Running summarizing tools on a new article

Abstract: Active galactic nuclei (AGN) vary on broad timescales, but are primarily thought to "flicker" on and off during episodes of high and low accretion. Supermassive black holes (SMBHs) which are actively accreting material often show optical spectra dominated by AGN emission in their nuclei and beyond. In this work we develop an empirical model to quantify the fractional AGN contribution to optical spectra using the Baldwin-Phillips-Terlevich (BPT) emission line diagnostic diagram. We leverage this model against integral field unit (IFU) data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to select 34 galaxies with weak nuclear AGN activity relative to their AGN signatures at large radii; this signifies past AGN activity which is fading with time (i.e., fading-AGN). The majority of our fading-AGN reside in edge-on systems undergoing minor mergers. We construct a simple model consisting of a disk galaxy with ionization cones and find that the orientation of such a system will impact its BPT classification in both single-fiber and IFU observations. The mean distance to the extended AGN emission in our sample corresponds to a light-travel time of $1.1\times10^4$ years. We use this to set a lower-limit on the typical lifetime of the AGN phase ($t_{\text{AGN}}>1.3\times10^5$ years) which is consistent with SMBHs growing via brief accretion bursts. These fading-AGN allow us to probe the elusive period of post-AGN galaxy evolution, and they give a $\sim10^4$ year baseline for understanding the accretion history in these systems.