2025A&A...694L..12H

Multiple galaxies hosting active galactic nuclei (AGNs) at kiloparsec separations from each other are exceedingly rare, and in fact, only one quadruple AGN is known so far. These extreme densities of AGNs are expected to pinpoint protocluster environments and therefore should be surrounded by large galaxy overdensities. In this Letter, we present another quadruple AGN candidate at z ∼ 3 including two SDSS quasars at a separation of roughly 480 kpc. The brighter quasar is accompanied by two AGN candidates (a type 1 AGN and a likely type 2 quasar) at a close (∼20 kpc) separation identified through emission line ratios, line widths, and high ionization lines, such as N Vλ1240. The extended Lyα emission associated with the close triple system is more modest in extent and brightness compared to similar multiple AGN systems and could be caused by ram-pressure stripping of the type 2 quasar host during infall into the central dark matter halo. The predicted evolution of the system into a z = 0 galaxy cluster with the AGN host galaxies forming the brightest cluster galaxy needs to be further tested by galaxy overdensity studies on large scales around the quadruple AGN candidate. If confirmed as a quadruple AGN with X-ray observations or rest-frame optical line ratios, this system would represent a second AGN quartet and be the highest-redshift multiplet and the closest high-redshift triplet known.