The response of a spheroid to a disc field or were bulges ever ellipticals?

A collisionless N-body code is used to study the response of a proto-bulge to the field of an exponential disc. In the limit where the time-scale of the disc's growth is long compared to the dynamical time-scale of the bulge, the final state of the bulge will depend on the final state of the disc, but not on the details of how the disc matter was assembled. The disc field acts to increase the surface brightness, velocity dispersion, rotation velocity and flattening of the bulge, but does not (directly) account for either the ‘thick disc’ or ‘box-shaped’ bulge phenomena. Our results suggest that the close similarity between bulges and ellipticals cannot result from disc accretion on to a pre-existing normal elliptical unless discs are considerably less massive than is currently believed.