Cosmic Train Wreck by Massive Black Holes: Discovery of a kpc-Scale Triple Active Galactic Nucleus

TL;DR

This paper reports the discovery of a kpc-scale triple active galactic nucleus, providing evidence for hierarchical galaxy mergers leading to triple supermassive black hole systems, with implications for galaxy evolution.

Contribution

The study presents the first systematic identification of a kpc-scale triple AGN, revealing a rare triple SMBH system and estimating its merger timescale.

Findings

Discovered a triple AGN with three obscured nuclei.

Estimated the SMBH masses to be greater than 10^8 solar masses.

Predicted the system will merge within approximately 40 million years.

Abstract

Hierarchical galaxy mergers will lead to the formation of binary and, in the case of a subsequent merger before a binary coalesce, triple supermassive black holes (SMBHs), given that most massive galaxies harbor SMBHs. A triple of SMBHs becomes visible as a triple active galactic nucleus (AGN) when the BHs accrete large amounts of gas at the same time. Here we report the discovery of a kpc-scale triple AGN, SDSSJ1027+1749 at z = 0.066, from our systematic search for hierarchical mergers of AGNs. The galaxy contains three emission-line nuclei, two of which are offset by ~ 450 and 110 km/s in velocity and by 2.4 and 3.0 kpc in projected separation from the central nucleus. All three nuclei are classified as obscured AGNs based on optical diagnostic emission line ratios, with black hole mass estimates M_BH ~> 10^8 M_sun from stellar velocity dispersions measured in the associated stellar components. Based on dynamical friction timescale estimates, the three stellar components in SDSSJ1027+1749 will merge in ~ 40 Myr, and their associated SMBHs may evolve into a gravitationally interacting triple system in ~< 200 Myr. Our result sets a lower limit of ~ 5 x 10^(-5) for the fraction of kpc-scale triples in optically selected AGNs at z ~ 0.1.