2020MNRAS.497.1654C

Using a 5-h adaptive-optics-assisted observation with Multi-Unit Spectroscopic Explorer, we have identified a doubly imaged Ly α source at a redshift of 5.975 behind the z = 0.222 lens galaxy J0946+1006 (the 'Jackpot'). The source separation implies an Einstein radius of ∼2.5 arcsec. Combined with the two previously known Einstein rings in this lens (radii 1.4 arcsec at z = 0.609 and 2.1 arcsec at z ≃ 2.4), this system is now a unique galaxy-scale triple-source-plane lens. We show that existing lensing models for J0946+1006 successfully map the two new observed images to a common point on the z = 5.975 source plane. The new source will provide further constraints on the mass distribution in the lens and in the two previously known sources. The third source also probes two new distance scaling factors that are sensitive to the cosmological parameters of the Universe. We show that detection of a new multiply imaged emission-line source is not unexpected in observations of this depth; similar data for other known lenses should reveal a larger sample of multiple-image-plane systems for cosmography and other applications.