A hidden population of high-redshift double quasars unveiled by astrometry

TL;DR

This paper reports the discovery of two high-redshift double quasars using a novel astrometric technique, providing potential evidence for kpc-scale dual SMBHs at z>2, which are crucial for understanding SMBH mergers.

Contribution

The study introduces a new astrometric method to systematically identify high-redshift double quasars, achieving a high success rate and offering insights into early universe SMBH evolution.

Findings

Discovered two double quasars at z>2 using a novel astrometric approach.

One quasar pair spatially resolved with optical spectroscopy, suggesting a physical SMBH pair.

Potential identification of the first kpc-scale dual SMBHs at high redshift.

Abstract

Galaxy mergers occur frequently in the early universe and bring multiple supermassive black holes (SMBHs) into the nucleus, where they may eventually coalesce. Identifying post-merger-scale (i.e., <~a few kpc) dual SMBHs is a critical pathway to understanding their dynamical evolution and successive mergers. While serendipitously discovering kpc-scale dual SMBHs at z<1 is possible, such systems are elusive at z>2, but critical to constraining the progenitors of SMBH mergers. The redshift z~2 also marks the epoch of peak activity of luminous quasars, hence probing this spatial regime at high redshift is of particular significance in understanding the evolution of quasars. However, given stringent resolution requirements, there is currently no confirmed <10 kpc physical SMBH pair at z>2. Here we report two sub-arcsec double quasars at z>2 discovered from a targeted search with a novel astrometric technique, demonstrating a high success rate (~50%) in this systematic approach. These high-redshift double quasars could be the long-sought kpc-scale dual SMBHs, or sub-arcsec gravitationally-lensed quasar images. One of these double quasars (at z=2.95) was spatially resolved with optical spectroscopy, and slightly favors the scenario of a physical quasar pair with a projected separation of 3.5 kpc (0.46"). Follow-up observations of double quasars discovered by this targeted approach will be able to provide the first observational constraints on kpc-scale dual SMBHs at z>2.