Primordial environment of super massive black holes: large-scale galaxy overdensities aroundz ~ 6 quasars with LBT

Abstract

Context. 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. Aims. 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. Methods. With the wide-field (∼23 × 25 ) 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 ∼2.3 × 10 4 to ∼2.9 × 10 4 objects, down to a 50% completeness limit of z = 25.0-25.2 AB mag. 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 ∼8 × 8 physical Mpc at z ∼ 6. The same selection criteria were then applied to z-band-selected sources in the ∼1 deg 2 wide-and-deep Subaru-XMM Newton Deep Survey (SXDS) to derive the expected number of dropouts over a blank LBC-sized field (∼0.14 deg 2 after removing masked regions). Results. 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 deg 2 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 0.14 deg 2 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.

Luminous quasars do not live in the most overdense regions of galaxies at z ∼ 4

“…For example, Husband et al (2013) found that three luminous quasars at z ∼ 5 reside in overdense regions of Lyman break galaxies (LBGs). Morselli et al (2014) found more idropout galaxies in four z ∼ 6 quasar fields than expected in a blank field. Adams et al (2015) reported that about 10 % of a sample of luminous quasars at z ∼ 4 reside in overdense regions of Lyα emitters (LAEs).…”

Tracing a high redshift cosmic web with quasar systems

“…12 M sun h −1 (Wold et al 2000;Trainor & Steidel 2012;Shen et al 2013;Karhunen et al 2014;Morselli et al 2014). The small space density of quasars and the mass limit of quasar host haloes tell us that quasars are rare events in small groups rather than common events in rare, very rich clusters (Trainor & Steidel 2012, 2013.…”

The diverse galaxy counts in the environment of high-redshift massive black holes in Horizon-AGN

“…They show that the common assumption of quasars being embedded in the most massive dark matter haloes can not be ruled out, nor confirmed, and that the galaxy density enhancement can actually extend to large scale of several tens of Mpc. In the BlueTides simulation, the most massive BHs are not found in haloes with the (Husband et al 2013), J0338+0021 (Husband et al 2013), J1204+0021 (Husband et al 2013), J0203+0012 (Bañados et al 2013), J1030+0524 (Kim et al 2009;Stiavelli et al 2005;Willott et al 2005;Morselli et al 2014), J1630+4012 (Kim et al 2009), J1048+4637 (Kim et al 2009;Willott et al 2005;Morselli et al 2014), J1148+5251 (Kim et al 2009;Willott et al 2005;Morselli et al 2014), J1306+0356 (Kim et al 2009), J0836+0054 (Zheng et al 2006), J1439+0342 (Schneider et al 2000). We have computed the separation distances with the cosmological parameters that have been used for the simulation Horizon-AGN.…”

“…In fact, some quasars have been observed several times, with different instruments, selection criteria, and FoV. This is the case for the quasars J1148+5251(Willott et al 2005;Kim et al 2009;Morselli et al 2014), J1048+4637(Willott et al 2005;Kim et al 2009;Morselli et al 2014), and J1030+0524(Stiavelli et al 2005;Willott et al 2005;Kim et al 2009;Morselli et al 2014)…”