  We use cosmological SPH simulations to study the effects of mergers in the
star formation history of galactic objects in hierarchical clustering
scenarios. We find that during some merger events, gaseous discs can experience
two starbursts: the first one during the orbital decay phase, due to gas
inflows driven as the satellite approaches, and the second one, when the two
baryonic clumps collide. A trend for these first induced starbursts to be more
efficient at transforming the gas into stars is also found. We detect that
systems which do not experience early gas inflows have well-formed stellar
bulges and more concentrated potential wells, which seem to be responsible for
preventing further gas inward transport triggered by tidal forces. Our results
constitute the first proof that bulges can form as the product of collapse,
collisions and secular evolution in a cosmological framework, and they are
consistent with a rejuvenation of the stellar population in bulges at
intermediate z with, at least, 50% of the stars (in SCDM) being formed at high
z. (Abridged)
