An Interacting Galaxy Pair at the Origin of a Light Echo

Abstract

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 bi-cone 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 (2.4 × 10 44 erg s −1 ) is a factor 20 lower than the power needed to excite the gas in the EELR (4.6×10 45 erg s −1 ) indicating a dramatic fading of the AGN in the last 3 × 10 4 yr. ShaSS 073-622 provides all the ingredients listed in the recipe of a light echo where a high-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.

Interrogating Seyferts with NebulaBayes: Spatially Probing the Narrow-line Region Radiation Fields and Chemical Abundances

“…Our new code NebulaBayes generalizes the methods used in IZI and BOND, providing parameter estimates and statistical errors from observed line fluxes. A first analysis of ENLR spectra with NebulaBayes was performed by Merluzzi et al (2018); the inferred parameters were consistent with the results produced by a more detailed analysis with tailored photoionization models. The NebulaBayes software is described in detail in the following section and in Appendix A.…”

Shapley Supercluster Survey: mapping the filamentary network connecting the clusters

“…The redshift survey enables the velocity limits of the Shapley supercluster to be robustly defined, and the confirmation of galaxies as supercluster members. This is a key step in the identification of promising targets for follow-up IFU spectroscopy through our ongoing WiFeS survey of ram-pressure stripped or interacting galaxies within the Shapley supercluster (Merluzzi et al 2013(Merluzzi et al , 2016(Merluzzi et al , 2018.…”

Ionizing Spotlight of Active Galactic Nucleus

“…Such 'ionizing footprint from a neighbour's even allow to find hidden AGN in an interacting system, as it was shown in Was 49 [71]. An inverse case is ShaSS 073 in the Shapley supercluster, where the EELR excited in the companion's disk gave evidences of a dramatic fading of AGN luminosity (in 20 times in the past 30,000 yr) hosted in the second component of the pair at projected separation 21 kpc [72]. The most extreme example of cross-ionization among nearby galaxies was recently found in Mrk 783.…”