It is still puzzling how a galaxy can grow a supermassive black hole at its center and how this growth affects the galaxy itself. Black holes of 10^9 solar masses already existed when the Universe was only ~1 Gyr old. To reach this mass in such a short time, they should have started as seed intermediate-mass black holes (IMBHs) of 100-10^6 solar masses and grow very fast via accretion and mergers. Such IMBHs should be present in the nucleus of low-mass galaxies and in the halos of large galaxies, e.g. in the form of ultraluminous X-ray sources (ULXs). Nevertheless, observational evidence of their existence is still scarce. To probe the existence of the initial seed IMBHs from which active galactic nuclei (AGN) grow, we studied jet radio emission in ULXs. The observations reveal two potential IMBH candidates, one of them in the nucleus of a dwarf galaxy, as well as the detection of the largest non-nuclear extragalactic jet ever discovered. Its location in the arm of a spiral galaxy likely undergoing a minor merger indicates that the nuclei of minor mergers remain amongst the best candidates for IMBHs. We thus carry out a photometric study of double-nucleus disk galaxies candidates to minor mergers. We find that AGN activity can be triggered by mergers, as expected from simulations, and define one of the largest samples of binary AGN candidates with physical separations below 1 kpc.