Hot graphite dust in the inner regime of NGC 4151

TL;DR

This study models the near-infrared spectral energy distribution of NGC 4151 using 3D radiative transfer to compare torus and ring-torus scenarios, revealing a hot graphite ring at 0.04 pc as the most plausible structure.

Contribution

It introduces a detailed 3D radiative transfer model comparing torus and ring-torus configurations for NGC 4151's infrared emission, identifying the hot graphite ring as key.

Findings

A hot graphite ring at 0.04 pc explains the NIR SED.

The torus alone cannot account for the observed NIR emission.

A smooth dust distribution in the ring fits the data best.

Abstract

We model the near infrared SED of NGC 4151 with a 3-D radiative transfer SKIRT code, using which torus only (TO) and Ring And Torus (RAT) scenarios are studied. In the RAT models, a graphite ring-like structure (clumpy or smooth), is incorporated between the torus and the accretion disk. We vary the inclination angle $(i)$, inner radius (of the torus and the ring, $R_{\rm in, t}$ and $R_{\rm in, r}$ respectively), torus half-opening angle ($\sigma $), optical depth ($\tau_{9.7, \rm t} $ of the torus and $\tau_{9.7, \rm r} $ of the ring ) and the dust clump size ($R_{\rm clump}$). We perform a statistical analysis of the parameter space and find that all the models are able to explain the flat NIR SED of NGC 4151 with minor differences in the derived parameters. For the TO model, we get, $R_{\rm in, t}=0.1$ pc, $\sigma = 30^\circ $, $i = 53^\circ $, $\tau_{9.7, \rm t}=10$ and the clumpsize, $R_{\rm clump}$ =0.4 pc. For the smooth RAT model, $R_{\rm in, \rm r}=0.04$ pc, $\tau_{9.7, \rm total}$ = 11 and for the clumpy RAT model, $R_{\rm in, r} = 0.04$ pc/0.06 pc and $\tau_{9.7, \rm total}=20$. The $R_{\rm in, t}$ from the TO model does not agree with the NIR observations ($\sim 0.04$ pc). Hence, the most likely scenario is that a hot graphite ring is located at a distance 0.04 pc from the centre, composed of a smooth distribution of dust followed by a dusty torus at 0.1 pc with ISM type of grains.