Large Scale Environment of a $z=6.61$ Luminous Quasar Probed by
  Ly$\alpha$ Emitters and Lyman Break Galaxies
  Quasars (QSOs) hosting supermassive black holes are believed to reside in
massive halos harboring galaxy overdensities. However, many observations
revealed average or low galaxy densities around $z\gtrsim6$ QSOs. This could be
partly because they measured galaxy densities in only tens of arcmin$^2$ around
QSOs and might have overlooked potential larger scale galaxy overdensities.
Some previous studies also observed only Lyman break galaxies (LBGs, massive
older galaxies) and missed low mass young galaxies like Ly$\alpha$ emitters
(LAEs) around QSOs. Here we present observations of LAE and LBG candidates in
$\sim700$ arcmin$^2$ around a $z=6.61$ luminous QSO using Subaru Telescope
Suprime-Cam with narrow/broadbands. We compare their sky distributions, number
densities and angular correlation functions with those of LAEs/LBGs detected in
the same manner and comparable data quality in our control blank field. In the
QSO field, LAEs and LBGs are clustering in 4-20 comoving Mpc angular scales,
but LAEs show mostly underdensity over the field while LBGs are forming
$30\times60$ comoving Mpc$^2$ large scale structure containing 3-$7\sigma$ high
density clumps. The highest density clump includes a bright (23.78 mag in the
narrowband) extended ($\gtrsim 16$ kpc) Ly$\alpha$ blob candidate, indicative
of a dense environment. The QSO could be part of the structure but is not
located exactly at any of the high density peaks. Near the QSO, LAEs show
underdensity while LBGs average to $4\sigma$ excess densities compared to the
control field. If these environments reflect halo mass, the QSO may not be in
the most massive halo, but still in a moderately massive one.
