  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.
