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

TL;DR

This study spectroscopically confirmed dual and offset quasars from Subaru HSC-SSP imaging, providing insights into their properties, fractions, and the challenges in their detection across different redshifts.

Contribution

First spectroscopic confirmation of dual quasars from Subaru HSC-SSP, with analysis of their properties and estimated occurrence rates across redshifts.

Findings

Confirmed six broad-line dual quasars and eleven offset quasars.

Dual quasars at z > 1.5 have high black hole masses and luminosities but moderate accretion rates.

Estimated dual quasar fractions are 0.2%-1.2% at z<0.8, lower than some theoretical predictions.

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 arcsec 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 ($M_{\rm BH}$ $=10^{8.5}$-$10^{10} M_{\odot}$) with high bolometric luminosities ($L_{bol}$ $=10^{45.5}$-$10^{47.5}$ 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%-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.