An Active Galactic Nucleus Caught in the Act of Turning Off and On

TL;DR

This paper reports the discovery of an active galactic nucleus in SDSS J1354+1327 that has episodically turned off and on again, evidenced by distinct outflows from separate accretion events likely triggered by galaxy interaction.

Contribution

It provides the first detailed observational evidence of an AGN that has turned off and on again, revealing episodic accretion events through multi-wavelength imaging and spectroscopy.

Findings

Detected two distinct outflows indicating separate AGN episodes

Identified shocked and photoionized gas structures associated with AGN activity

Supported the episodic AGN activity hypothesis with multi-instrument data

Abstract

We present the discovery of an active galactic nucleus (AGN) that is turning off and then on again in the z=0.06 galaxy SDSS J1354+1327. This episodic nuclear activity is the result of discrete accretion events, which could have been triggered by a past interaction with the companion galaxy that is currently located 12.5 kpc away. We originally targeted SDSS J1354+1327 because its Sloan Digital Sky Survey spectrum has narrow AGN emission lines that exhibit a velocity offset of 69 km s$^{-1}$ relative to systemic. To determine the nature of the galaxy and its velocity-offset emission lines, we observed SDSS J1354+1327 with Chandra/ACIS, Hubble Space Telescope/Wide Field Camera 3, Apache Point Observatory optical longslit spectroscopy, and Keck/OSIRIS integral-field spectroscopy. We find a ~10 kpc cone of photoionized gas south of the galaxy center and a ~1 kpc semi-spherical front of shocked gas, which is responsible for the velocity offset in the emission lines, north of the galaxy center. We interpret these two outflows as the result of two separate AGN accretion events; the first AGN outburst created the southern outflow, and then $<10^5$ yrs later the second AGN outburst launched the northern shock front. SDSS J1354+1327 is the galaxy with the strongest evidence for an AGN that has turned off and then on again, and it fits into the broader context of AGN flickering that includes observations of AGN light echoes.