Unravelling the nuclear dust morphology of NGC 1365: a two-phase polar–RAT model for the ultraviolet to infrared spectral energy distribution

Abstract

We present a 3D radiative transfer model for the spectral energy distribution (SED) of NGC 1365, which is a ‘changing look’ Seyfert 1.8 type active galactic nucleus (AGN). The SED from the ultraviolet (UV) to the infrared (IR) is constructed using archival data from the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat, along with IR data from the literature. The SKIRT radiative transfer code is used to model the SED and derive the geometry and composition of dust in this AGN. Similar to our earlier SED model of NGC 4151, the nuclear region of NGC 1365 is assumed to contain a ring or disc-like structure concentric to the accretion disc, composed of large (0.1–1 μm) graphite grains in addition to the two-phase dusty torus made up of interstellar-medium-type grains (Ring And Torus or RAT model). We also include, for the first time, an additional component of dusty wind in the form of a bipolar cone. We carry out a detailed analysis and derive the best-fitting parameters from a χ2 test to be Rin, r = 0.03 pc, σ = 26◦, and τ total = 20 for the assumed ring–torus–polar wind geometry. Our results suggest the presence of hot dust at a temperature T ∼ 1216 K at the location of the ring that absorbs and scatters the incident UV radiation and emits in the near-IR. In the mid-IR, the major contributors are the polar cone and warm dust with T ∼ 914 K at Rin, t = 0.1 pc. Not only are our model radii in agreement with IR interferometric observations, but also our study reiterates the role of high-resolution UV observations in constraining the dust grain size distribution in the nuclear regions of AGN.