Subaru and e-Merlin observations of NGC 3718

Diaries of a supermassive black hole recoil?^⋆,⋆⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: markakis@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9P, UK
⁴ Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
⁵ Göttingen eResearch Alliance, State and University Library Göttingen, Papendiek 14, 37073 Göttingen  

Received: 29 September 2014
Accepted: 13 April 2015

Abstract

NGC 3718 is a low-ionization nuclear emission line region (LINER) L1.9 galaxy, lying at a distance of about ~17.4 Mpc from the Earth; its similarities with NGC 5128 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 NGC 3718 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 NGC 3729, while other authors favor a merging event with mass ratio ≥(3−4):1 as the origin of NGC3718. We use high angular resolution (~100 mas) e-Merlin radio and Subaru near-IR (NIR) (~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 (~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 ~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) ~1 (~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 NGC 3718 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 Hα 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.