Adaptive Optics Imaging of QSOs with Double-Peaked Narrow Lines: Are they Dual AGNs?

TL;DR

This study uses adaptive optics imaging to identify and analyze dual AGN candidates among SDSS QSOs with double-peaked emission lines, providing insights into SMBH growth during galaxy mergers.

Contribution

It presents the first high-resolution imaging survey of a complete sample of double-peaked SDSS QSOs to constrain the fraction of dual SMBHs and their role in galaxy evolution.

Findings

Approximately 0.3%-0.65% of SDSS QSOs host dual SMBHs on kpc scales.

The dual AGN phase duration is shorter than previous estimates based on galaxy merger rates.

Little difference in SMBH mass distributions between single and dual AGN, suggesting SMBH growth occurs later in mergers.

Abstract

Active galaxies hosting two accreting and merging super-massive black holes (SMBHs) -- dual Active Galactic Nuclei (AGN) -- are predicted by many current and popular models of black hole-galaxy co-evolution. We present here the results of a program that has identified a set of probable dual AGN candidates based on near Infra-red (NIR) Laser Guide-Star Adaptive Optics (LGS AO) imaging with the Keck II telescope. These candidates are selected from a complete sample of radio-quiet Quasi-stellar Objects (QSOs) drawn from the Sloan Digital Sky Survey (SDSS), which show double-peaked narrow AGN emission lines. Of the twelve AGNs imaged, we find six with double galaxy structure, of which four are in galaxy mergers. We measure the ionization of the two velocity components in the narrow AGN lines to test the hypothesis that both velocity components come from an active nucleus. The combination of a well-defined parent sample and high-quality imaging allows us to place constraints on the fraction of SDSS QSOs that host dual accreting black holes separated on kiloparsec (kpc) scales: ~0.3%-0.65%. We derive from this fraction the time spent in a QSO phase during a typical merger and find a value that is much lower than estimates that arise from QSO space densities and galaxy merger statistics. We discuss possible reasons for this difference. Finally, we compare the SMBH mass distributions of single and dual AGN and find little difference between the two within the limited statistics of our program, hinting that most SMBH growth happens in the later stages of a merger process.