Spectroscopic confirmation of dual and offset quasars from the Subaru HSC-SSP program

ABSTRACT We present a spectroscopic follow-up program targeting closely separated dual quasar candidates selected from imaging of Sloan Digital Sky Survey (SDSS) quasars with the Subaru Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using 2D image decomposition, our selection identifies point spread function-like companions within 0.6–4 arcsec separation ($\lesssim$ 30 kpc) around the SDSS quasar. We newly confirm 6 broad-line dual quasars and 11 offset quasars (quasar–galaxy pairs), spanning $1.5 \lt z \lt 3.3$ for the duals and predominantly $z \lt 0.6$ for the offset systems. No obvious lensed quasars were discovered from this program. We obtained 99 spectra of these candidates from NTT/EFOSC2 (New Technology Telescope ESO Faint Object Spectrograph and Camera version 2), Gemini/GMOS-N (Gemini Multi-Object Spectrograph on Gemini North), Keck/NIRES (Near-Infrared Echellette Spectrometer), and Subaru/FOCAS (Faint Object Camera and Spectrograph). From the spectra, we measure the emission-line properties of these dual black holes (BHs). At $z\gt 1.5$, the confirmed duals exhibit high BH mass ($M_{\rm BH}$ $=10^{8.5}$–$10^{10} {\rm M}_{\odot }$) with high bolometric luminosities ($L_{\rm bol}$ $=10^{45.5}$–$10^{47.5}$ erg s$^{-1}$), yet accrete at moderate Eddington ratios ($\lambda _{\rm Edd}=$0.01–0.4). From the spectroscopically confirmed samples, we estimate the dual fraction of SDSS quasars with separations of $0.6$–$4$ arcsec to be 0.2 per cent–1.2 per cent at $z\lt 0.8$, 0.08 per cent–0.24 per cent at $0.8\lt z\lt 1.5$, and 0.06 per cent at $1.5\lt z\lt 3.3$. These values are broadly consistent with other recent optical studies, but lower than theoretical expectations of a rising dual fraction at cosmic noon. However, we note that these fractions, especially at high z, still need a more accurate assessment of selection and observation effects.