Detailed accretion history of the supermassive black hole in NGC 5972 over the past $\gtrsim$10$^4$ years through the extended emission line region

TL;DR

This study uses integral field spectroscopy to analyze the extended emission line region of NGC 5972, revealing the galaxy's supermassive black hole's accretion history over the past 10,000 years, indicating a gradual fading of the AGN.

Contribution

It provides the first detailed accretion history of a supermassive black hole over 10,000 years using spatially-resolved emission line diagnostics and photoionization modeling.

Findings

NGC 5972's AGN has decreased in luminosity by a factor of ~100 over 50,000 years.

The EELR is primarily ionized by the AGN, consistent with Seyfert-like emission.

Multiple kinematic components suggest a history of galaxy interaction.

Abstract

We present integral field spectroscopic observations of NGC 5972 obtained with the Multi Unit Spectroscopic Explorer (MUSE) at VLT. NGC 5972 is a nearby galaxy containing both an active galactic nucleus (AGN), and an extended emission line region (EELR) reaching out to $\sim 17$ kpc from the nucleus. We analyze the physical conditions of the EELR using spatially-resolved spectra, focusing on the radial dependence of ionization state together with the light travel time distance to probe the variability of the AGN on $\gtrsim 10^{4}$ yr timescales. The kinematic analysis suggests multiple components: (a) a faint component following the rotation of the large scale disk; (b) a component associated with the EELR suggestive of extraplanar gas connected to tidal tails; (c) a kinematically decoupled nuclear disk. Both the kinematics and the observed tidal tails suggest a major past interaction event. Emission line diagnostics along the EELR arms typically evidence Seyfert-like emission, implying that the EELR was primarily ionized by the AGN. We generate a set of photoionization models and fit these to different regions along the EELR. This allows us to estimate the bolometric luminosity required at different radii to excite the gas to the observed state. Our results suggests that NGC 5972 is a fading quasar, showing a steady gradual decrease in intrinsic AGN luminosity, and hence the accretion rate onto the SMBH, by a factor $\sim 100$ over the past $5 \times 10^{4}$ yr.