The effects of a disc field on bulge surface brightness

Collisionless N ‐body simulations are used in an effort to reproduce the observed tendency of the surface brightness profile of bulges to change progressively from an R 1/4 law to an exponential, going from early‐ to late‐type spirals. A possible cause for this is the formation of the disc, later in the history of the galaxy, and this is simulated by applying on the N ‐body bulge the force field of an exponential disc the surface density of which increases with time. It is shown that n , the index of the Sersic law Σ n  ( r ) ∝ exp [−( r / r 0 ) 1/ n ] that best describes the surface brightness profile, does indeed decrease from 4 (de Vaucouleurs law) to smaller values; this decrease is larger for more massive and more compact discs. A large part of the observed trend of n with B/D ratio is explained, and many of the actual profiles can be matched exactly by the simulations. The correlation between the disc scalelength and bulge effective radius, used recently to support the ‘secular evolution’ origin for bulges, is also shown to arise naturally in a scenario like this. This mechanism, however, saturates at around n  = 2 and exponential bulges cannot be produced; as n gets closer to 1, the profile becomes increasingly robust against a disc field. These results provide strong support to the old‐bulge hypothesis for the early‐type bulges. The exponential bulges, however, remain essentially unexplained; the results here suggest that they did not begin their lives as R 1/4 spheroids, and hence were probably formed, at least in part, by different processes from those of early‐type spirals.