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
( ≲ 30 kpc) around the SDSS quasar . We newly confirm 6 broad-line dual quasars and 11 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 discov ered 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 N orth), Keck/NIRES (N ear-Infrar ed Echellette Spectrometer), and Subaru /FOCAS (Faint Object Camera and
Spectrogr aph) . F rom the spectr a, we measure the emission-line properties of these dual black holes (BHs). At z > 1 . 5 ,
the confirmed duals exhibit high BH mass ( MBH = ^{10 8 . 5} –10 ¹⁰ M ) with high bolometric luminosities ( L _bol = 10 ^{45 . 5} –10 ^{47 . 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 arcsec to be 0.2 per cent–1.2 per cent at z < 0 . 8 , 0.08
per cent–0.24 per cent at 0 . 8 < z < 1 . 5 , and 0.06 per cent 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. How ever, w e not e
that these fractions, especially at high z, still need a more accurate assessment of selection and observation effects.

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Identifiers

ORCID for Manda Banerji: orcid.org/0000-0002-0639-5141

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