The clustering of narrow-line AGN in the local Universe

We have analyzed the clustering of ∼ 90, 000 narrow-line AGN drawn from the Data Release 4 (DR4) of the Sloan Digital Sky Survey. Our analysis addresses the following questions: a) How do the locations of galaxies within the large-scale distribution of dark matter influence ongoing accretion onto their central black holes? b) Is AGN activity triggered by interactions or mergers between galaxies? We compute the crosscorrelation between AGN and a reference sample of galaxies drawn from the DR4. We compare this to results for control samples of inactive galaxies matched simultaneously in redshift, stellar mass, concentration, velocity dispersion and mean stellar age, as measured by the 4000 Å break strength. We also compare near-neighbour counts around AGN and around the control galaxies. On scales larger than a few Mpc, AGN have almost the same clustering amplitude as the control sample. This demonstrates that AGN host galaxies and inactive control galaxies populate dark matter halos of similar mass. On scales between 100 kpc and 1 Mpc, AGN are clustered more weakly than the control galaxies. We use mock catalogues constructed from high-resolution Nbody simulations to interpret this anti-bias, showing that the observed effect is easily understood if AGN are preferentially located at the centres of their dark matter halos. On scales less than 70 kpc, AGN cluster marginally more strongly than the control sample, but the effect is weak. When compared to the control sample, we find that only one in a hundred AGN has an extra neighbour within a radius of 70 kpc. This excess increases as a function of the accretion rate onto the black hole, but it does not rise above the few percent level. Although interactions between galaxies may be responsible for triggering nuclear activity in a minority of nearby AGN, some other mechanism is required to explain the activity seen in the majority of the objects in our sample.