Growth of galactic bulges by mergers I. Dense satellites

Andredakis et al. (1995) fit Sersic's law mu (r) similar to r(1/n) to the b ulges of the Balcells & Peletier (1994) galaxy sample, and infer that n dro ps with morphological type T from n approximate to 4-6 for SO to n = 1 (exp onential) for Sc's. We use collisionless N body simulations to test the ass umption that initially the surface brightness profiles of all bulges were e xponential, and that the steepening of the profiles toward the early-types is due to satellite accretion. The results are positive. After the accretio n of a satellite. bulge-disk fits show that the bulge grows and that the bu lge profile index n increases proportional to the satellite mass. For a sat ellite as massive as the bulge, n rises from 1 to 4. We present kinematic d iagnostics on the remnants and disk thickening. The latter suggests that th e bulge growth must have occurred before the last formation of a thin disk in the galaxy. The thick disks created by the merger are reminiscent of thi ck disks seen in early-type edge-on galaxies. The efficiency of the process suggests that prevent day bulges of late-type spirals showing exponential profiles cannot have grown significantly by collisionless mergers.