# Cooling timescale of dust tori in dying active galactic nuclei

## TL;DR

This paper estimates the dust torus cooling timescale in dying AGN, showing it is mainly governed by light crossing time, leading to rapid fading of mid-infrared emission within 100 years after quenching.

## Contribution

It provides the first detailed estimate of dust torus cooling timescale in AGN post-quenching, highlighting the rapid decline of infrared emission and proposing observational methods to identify dying AGN.

## Key findings

- Dust torus cools from 1000 K to 10 K in less than 10 years.
- Mid-infrared emission drops over two orders of magnitude within 100 years.
- Cooling timescale is primarily determined by light crossing time of the torus.

## Abstract

We estimate the dust torus cooling timescale once the active galactic nucleus (AGN) is quenched. In a clumpy torus system, once the incoming photons are suppressed, the cooling timescale of one clump from $T_{\rm dust}=1000$ K to several $10$ K is less than $10$ years, indicating that the dust torus cooling time is mainly governed by the light crossing time of the torus from the central engine. After considering the light crossing time of the torus, the AGN torus emission at $12~\mu$m becomes over two orders of magnitude fainter within $100$ years after the quenching. We also propose that those "dying" AGN could be found using the AGN indicators with different physical scale $R$ such as $12~\mu$m band luminosity tracing AGN torus ($R \sim 10$ pc) and the optical [OIII]$\lambda5007$ emission line narrow line regions ($R=10^{2-4}$ pc).