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 SDSS quasars with the Subaru Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using two-dimensional image decomposition, our selection identifies PSF-like companions within 0″.6–4″separation (≲ 30 kpc) around the SDSS quasar. We newly confirm six broad-line dual quasars and eleven offset quasars (quasar-galaxy pairs), spanning 1.5 < z < 3.3 for the duals and predominantly z < 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, Gemini/GMOS-N, Keck/NIRES, and Subaru/FOCAS. From the spectra, we measure the emission-line properties of these dual black holes (BH). At z > 1.5, the confirmed duals exhibit high black hole mass (MBH =108.5–1010M⊙) with high bolometric luminosities (Lbol =1045.5–1047.5 erg s−1), yet accrete at moderate Eddington ratios (λEdd =0.01–0.4). From the spectroscopically-confirmed samples, we estimate the dual fraction of SDSS quasars with separations of 0.6″–4″ to be 0.2 %-1.2 % at z < 0.8, 0.08 %-0.24 % at 0.8 < z < 1.5, and 0.06 % at 1.5 < z < 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.