Did galaxy assembly and supermassive black-hole growth go hand-in-hand?

In this paper, we address whether the growth of supermassive black-holes has kept pace with the process of galaxy assembly. For this purpose, we first searched the Hubble Ultra Deep Field (HUDF) for tadpole galaxies, which have a knot at one end plus an extended tail. They appear dynamically unrelaxed-presumably early-stage mergers-and make up similar to 6% of the field galaxy population. Their redshift distribution follows that of field galaxies, indicating that-if tadpole galaxies are indeed dynamically young the process of galaxy assembly generally kept up with the reservoir of field galaxies as a function of epoch. Next, we present a search for HUDF objects with point-source components that are optically variable (at the >= 3.0 sigma level) on timescales of weeks months. Among 4644 objects to i(AB)' less than or similar to 28.0 mag (10 sigma), 45 have variable point-like components, which are likely weak AGN. About similar to 1% of all field objects show variability for 0.1 less than or similar to z less than or similar to 4.5, and their redshift distribution is similar to that of field galaxies. Hence, supermassive black-hole growth in weak AGN likely also kept up with the process of galaxy assembly. However, the faint AGN sample has almost no overlap with the tadpole sample, which was predicted by recent hydrodynamical numerical simulations. This suggests that tadpole galaxies are early-stage mergers, which likely preceded the turn-on of the AGN component and the onset of visible point-source variability by greater than or similar to 1 Gyr. (c) 2006 Elsevier B.V. All rights reserved.