Measuring the Density Fields around Bright Quasars at $z\sim 6$ with XQR-30 Spectra

Abstract

Measuring the density of the intergalactic medium using quasar sightlines in the epoch of reionization is challenging due to the saturation of Lyman-α absorption. Near a luminous quasar, however, the enhanced radiation creates a proximity zone observable in the quasar spectra where the Lyman-α absorption is not saturated. In this study, we use 10 high-resolution (R 10, 000) z ∼ 6 quasar spectra from the extended XQR-30 sample to measure the density field in the quasar proximity zones. We find a variety of environments within 3 pMpc distance from the quasars. We compare the observed density cumulative distribution function (CDF) with models from the Cosmic Reionization on Computers simulation, and find a good agreement between 1.5 to 3 pMpc from the quasar. This region is far away from the quasar hosts and hence approaching the mean density of the universe, which allows us to use the CDF to set constraints on the cosmological parameter σ 8 = 0.6 ± 0.3. The uncertainty is mainly due to the limited number of high-quality quasar sightlines currently available. Utilizing the more than > 200 known quasars at z 6, this method will allow us in the future to tighten the constraint on σ 8 to the percent level. In the region closer to the quasar within 1.5 pMpc, we find the density is higher than predicted in the simulation by 1.23 ± 0.17, suggesting the typical host dark matter halo mass of a bright quasar (M 1450 < −26.5) at z ∼ 6 is log 10 (M/M ) = 12.5 +0.4 −0.7 .

Conditions for direct black hole seed collapse near a radio-loud quasar 1 Gyr after the Big Bang

“…If we assume a typical stellar-to-dark-matter mass ratio of ∼ 30 − 50 × 10 −3 appropriate for the high mass end of the halo mass function at z ∼ 6 (Stefanon et al 2021), this would imply a quasar host halo mass of (1 − 2) × 10 12 M . Although the uncertainty of this estimate is large, it is consistent with recent derivations based on the density profile in the proximity zones of z ∼ 6 quasars by Chen et al (2021), who found a typical host halo mass of log 10 (M h /M ) = 12.5 +0.4 −0.7 . (2006).…”

“…One could argue that in principle each quasar at z 6 must have formed in such an overdense environment given their high halo masses (Chen et al 2021). However, the finding of relatively rich environments around quasars such as J0836 and J1030 compared to other quasars must then imply that the conditions required for DCBH formation as suggested by theory may have been particularly well met for the progenitor regions of these sources.…”