Discovery of Dying Active Galactic Nucleus in Arp 187: Experience of Drastic Luminosity Decline within $10^4$ years
2018
Arp187 is one of the fading active galactic nuclei (AGN), whose AGN activity is currently decreasing in
luminosity. We investigate the observational signatures of AGN in
Arp187, which trace various physical scales from less than 0.1 pc to the nearly 10 kpc, to estimate the longterm
luminositychange over $10^{4}$ years. The VLA 5 GHz, 8 GHz, and the ALMA 133 GHz images reveal bimodal jet lobes with $\sim$5 kpc size and the absence of the central radio-core. The 6dF optical spectrum shows that
Arp187 hosts narrow line region with the estimated size of $\sim$1 kpc, and the line strengths give the AGN
luminosityof $L_{\rm bol}=1.5 \times 10^{46}$
ergs$^{-1}$. On the other hand, the current AGN activity estimated from the AGN torus emission gives the upper bound of $L_{\rm bol} < 2.2 \times 10^{43}$
ergs$^{-1}$. The absence of the radio-core gives the more strict upper bound of the current AGN
luminosityof $L_{\rm bol} < 8.0 \times 10^{40}$
ergs$^{-1}$, suggesting that the central engine is already quenched. These multi-wavelength signatures indicate that
Arp187 hosts a "dying" AGN: the central engine is already dead, but the large scale AGN indicators are still observable as the remnant of the past AGN activity. The central engine has experienced the drastic
luminositydecline by a factor of $\sim10^{3-5}$ fainter within $\sim10^{4}$ years, which is roughly consistent with the viscous timescale of the inner part of the accretion disk within $\sim$500 years.
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