Paper detail

AGN environments: is the viewing angle sufficient to explain the difference between broad-line and narrow-line AGN? -- A low-redshift study of close AGN neighbours. Paper I

The unification of active galactic nuclei (AGN) is a model that has been difficult to test due to the lack of knowledge on the intrinsic luminosities of the objects. We present a test were we probe the model by statistical investigation of the neighbours to AGN at redshifts 0.03 < z < 0.2 within a projected distance of 350 kpc and |Δz|<0.001, 0.006, 0.012 and 0.03 between AGN and neighbour. 1658 Type-1 (broad-line) AGN-galaxy pairs and 5698 Type-2 AGN-galaxy pairs with spectroscopic redshifts from the Data Release 7 of Sloan Digital Sky Survey were used together with a complementary set of pairs with photometric redshifts on the neighbour galaxies (13519 Type-1 AGN-galaxy and 58743 Type-2 AGN-galaxy pairs). Morphologies for the AGN host galaxies were derived from the Galaxy Zoo project. Our results suggest that broad-line AGN and narrow-line AGN reside in widely different environments where the neighbours to Type-2 AGN are more star-forming and bluer than those of Type-1 AGN. There is a colour-dependency only detectable in the neighbours with photometric redshifts for the Type-2 AGN. We see that the ratio between Type-1/Type-2 neighbours to Type-2 AGN decreases steadily at short separations with a statistical significance of 4.5 sigma. The lack of change in the morphology of the Type-2 AGN hosts having a close companion (contrary to the case of Type-1 AGN hosts) suggests that the innate state of Type-2 AGN is extremely short-lived and is not preserved in subsequent mergers. Finally, we perform a hypothetical luminosity test to investigate whether a mass bias in our selection could explain the observed differences in our samples. Our conclusion is that AGN unification is consistently not supported by the environment of the two types of AGN, but that an evolutionary connection between them might exist.