The off-centered Seyfert-like compact emission in the nuclear region of NGC 3621

TL;DR

This study reveals an off-centered Seyfert-like emission region in NGC 3621, suggesting past AGN activity, obscuration effects, or a recoiling black hole, based on integral field spectroscopy analysis.

Contribution

It is the first detailed analysis of an off-centered emission blob in NGC 3621, proposing multiple scenarios for its origin and AGN activity history.

Findings

The emission blob is 70 pc from the nucleus with Seyfert 2 spectrum.

The AGN luminosity decreased by a factor of 13-500 over 230 years.

Multiple scenarios, including obscuration and recoiling black hole, are consistent with observations.

Abstract

We analyze an optical data cube of the nuclear region of NGC 3621, taken with the integral field unit of the Gemini Multi-object Spectrograph. We found that the previously detected central line emission in this galaxy actually comes from a blob, located at a projected distance of 2.14" +/- 0.08" (70.1 +/- 2.6 pc) from the stellar nucleus. Only diffuse emission was detected in the rest of the field of view, with a deficit of emission at the position of the stellar nucleus. Diagnostic diagram analysis reveals that the off-centered emitting blob has a Seyfert 2 spectrum. We propose that the line-emitting blob may be a "fossil" emission-line region or a light "echo" from an active galactic nucleus (AGN), which was significantly brighter in the past. Our estimates indicate that the bolometric luminosity of the AGN must have decreased by a factor of ~13 - 500 during the last ~230 years. A second scenario to explain the morphology of the line-emitting areas in the nuclear region of NGC 3621 involves no decrease of the AGN bolometric luminosity and establishes that the AGN is highly obscured toward the observer but not toward the line-emitting blob. The third scenario proposed here assumes that the off-centered line-emitting blob is a recoiling supermassive black hole, after the coalescence of two black holes. Finally, an additional hypothesis is that the central X-ray source is not an AGN, but an X-ray binary. This idea is consistent with all the scenarios we proposed.