A close quasar pair in a disk-disk galaxy merger at z = 2.17

TL;DR

This paper reports the discovery of a dual-quasar system at cosmic noon ($z=2.17$), providing rare observational evidence of galaxy mergers hosting supermassive black holes during peak cosmic activity.

Contribution

It presents the first confirmed kpc-scale dual quasar system at $z oughly2$, revealing galaxy merger features and host galaxy properties at this epoch.

Findings

Discovery of a dual-quasar system at $z=2.17$ with 3.8 kpc separation.

Host galaxies are massive, compact, disk-dominated, with tidal features.

The SMBHs follow the local mass--host relation and may form a binary in 0.22 Gyr.

Abstract

Galaxy mergers produce pairs of supermassive black holes (SMBHs), which may be witnessed as dual quasars if both SMBHs are rapidly accreting. The kiloparsec (kpc)-scale separation represents a physical regime sufficiently close for merger-induced effects to be important yet wide enough to be directly resolvable with the facilities currently available. Whereas many kpc-scale dual active galactic nuclei--the low-luminosity counterparts of quasars--have been observed in low-redshift mergers, no unambiguous dual quasar is known at cosmic noon ($z\approx2$), the peak of global star formation and quasar activity. Here we report multiwavelength observations of SDSS J0749+2255 as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon ($z=2.17$). We discover extended host galaxies associated with the much brighter compact quasar nuclei (separated by 0.46" or 3.8 kpc) and low-surface-brightness tidal features as evidence for galactic interactions. Unlike its low-redshift and low-luminosity counterparts, SDSS J0749+2255 is hosted by massive compact disk-dominated galaxies. The apparent lack of stellar bulges and the fact that SDSS J0749+2255 already follows the local SMBH mass--host stellar mass relation, suggest that at least some SMBHs may have formed before their host stellar bulges. While still at kpc-scale separations where the host-galaxy gravitational potential dominates, the two SMBHs may evolve into a gravitationally bound binary system in around 0.22 Gyr.