Optical Spectroscopy of Dual Quasar Candidates from the Subaru HSC-SSP program

TL;DR

This study uses optical spectroscopy to identify and analyze dual quasar systems during galaxy mergers, revealing their properties, host galaxy relations, and implications for black hole growth prior to coalescence.

Contribution

It reports the discovery of new dual quasars, measures their properties, and discusses their black hole and host galaxy mass relations, providing insights into black hole growth during mergers.

Findings

Discovered three new dual quasars with separations less than 20 kpc.

Dual SMBHs have higher masses than their host galaxies, indicating early black hole growth.

Success rate of identifying dual quasars is approximately 19%.

Abstract

We report on a spectroscopic program to search for dual quasars using Subaru Hyper Suprime-Cam (HSC) images of SDSS quasars which represent an important stage during galaxy mergers. Using Subaru/FOCAS and Gemini-N/GMOS, we identify three new physically associated quasar pairs having projected separations less than 20 kpc, out of 26 observed candidates. These include the discovery of the highest redshift ($z=3.1$) quasar pair with a separation $<$ 10 kpc. Based on the sample acquired to date, the success rate of identifying physically associated dual quasars is $19\%$ when excluding stars based on their HSC colors. Using the full sample of six spectroscopically confirmed dual quasars, we find that the black holes in these systems have black hole masses ($M_{BH} \sim 10^{8-9}M_{\odot}$) similar to single SDSS quasars as well as their bolometric luminosities and Eddington ratios. We measure the stellar mass of their host galaxies based on 2D image decomposition of the five-band ($grizy$) optical emission and assess the mass relation between supermassive black holes (SMBHs) and their hosts. Dual SMBHs appear to have elevated masses relative to their host galaxies. Thus mergers may not necessarily align such systems onto the local mass relation, as suggested by the Horizon-AGN simulation. This study suggests that dual luminous quasars are triggered prior to the final coalescence of the two SMBHs, resulting in early mass growth of the black holes relative to their host galaxies.