Subaru and e-Merlin observations of NGC 3718

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 largescale 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.

Optical spectroscopy of local type-1 AGN LINERs

“…This spiral galaxy has a prominent dust lane, which runs across the entire stellar bulge, and a warped molecular and atomic gas disc (Sparke et al 2009;Krips et al 2005). It also shows signs of a past interaction (probably with NGC 3729, Markakis et al 2015). The elongated structures seen in the HST images and e-MERLIN data are indicative of a small-scale bipolar jets or outflows.…”

The Milky Way’s Supermassive Black Hole: How Good a Case Is It?

“…in nuclei of galaxies of different types and masses); hence, we can quasi exploit them in a statistical way by observing samples of SMBHs and SgrA* at the center of the Milky Way is one of them. Examples for such studies are the scaling relations for SMBHs (Graham et al, 2015;Ferré-Mateu et al, 2015), merger scenarii for gravitational waves detection (Schnittman, 2013), reverberation analyses of black hole masses Matsuoka et al, 2015), recoil scenarii in galactic nuclei (Markakis et al, 2015;Guedes et al, 2011).…”

The Quest for the Largest Depleted Galaxy Core: Supermassive Black Hole Binaries and Stalled Infalling Satellites

“…Khan et al 2013;Merritt 2015;Holley-Bockelmann & Khan 2015;Vasiliev et al 2015). The SMBH recoil scenario has been recently advocated by Markakis et al (2015) to explain the ∼0.2 kpc core they observed in the peculiar galaxy NGC 3718.…”