An off-centered active galactic nucleus in NGC 3115

TL;DR

This paper reports the discovery of an off-centered type 1 AGN in NGC 3115, revealing a previously undetected emission-line spectrum and suggesting possible black hole binary or merger remnant scenarios.

Contribution

It is the first detection of an emission-line spectrum from an off-centered AGN in NGC 3115, providing evidence for a displaced black hole and its potential implications.

Findings

Detected a type 1 AGN with Hα luminosity of ~4.2×10^37 erg/s

Located ~14.3 pc from the galaxy's stellar bulge center

Possible black hole binary or merger remnant scenario

Abstract

NGC 3115 is an S0 galaxy that has always been considered to have a pure absorption-line spectrum. Some recent studies have detected a compact radio-emitting nucleus in this object, coinciding with the photometric center and with a candidate for the X-ray nucleus. This is evidence of the existence of a low-luminosity active galactic nucleus (AGN) in the galaxy, although no emission line has ever been observed. We report the detection of an emission-line spectrum of a type 1 AGN in NGC 3115, with an H$\alpha$ luminosity of $L_{H\alpha} = (4.2 \pm 0.4) \times 10^{37}$ erg s$^{-1}$. Our analysis revealed that this AGN is located at a projected distance of $\sim 0.29" \pm 0.05"$ (corresponding to $\sim 14.3 \pm 2.5$ pc) from the stellar bulge center, which is coincident with the kinematic center of this object's stellar velocity map. The black hole corresponding to the observed off-centered AGN may form a binary system with a black hole located at the stellar bulge center. However, it is also possible that the displaced black hole is the merged remnant of the binary system coalescence, after the "kick" caused by the asymmetric emission of gravitational waves. We propose that certain features in the stellar velocity dispersion map are the result of perturbations caused by the off-centered AGN.