Unravelling the Nuclear Dust Morphology of NGC 1365: A Two Phase Polar - RAT Model for the Ultraviolet to Infrared Spectral Energy Distribution

TL;DR

This paper develops a detailed 3D radiative transfer model for NGC 1365's spectral energy distribution, revealing complex dust structures including a ring, torus, and bipolar wind, consistent with IR observations.

Contribution

Introduces a novel two-phase dust model with a bipolar wind component for NGC 1365, enhancing understanding of AGN dust morphology and SED fitting.

Findings

Best-fit dust ring at 0.03 pc with hot dust at 1216 K

Mid-IR emission dominated by polar cone and warm dust at 0.1 pc

Model radii agree with IR interferometric data

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 AGN. The SED from the ultraviolet (UV) to the infrared (IR) is constructed using archival data from the UVIT on-board $AstroSat$, 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 disk-like structure concentric to the accretion disk, composed of large (0.1$\mu$m - 1$\mu$m) graphite grains in addition to the two-phase dusty torus made up of ISM-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-fit parameters from a $\chi^2 $ test to be $R_{\rm in,r}$ = 0.03 pc, $\sigma$ = 26$^\circ$ and $\tau$ = 20 for the assumed ring-torus-polar wind geometry. Our results suggest the presence of hot dust at a temperature T $\sim$ 1216 K at the location of the ring which absorbs and scatters the incident UV radiation and emits in the near-IR (NIR). In the mid-IR (MIR) the major contributors are the polar cone and warm dust with T $\sim$ 916 K at $R_{\rm in,t}$ = 0.1 pc. Not only are our model radii in agreement with IR interferometric observations, our study also reiterates the role of high resolution UV observations in constraining the dust grain size distribution in the nuclear regions of AGN.