AGN host galaxies at redshift z ≈ 0.7: peculiar or not?

Aims. We perform a quantitative morphological comparison between the hosts of active galactic nuclei (AGN) and quiescent galaxies at intermediate redshifts (z ≈ 0.7). The imaging data are taken from the large HST/ACS mosaics of the GEMS and STAGES surveys. Our main aim is to test whether nuclear activity at this cosmic epoch is triggered by major mergers.

Methods. Using images of quiescent galaxies and stars, we created synthetic AGN images to investigate the impact of an optical nucleus on the morphological analysis of AGN hosts. Galaxy morphologies are parameterized using the asymmetry index A, the concentration index C, the Gini coefficient G, and the M²⁰ index. A sample of ~200 synthetic AGN was matched to 21 real AGN in terms of redshift, host brightness, and host-to-nucleus ratio to ensure a reliable comparison between active and quiescent galaxies.

Results. The optical nuclei strongly affect the morphological parameters of the underlying host galaxy. Taking these effects into account, we find that the morphologies of the AGN hosts are clearly distinct from galaxies undergoing violent gravitational interactions. Indeed, the host galaxy distributions in morphological descriptor space are more similar to undisturbed galaxies than to major mergers. Conclusions. Intermediate-luminosity (LX ≲ 10⁴⁴   erg/s) AGN hosts at z ≈ 0.7 show morphologies similar to the general population of massive galaxies with significant bulges at the same redshifts. If major mergers are the driver of nuclear activity at this epoch, the signatures of gravitational interactions fade rapidly before the optical AGN phase starts, making them undetectable on single-orbit HST images, at least with usual morphological descriptors. This could be investigated in future synthetic observations created from numerical simulations of galaxy-galaxy interactions.