An interacting galaxy pair at the origin of a light echo
  In a low-density region of the Shapley supercluster we identified an
interacting galaxy pair at redshift z=0.04865 in which the Seyfert-2 nucleus of
the main galaxy (ShaSS 073) is exciting an extended emission line region (EELR,
~170 kpc^2) in the disk of the less massive companion (ShaSS 622). New
integral-field spectroscopy and the multi-band data-set, spanning from
far-ultraviolet to far-infrared and radio wavelengths, allowed us to obtain a
detailed description of the ShaSS 622-073 system. The gas kinematics shows
hints of interaction, although the overall velocity field shows a quite regular
rotation in both galaxies, thus suggesting that we are observing their first
encounter as confirmed by the estimated distance of 21 kpc between the two
galaxy centers. The detected ~2-3 kpc AGN outflow and the geometry of the EELR
in ShaSS 622 support the presence of a hollow bicone structure. The status and
sources of the ionization across the whole system have been analysed through
photoionization models and a Bayesian approach which prove a clear connection
between the AGN and the EELR. The luminosity of the AGN (2.4x10^44 erg/s) is a
factor 20 lower than the power needed to excite the gas in the EELR (4.6x10^45
erg/s) indicating a dramatic fading of the AGN in the last 3x10^4 yr. ShaSS
073-622 provides all the ingredients listed in the recipe of a light echo where
a highly ionised region maintains memory of a preceding more energetic phase of
a now faded AGN. This is the first case of a light echo observed between two
galaxies.
