Do z>6 quasars reside in protoclusters?

TL;DR

The paper tests whether luminous quasars at z>6 trace the progenitors of present-day massive halos by applying the GAEA semi-analytic framework to Planck Millennium Simulation merger trees and selecting 56 QSOs with L_bolo > 10^{46.25} erg s^{-1}. It analyzes their local environments within a 7.5 h^{-1} Mpc box and tracks their descendants to z=0, comparing to JWST/ASPIRE results. The findings show a wide range of environments and evolutionary paths, with about half triggered by disc instabilities and many fields hosting AGN companions, yet only a minority end up in the most massive clusters; bright high-z QSOs are therefore not robust signposts of proto-clusters. The results imply that extra environmental information is needed to identify the most promising proto-cluster candidates, shaping how high-z QSO fields are used to study early structure formation.

Abstract

We discuss the properties of a sample of z>6 bright (bolometric luminosity L$_{\rm bolo}$>10$^{46.25}$ erg/s) Quasars drawn from a realization of the GAlaxy Evolution and Assembly (GAEA) model coupled with the Planck Millennium Simulation. We focus on the properties and environment of host galaxies, and their evolution down to z=0, with the aim of assessing how well the bright high redshift QSOs population traces the progenitors of most massive haloes in the local Universe. Our results show that at z>6 bright QSOs live in a variety of environments, and that secular processes like disc instability are responsible for triggering roughly the same number of QSOs as galaxy mergers. Half of cubic (7.5 $h^{-1}$ cMpc size) mock fields built around these high-z QSOs include other active galaxies (with L$_{\rm bolo}$>10$^{44}$ erg/s) in sizeable number, the other host galaxies being relatively isolated. The large field-to-field variance in the the number of companions (both active and non-active) recently reported from JWST observations is fairly well reproduced by GAEA predictions. Descendants of host galaxies at z=0 cover a wide range of physical properties and environments with only a small fraction of the hosts of high-z QSOs ending up in massive galaxy clusters. Viceversa, GAEA predicts that only a small fraction of Bright Central Galaxies have a bright z>6 QSOs among their progenitors. Our results suggest that luminous high-z QSO loosely trace the progenitors of low-z galaxy clusters, and that additional information about the environment of high-z QSOs are required to identify the most promising proto-cluster candidates.

Figures (7)

• Figure 1: Properties of the most luminous z>6 QSOs and their host galaxies in gaea. Blue empty histograms refer to the properties of the full sample of 56 high-z QSOs, while the cyan filled histograms show the same properties for the subsample of QSOs triggered by disc instability events. Vertical dashed and dotted lines mark the position of our reference objects (see main text for more details).
• Figure 2: Upper panels: 3D galaxy distribution around two representative high-z QSOs. Left-hand panels correspond to QSO-B, while right-hand panels to QSO-A (see text for more details). Circles mark the position of galaxy companions with M$_\star$>10$^{8} \, {\rm M}_\odot$ in a 7.5 $h^{-1}$ cMpc size box centred on the high-z QSO. The size of the symbols is proportional to the galaxy stellar mass, and they are colour coded according to their SFR as in the colorbar on the right of the lower panels. Lower panels: projected galaxy distribution around reference high-z QSOs. The depth of the projection is 7.5 $h^{-1}$ cMpc. Purple pluses and crosses mark the position of AGN companions with L$_{\rm bolo}$ larger than 10$^{42.5}$ erg/s and 10$^{44}$ erg/s, respectively. In all panels, the grey shading correspond to the underlying projected distribution of dark matter, computed from the distribution of DMHs in the simulated volume.
• Figure 3: Upper panel: Fraction of fields $f_{\rm fields}$ hosting a given number of M$_\star$>10$^{8} \, {\rm M}_\odot$ AGN companions brighter than L$_{\rm bolo}$> 10$^{42.5}$ (blue dot-dashed histogram) and 10$^{44}$ erg/s (red dashed histogram and Orange filled area). The black histogram shows the fraction of fields in the ASPIRE survey, hosting a given number of [O$_{\rm III}$] emitters Wang25. Lower panel: M$_\star$>10$^{8} \, {\rm M}_\odot$ galaxy surface number density distribution as a function of projected galaxy separation from the central QSO. Solid black, red dashed and blue dot-dashed lines refer to the total galaxy population and the AGN companions (defined using a with L$_{\rm bolo}$> 10$^{42.5}$ and 10$^{44}$ erg/s AGN luminosity threshold) respectively.
• Figure 4: Redshift evolution of selected properties of the host galaxies for the two reference high-z QSOs (thick lines). In all panels, the evolutionary tracks for the entire sample of high-z QSOs is shown with thin lines. Left Panels correspond to the redshift evolution for QSO-B, while the right panel for QSO-A (see text for more details). Upper panels show the redshift evolution of the parent DMH mass, stellar mass and SMBH mass. Lower panels show the redshift evolution for the sSFR and bolometric luminosity. In the sSFR panel, the dotted line represent a classical threshold for defining active galaxies in theoretical models, larger than 0.3/t$_H$, where t$_H$ is the Hubble time at the redshift under consideration. In the L$_{\rm bolo}$ panel, the horizontal line marks the detection threshold for high-z QSOs we assume in this work.
• Figure 5: Evolution of the M$_{\rm BH}$-M$_{\star}$ relation for the host galaxies of high-z QSOs. Blue stars represent the relation at the redshift of detection of the sample, while cyan circles mark the position at z$\sim$0. Thin lines show the evolution of the relation along the main progenitor of the host galaxy. Red open symbols and blue thick lines show the evolution for QSOA-A and QSO-B (dotted and dashed line respectively). Orange lines refer to observed relations at different redshifts, as estimated by Pacucci23, KormendyHo13 and McConnellMa13.