Correlated orientations of the axes of large quasar groups on Gpc scales

Abstract

Correlated orientations of quasar optical and radio polarisation, and of radio jets, have been reported on Gpc scales, possibly arising from intrinsic alignment of spin axes. Optical quasar polarisation appears to be preferentially either aligned or orthogonal to the host large-scale structure, specifically large quasar groups (LQGs). Using a sample of 71 LQGs at redshifts $1.0 \leq z \leq 1.8$, we investigate whether LQGs themselves exhibit correlated orientation. We find that LQG position angles (PAs) are unlikely to be drawn from a uniform distribution ($p$-values $0.008 \lesssim p \lesssim 0.07$). The LQG PA distribution is bimodal, with median modes at $\bar{\theta}\sim45\pm2^{\circ}, 136\pm2^{\circ}$, remarkably close to the mean angles of quasar radio polarisation reported in two regions coincident with our LQG sample. We quantify the degree of alignment in the PA data, and find that LQGs are aligned and orthogonal across very large scales. The maximum significance is $\simeq 0.8\%$ ($2.4\sigma$) at typical angular (proper) separations of $\sim 30^{\circ}$ (1.6 Gpc). If the LQG orientation correlation is real, it represents large-scale structure alignment over scales larger than those predicted by cosmological simulations and at least an order of magnitude larger than any so far observed, with the exception of quasar-polarisation / radio-jet alignment. We conclude that LQG alignment helps explain quasar-polarisation / radio-jet alignment, but raises challenging questions about the origin of the LQG correlation and the assumptions of the concordance cosmological model.