Exploring the inner parsecs of Active Galactic Nuclei using near-infrared high resolution polarimetric simulations with MontAGN

Abstract

Aims: In this paper, we aim to constrain the properties of dust structures in the central first parsecs of Active Galactic Nuclei (AGN). Our goal is to study the required optical depth and composition of different dusty and ionised structures. Methods: We developed a radiative transfer code, MontAGN, optimised for polarimetric observations in the infrared. With both this code and STOKES, designed to be relevant from the hard X-ray band to near-infrared wavelengths, we investigate the polarisation emerging from a characteristic model of the AGN environment. For that purpose, we compare predictions of our models with previous infrared observations of NGC 1068, and try to reproduce several key polarisation patterns revealed by polarisation mapping. Results: We constrain the required dust structures as well as their densities. More precisely, we find out that the electron density inside the ionisation cone is about $2.0 \times 10^9$ m$^{-3}$ . With structures constituted of spherical grains of constant density, we also highlight that the torus should be thicker than 20 in term of K band optical depth to block direct light from the centre. It should also have a stratification in density, with a lesser dense outer rim with an optical depth at 2.2 {\mu}m typically between 0.8 and 4 for observing the double scattering effect previously proposed. Conclusions: We bring constraints on the dust structures in the inner parsecs of an AGN model supposed to describe NGC 1068. When compared to observations, this leads to optical depth of at least 20 in Ks band for the torus of NGC 1068, corresponding to $\tau_V \approx 170$, which is within the range of current estimation based on observations. In the future, we will improve our study by including non uniform dust structures and aligned elongated grains to constrain other possible interpretations of the observations.