The feedback of active galatic nuclei (AGN) can severely affect the evolution of its host galaxy by truncating or triggering/amplifying the star formation. Therefore, it is of utmost interest to understand the processes involved in the growth of black holes. For decades, it was a widely accepted scenario that the occurrence of AGNs are a result of a merger of gas-rich galaxies. However, over the last years the relevance of major interactions for the growth of black holes has been extensively tested, leading to a model in which the merging of two gas-rich galaxies is playing a sub-dominant part for the occurrence of an AGN.
With my work I focus on the question whether this conclusion is also valid for black holes with highest accretion rates. As large amounts of gas and its transfer to the central region is needed, it may very well be that for this particular population of black holes major mergers are still the best or maybe only suitable mechanism.
To examine this question we analyze the merger fractions of AGN host galaxies at z∼2 (peak of black hole growth) observed by HST, and at z∼0.2 observed with VLT/FORS2 and subsequently compare them to the fractions of two matched samples of inactive galaxies. As all other parameters (mass, redshift, S/N, instrument and filters, residuals) between the AGN samples and the respective selections of inactive galaxies are comparable, a significant increase of the AGN merger fractions means that major merging is in those particular cases a dominant trigger for the growth of black holes.
The plot shows our result for our z=2 sample. We derived merger fractions for both samples, amounting to fm,AGN = 0.24 ± 0.09 for the AGN host galaxy sample and fm, ina = 0.19 ± 0.04 for the inactive galaxies. We find no significant enhancement of the merger fractions for the AGN host galaxies with respect to the sample of inactive galaxies, which leads us to the conclusion that even black holes showing highest accretion rates at the peak of cosmic AGN activity are not fueled by major mergers, but rather by secular evolution, bar instabilities or minor merger events. More details can be found in Marian et al., 2019.