The different neighbours around Type-1 and Type-2 active galactic nuclei
  One of the most intriguing open issues in galaxy evolution is the structure
and evolution of active galactic nuclei (AGN) that emit intense light believed
to come from an accretion disk near a super-massive black hole (Rees 1984,
Lynden-Bell 1969). To understand the zoo of different AGN classes, it has been
suggested that all AGN are the same type of object viewed from different angles
(Antonucci 1993). This model -- called AGN unification -- has been successful
in predicting e.g. the existence of hidden broad optical lines in the spectrum
of many narrow-line AGN. But this model is not unchallenged (Tran 2001) and it
is an open problem whether more than viewing angle separates the so-called
Type-1 and Type-2 AGN. Here we report the first large-scale study that finds
strong differences in the galaxy neighbours to Type-1 and Type-2 AGN with data
from the Sloan Digital Sky Survey (SDSS) (York et al. 2000) Data Release 7
(DR7) (Abazajian et al. 2008) and Galaxy Zoo (Lintott et al, 2008, Lintott et
al 2011). We find strong differences in the colour and AGN activity of the
neighbours to Type-1 and Type-2 AGN and in how the fraction of AGN residing in
spiral hosts changes depending on the presence of a neighbour or not. These
findings suggest that an evolutionary link between the two major AGN types
might exist.
