The Quasar Proximity Effect as an Alternative Probe of Quasar Pair Distances

Abstract Recently discovered quasar pairs at high redshifts ( z ≳ 5) are likely precursors to supermassive black hole mergers, providing a promising window to high-redshift quasar growth mechanisms. However, the large uncertainties on their relative distances along the line of sight ( d LOS ) limit our ability to characterize quasar pairs. In this study, we explore synthetic quasar proximity zone spectra as an alternative method to constrain the line-of-sight distance of quasar pairs. We find that for small sky-plane separations ( d sky ≈ 10–100 pkpc), a simple peak-finding algorithm can easily distinguish between scenarios of d LOS ≲ 1 pMpc and ≳1 pMpc. For cases where the true d LOS ≥ 3 pMpc, the accuracy of d LOS estimation is ≈0.2 pMpc. Large sky-plane separations of d sky = 1 pMpc have larger absolute uncertainties in d LOS estimates, but the method can still easily distinguish between scenarios where d LOS ≲ 4 pMpc and ≳4 pMpc. The d LOS estimates have an uncertainty of ≈0.5 pMpc when true d LOS ≳ 4 pMpc. Our proof-of-concept study illustrates the potential use of quasar proximity zones to constrain the three-dimensional quasar pair configuration, providing an avenue to characterize quasar pairs.