The high redshift Integrated Sachs-Wolfe effect

Abstract

In this paper we rely on the quasar (QSO) catalog of the SloanDigital Sky Survey Data Release Six (SDSS DR6) of about one millionphotometrically selected QSOs to compute the Integrated Sachs-Wolfe(ISW) effect at high redshift, aiming at constraining the behaviorof the expansion rate and thus the behaviour of dark energy at thoseepochs. This unique sample significantly extends previous catalogsto higher redshifts while retaining high efficiency in the selectionalgorithm. We compute the auto-correlation function (ACF) of QSOnumber density from which we extract the bias and the stellarcontamination. We then calculate the cross-correlation function(CCF) between QSO number density and Cosmic Microwave Background(CMB) temperature fluctuations in different subsamples: at highz > 1.5 and low z < 1.5 redshifts and for two different choices ofQSO in a conservative and in a more speculative analysis. We find anoverall evidence for a cross-correlation different from zero at the2.7 σ level, while this evidence drops to 1.5 σ at z > 1.5. We focus on the capabilities of the ISW to constrain thebehaviour of the dark energy component at high redshift both in the Λ CDM and Early Dark Energy cosmologies, when the dark energyis substantially unconstrained by observations. At present, theinclusion of the ISW data results in a poor improvement compared tothe obtained constraints from other cosmological datasets. We studythe capabilities of future high-redshift QSO survey and find thatthe ISW signal can improve the constraints on the most importantcosmological parameters derived from Planck CMB data, including thehigh redshift dark energy abundance, by a factor ∼ 1.5.