Discovery of merging twin quasars at z = 6.05

Abstract

We report the discovery of two quasars at a redshift of $z$ = 6.05, in the process of merging. They were serendipitously discovered from the deep multi-band imaging data collected by the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey. The quasars, HSC $J$121503.42$-$014858.7 (C1) and HSC $J$121503.55$-$014859.3 (C2), both have luminous ($>$10${}^{43}$ erg s${}^{-1}$) Ly$\alpha$ emission with a clear broad component (full width at half maximum $>$1000 km s${}^{-1}$). The rest-frame ultraviolet (UV) absolute magnitudes are $M_{1450}=-23.106\pm 0.017$ (C1) and $-22.662\pm 0.024$ (C2). Our crude estimates of the black hole masses provide $\log (M_{\mathrm{BH}}/M_{\odot })=8.1\pm 0.3$ in both sources. The two quasars are separated by 12 kpc in projected proper distance, bridged by a structure in the rest-UV light suggesting that they are undergoing a merger. This pair is one of the most distant merging quasars reported to date, providing crucial insight into galaxy and black hole build-up in the hierarchical structure formation scenario. A companion paper will present the gas and dust properties captured by Atacama Large Millimeter/submillimeter Array observations, which provide additional evidence for and detailed measurements of the merger and also demonstrate that the two sources are not gravitationally-lensed images of a single quasar.

arXiv commentPublished from ApJL. A companion paper (T. Izumi et al.) presents the associated ALMA observations

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