High angular resolution polarimetric imaging of the nucleus of NGC 1068:
  Disentangling the polarising mechanisms
  Polarisation is a decisive method to study the inner region of active
galactic nuclei (AGNs) since it is not affected by contrast issues similarly to
how classical imaging is. When coupled to high angular resolution (HAR),
polarisation can help to disentangle the location of the various polarising
mechanisms and then give an insight on the physics taking place on the core of
AGNs. We obtained a new data set of HAR polarimetric images of the archetypal
Seyfert 2 nucleus of NGC 1068 observed with SPHERE/VLT and we aim in this paper
at presenting the polarisation maps and at spatially separating the location of
the polarising mechanisms, thus deriving constraints on the organisation of the
dust material in the inner region of this AGN. We then compared these
measurements to radiative transfer simulations of scattering and dichroic
absorption processes, using the Monte-Carlo code MontAGN. We establish a
detailed table of the relative importance of the polarising mechanism as a
function of the aperture and of the wavelength. We are able to separate the
dominant polarising mechanisms in the three regions of the ionisation cone, the
extended envelop of the torus and the very central bright source of the AGN.
Thus, we estimate the contribution of the different polarisation mechanisms to
the observed polarisation flux in these regions. Dichroic absorption is
estimated to be responsible for about 99 % of the polarised flux coming from
the photo-centre. However, this contribution would be restricted to this
location only, double scattering process being the most important contributor
to polarisation in the equatorial plane of the AGN and single scattering being
dominant in the polar outflow bi-cone.
