Abstract
Context. Polarisation is a decisive method to study the inner region of active galactic nuclei (AGNs) since, unlike classical imaging, it is not affected by contrast issues. When coupled with high angular resolution (HAR), polarisation can help to disentangle the location of the different polarising mechanisms and then give insight into the physics taking place in the core of AGNs. Aims. We obtained a new data set of HAR polarimetric images of the archetypal Seyfert 2 nucleus of NGC 1068 observed with SPHERE/VLT. We aim in this paper to present the polarisation maps and to spatially separate the location of the polarising mechanisms, thereby deriving constraints on the organisation of the dust material in the inner region of this AGN. Methods. With four new narrow-band images between the visible and the near-infrared combined with older broad-band observations, we studied the wavelength dependence of the polarisation properties from 0.7 to 2.2 μm of three selected regions within the inner 2″ surrounding the central engine. We then compared these measurements to radiative transfer simulations of scattering and dichroic absorption processes, using the Monte Carlo code MontAGN. Results. We establish a detailed table of the relative importance of the polarising mechanism as a function of aperture and wavelength. We are able to separate the dominant polarising mechanisms in the three regions of the ionisation cone, the extended envelope 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 only be restricted to this location because the double-scattering process would be the most important contributor to polarisation in the equatorial plane of the AGN and single scattering is dominant in the polar outflow bi-cone. Conclusions. Even though results are in good agreement with larger apertures measurements, the variety of situations with different mechanisms at play highlights the importance of spatial resolution for the interpretation of polarisation measurements. We also refine the estimation of the integrated optical depth in the visible of the obscuring structure to a range of 20−100, constraining the geometry of the inner region of this AGN.
Highlights
As a consequence of the continuous progress of instrumentation, our understanding of the organisation of the centre of active galactic nuclei (AGNs) has been growing rapidly in the last few years
This contribution would only be restricted to this location because the double-scattering process would be the most important contributor to polarisation in the equatorial plane of the AGN and single scattering is dominant in the polar outflow bi-cone
We are resolving the core of nearby AGNs at an unprecedented parsec size spatial scale owing to the development of high angular resolution (HAR) techniques, in particular interferometry and extreme adaptive optics (AO), with the help of polarisation
Summary
As a consequence of the continuous progress of instrumentation, our understanding of the organisation of the centre of active galactic nuclei (AGNs) has been growing rapidly in the last few years. Using the GRAVITY interferometer (at VLTI), the GRAVITY Collaboration (2018) was able to constrain the shape of the broad-line region of nearby quasar 3C 273 to a rotating thick disc and identified the sublimation region in NGC 1068 (GRAVITY Collaboration 2020). This view is completed owing to the high-contrast imaging coupled with polarimetry, as shown for instance by Packham et al (1997, 2007) and Lopez-Rodriguez et al (2015) with different instruments in the near to mid infrared range (Gratadour et al 2015; Grosset et al 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
