

Mergers of identical galaxies consisting of self-gravitating disks and halos are studied in the context of the suggestion that such events may form elliptical galaxies. It is shown that the luminous remnants of such mergers do, indeed, share many common properties with observed ellipticals. In particular, the end states of the simulations reported here rotate slowly in regions of relatively high surface density, having typical values v/σ <~0.2 there. Rotation velocities rise sharply at large radii, but it is difficult to quantify this effect accurately owing to sampling errors. Nevertheless, kinematic studies of the outer envelopes of elliptical galaxies should elucidate the relevance of spiral mergers to their formation. Morphologically, the remnants display a variety of interesting structures, including shells and loops comprising loosely bound material and boxy and disky isophotes. The luminous matter is well-fitted by ellipsoidal generalizations of Hernquist's model for elliptical galaxies, implying that the surface brightness profiles are essentially de Vaucouleurs-like over a large radial interval. Unfortunately, the core radii of the remnants, relative to their effective radii, are much larger than those inferred for real ellipticals. In view of this result, it seems that mergers of pure stellar disks do not represent an attractive mechanism for the production of massive elliptical galaxies.