2014A&A...568A...1M

In the current model of structure formation, bright quasars (QSOs) at z∼6 are assumed to be hosted by the most massive dark matter halos that collapsed at that time. The large-scale structures in which these halos are embedded may extend up to 10 physical Mpc, and probably can be traced by overdensities of star-forming galaxies. To date, the search for these overdensities has been limited to scales of 1-2 physical Mpc around the QSO and did not produce coherent results. We aim at studying the environment of z∼6 QSOs and verify whether they are associated with large-scale overdensities of galaxies selected at the same redshift as i-band dropouts. With the wide-field (∼23'x25') Large Binocular Camera (LBC) at the Large Binocular Telescope (LBT), we obtained deep r-, i- and z-band imaging of the fields around four high-z QSOs, namely SDSS J1030+0524 (z=6.28), SDSS J1148+5251 (z=6.41), SDSS J1048+4637 (z=6.20), and SDSS J1411+1217 (z=5.95). Our photometric catalogs are based on source detection in the z-band image (5σ) and contain from ∼2.3x10⁴ to ∼2.9x10⁴ objects, down to a 50% completeness limit of z=25.0-25.2ABmag. We adopted color-color selections within the i-z vs. r-z plane to identify samples of i-band dropouts at the QSO redshift and measured their relative abundance and spatial distribution in the four LBC fields, each covering ∼8x8 physical Mpc at z∼6. The same selection criteria were then applied to z-band-selected sources in the ∼1deg² wide-and-deep Subaru-XMM Newton Deep Survey (SXDS) to derive the expected number of dropouts over a blank LBC-sized field (∼0.14deg² after removing masked regions). The four observed QSO fields host more candidates than expected in a blank field. By defining objects with z_AB<25 and i-z>1.4 that are undetected in the r-band as i-band dropouts, we found 16, 10, 9, and 12 dropouts in SDSS J1030+0524, SDSS J1148+5251, SDSS J1048+4637, and SDSS J1411+1217, respectively, whereas only 4.3 such objects are expected over a 0.14 deg² blank field. This corresponds to overdensity significances of 3.3, 1.9, 1.7, and 2.5σ, respectively, after accounting for cosmic variance and for the contamination by bluer objects in our dropout samples produced by photometric errors. By considering the total number of dropouts in the four LBC fields and comparing it with what is expected in four blank fields of 0.14deg² each, we find that high-z QSOs reside in overdense environments at the 3.7σ level. This is the first direct and unambiguous measurement of the large-scale structures around z∼6 QSOs.