The "Green Bean" Galaxy SDSS J224024.1--092748: Unravelling the emission
  signature of a quasar ionization echo
  "Green Bean" Galaxies (GBs) are the most [O III]-luminous type-2 active
galactic nuclei (AGN) at z~0.3. However, their infrared luminosities reveal AGN
in very low activity states, indicating that their gas reservoirs must be
ionized by photons from a recent high activity episode - we are observing
quasar ionization echoes. We use integral field spectroscopy from the Gemini
Multi-Object Spectrograph to analyse the 3D kinematics, ionization state,
temperature and density of ionized gas in the GB SDSS J224024.1-092748. We
model the emission line spectrum of each spaxel as a superposition of up to
three Gaussian components and analyse the physical properties of each component
individually. Two narrow components, tracing the velocity fields of the disc
and an ionized gas cloud, are superimposed over the majority of the galaxy.
Fast shocks produce hot ($T_e$ $\geq$ 20,000 K), dense ($n_e$ $\geq$ 100
cm$^{-3}$), turbulent ($\sigma$ $\geq$ 600 km s$^{-1}$), [O III]-bright regions
with enhanced [N II]/H$\alpha$ and [S II]/H$\alpha$ ratios. The most prominent
such spot is consistent with a radio jet shock-heating the interstellar medium.
However, the AGN is still responsible for $\geq$ 82 per cent of the galaxy's
total [O III] luminosity, strengthening the case for previous quasar activity.
The ionized gas cloud has a strong kinematic link to the central AGN and is
co-rotating with the main body of the galaxy, suggesting that it may be the
remnant of a quasar-driven outflow. Our analysis of J224024.1-092748 indicates
that GBs provide a unique fossil record of the transformation from the most
luminous quasars to weak AGN.
