Modelling Grain Rotational Disruption by Radiative Torques and Extinction of Active Galactic Nuclei

Abstract: Extinction curves observed toward individual Active Galactic Nuclei (AGN) usually show a steep rise toward far-ultraviolet (FUV) wavelengths and can be described by the Small Magellanic Cloud (SMC)-like dust model. This feature suggests the dominance of small dust grains of size $a \leq 0.1 \mu m$ in the local environment of AGN, but the origin of such small grains is unclear. In this paper, we aim to explain this observed feature by applying the RAdiative Torque Disruption (RATD) to model the extinction of AGN radiation from FUV to mid-infrared (MIR) wavelengths. We find that in the intense radiation field of AGN, large composite grains of size $a \geq 0.1 \mu m$ are significantly disrupted to smaller sizes by RATD up to $d_{\rm RATD} > 100$ pc in the polar direction and $d_{\rm RATD} \sim 10$ pc in the torus region. Consequently, optical-MIR extinction decreases, whereas FUV-near-ultraviolet (NUV) extinction increases, producing a steep far-UV rise extinction curve. The resulting total-to-selective visual extinction ratio thus significantly drops to $R_{\rm V} < 3.1$ with decreasing distances to AGN center due to the enhancement of small grains. The dependence of $R_{\rm V}$ with the efficiency of RATD will help us to study the dust properties in the AGN environment via photometric observations. In addition, we suggest that the combination of the strength between RATD and other dust destruction mechanisms which are responsible for destroying very small grains of $a \leq 0.05 \mu m$ is the key for explaining the dichotomy observed 'SMC' and 'gray' extinction curve toward many AGN.