Cooling timescale of dust tori in dying active galactic nuclei

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 $T_{\mathrm{dust}}=1000$ 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]$\lambda 5007$ emission line narrow line regions ($R=10^{2-4}$ pc).

8 pages, 4 figures, accepted for publication in ApJ

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• CO Emission in Infrared-Selected Active Galactic Nuclei
• NuSTAR Observations of AGN with Low Observed X-ray to [OIII] Luminosity Ratios: Heavily Obscured AGN or Turned-Off AGN?
• Modeling the Infrared Reverberation Response of the Circumnuclear Dusty Torus in AGNs: An Investigation of Torus Response Functions
• Discovery of a strong ionized-gas outflow in an AKARI-selected Ultra-luminous Infrared Galaxy at z = 0.5
• Finding of a population of active galactic nuclei showing a significant luminosity decline in the past $\sim 10^{3-4}$ yrs
• NuSTAR hard X-ray data and Gemini 3D spectra reveal powerful AGN and outflow histories in two low-redshift Lyman-$\alpha$ blobs
• NuSTAR Non-detection of a Faint Active Galactic Nucleus in an Ultraluminous IR Galaxy with Kpc-scale Fast Wind
• An interacting galaxy pair at the origin of a light echo
• Implication of a galaxy-scale negative feedback by one of the most powerful multi-phase outflows in a hyper-luminous infrared galaxy at the intermediate redshift