  Many cosmological studies predict that early supermassive black holes (SMBHs)
can only form in the most massive dark matter halos embedded within large scale
structures marked by galaxy over-densities that may extend up to 10 physical
Mpc. This scenario, however, has not been confirmed observationally, as the
search for galaxy over-densities around high-z quasars has returned conflicting
results. The field around the z=6.28 quasar SDSSJ1030+0524 (J1030) is unique
for multi-band coverage and represents an excellent data legacy for studying
the environment around a primordial SMBH. In this paper we present wide-area
(25x25 arcmin), Y- and J-band imaging of the J1030 field obtained with the near
infrared camera WIRCam at the Canada-France-Hawaii Telescope (CFHT). We built
source catalogues in the Y- and J-band, and matched those with our photometric
catalogue in the r, z, i bands presented in Morselli et al. (2014). We used
these new infrared data together with H and K and Spitzer/IRAC data to refine
our selection of Lyman Break Galaxies (LBGs), extending our selection criteria
to galaxies in the range 25.2<zAB<25.7. We selected 21 robust high-z candidates
in the J1030 field with photometric redshift around 6 and colors i-z>=1.3. We
found a significant asymmetry in the distribution of the high-z galaxies in
J1030, supporting the existence of a coherent large-scale structure around the
quasar. We compared our results with those of Bowler et al. (2015), who adopted
similar LBGs selection criteria, and estimated an over-density of galaxies in
the field of delta = 2.4, which is significant at >4 sigma. The over-density
value and its significance are higher than those found in Morselli et al.
(2014), and we interpret this as evidence of an improved LBG selection.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics