Variability-selected AGN in dwarf galaxies: the incidence of AGN in dwarf and massive galaxies is similar

ABSTRACT We use the VST-COSMOS survey to identify, via their optical broad-band variability, 30 active galactic nuclei (AGN) in nearby ($z< 0.4$) dwarf (10$^{8}$ M$_{\odot }$ < M$_{\rm {\star }}$ < 10$^{10}$ M$_{\odot }$) galaxies. VST-COSMOS offers a 1 deg$^2$ survey footprint, a single visit depth of 24.6 mag and 68 r-band visits spanning an 11-yr temporal baseline. Compared to a control sample matched in stellar mass and redshift, the dwarf AGN population shows an elevated fraction of early-type galaxies but a similar fraction of interacting objects, suggesting that interactions do not play a significant role in triggering these AGN. Dwarf AGN hosts do not show strong differences in their distances to nodes, filaments, and massive galaxies compared to the controls, which indicates that AGN triggering, at least in this sample, is not strongly correlated with environment. Finally, by combining the true number of galaxies, the detectability of AGN and the measured numbers of variable sources in dwarf and massive (M$_{\rm {\star }}$ > 10$^{10}$ M$_{\odot }$) galaxies, we estimate the relative frequency of AGN in these two mass regimes. Our results suggest that the incidence of AGN in dwarfs and massive galaxies is similar (within less than a factor of 2 of each other), with some evidence that the AGN fraction increases with stellar mass in the dwarf population.