2017A&A...606A..23B

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 overdensities that may extend up to ∼10 physical Mpc. This scenario, however, has not been confirmed observationally, as the search for galaxy overdensities around high-z quasars has returned conflicting results. The field around the z=6.31 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 (∼25'x25') 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 catalogs in the Y- and J-band, and matched those with our photometric catalog in the r, z, and i bands presented in our previous paper and based on sources with z_AB<25.2 detected using z-band images from the the Large Binocular Cameras (LBC) at the Large Binocular Telescope (LBT) over the same field of view. We used these new infrared data together with H and K photometric measurements from the MUlti-wavelength Survey by Yale-Chile (MUSYC) and with the Spitzer Infrared Array Camera (IRAC) data to refine our selection of Lyman break galaxies (LBGs), extending our selection criteria to galaxies in the range 25.2<z_AB<25.7. We selected 21 robust high redshift candidates in the J1030 field with photometric redshift z∼6 and colors i-z≥1.3. We found a significant asymmetry in the distribution of the high redshift galaxies in J1030, supporting the existence of a coherent large-scale structure around the quasar. We estimated an overdensity of z∼6 galaxies in the field of δ=2.4, which is significant at >4σ. The overdensity value and its significance are higher than those found in our previous paper and we interpret this as evidence of an improved LBG selection.