Subaru and e-Merlin observations of NGC3718 Diaries of a supermassive black hole recoil?

NGC3718 is a low-ionization nuclear emission line region (LINER) L1.9 galaxy, lying at a distance of about similar to 17.4 Mpc from the Earth; its similarities with NGC5128 often award it the name northern Centaurus A. The presence of a compact radio source with a candidate jet structure, a prominent dust lane, and a strongly warped molecular and atomic gas disk are indications that NGC3718 has undergone some sort of a large-scale gravitational interaction sometime in the recent past, which channeled gas towards the center, feeding the black hole and igniting the central engine. One proposed scenario involves an encounter with the close neighboring galaxy NGC3729, while other authors favor a merging event with mass ratio >=(3 4): 1 as the origin of NGC3718. We use high angular resolution (similar to 100 mas) e-Merlin radio and Subaru near-IR (NIR) (similar to 170 mas) data to take a detailed view of the processes taking place in its central region. In order to preserve some objectivity in our interpretation, we combine our results with literature values and findings from previous studies. Our NIR maps suggest, on the one hand, that towards the stellar bulge there are no large-scale absorption phenomena caused by the apparent dust lane and, on the other, that there is a significant (local) contribution from hot (similar to 1000 K) dust to the nuclear NIR emission. The position where this takes place appears to be closer to the offset compact radio emission from our e-Merlin 6 cm map and is offset by similar to 4.25 pc from the center of the underlying stellar bulge. The shape of the radio map suggests the presence of one (or possibly two, forming an X-shape) bipolar structure(s) similar to 1 (similar to 0.6) arcsec across, which combined with the balance between the gas and the stellar velocity dispersions and the presence of hard X-ray emission, point towards effects expected by AGN feedback. We also argue that NGC3718 has a core in its surface brightness profile, although it is a gas-rich galaxy and we discuss its mixed photometric and spectroscopic characteristics. These characteristics combined with the observed spatial NIR and radio emission offsets, the relative redshift between the broad and the narrow H-alpha line, the limited star formation activity, and AGN feedback strongly imply the existence of a supermassive black hole recoil. Finally, we discuss a possible interpretation that could naturally incorporate all these findings into one physically consistent picture.