Discovery of Dying Active Galactic Nucleus in Arp 187: Experience of
  Drastic Luminosity Decline within $10^4$ years
  Arp 187 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 Arp 187, which trace various physical scales from less
than 0.1 pc to the nearly 10 kpc, to estimate the longterm luminosity change
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 Arp 187 hosts narrow line
region with the estimated size of $\sim$1 kpc, and the line strengths give the
AGN luminosity of $L_{\rm bol}=1.5 \times 10^{46}$ erg s$^{-1}$. On the other
hand, the current AGN activity estimated from the AGN torus emission gives the
upper bound of $L_{\rm bol} < 2.2 \times 10^{43}$ erg s$^{-1}$. The absence of
the radio-core gives the more strict upper bound of the current AGN luminosity
of $L_{\rm bol} < 8.0 \times 10^{40}$ erg s$^{-1}$, suggesting that the central
engine is already quenched. These multi-wavelength signatures indicate that Arp
187 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 luminosity decline 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.
