A fully-spectroscopic triple-source-plane lens: the Jackpot completed

TL;DR

This paper reports the spectroscopic redshift measurement of the second source in the Jackpot gravitational lens system, refining cosmological constraints and demonstrating the importance of spectroscopic data for accurate lens modeling.

Contribution

The study provides the first spectroscopic redshift for the second source in J0946+1006, improving the accuracy of cosmological parameter estimates derived from this lens system.

Findings

Spectroscopic redshift of the second source is z=2.035.

Revised redshift favors less negative dark energy equation-of-state parameter w.

Spectroscopic data marginally improves lens model accuracy.

Abstract

We present a spectroscopic determination of the redshift of the second source in the Jackpot gravitational lens system J0946+1006, for which only a photometric estimate of $z_{\rm phot}$ = 2.41$^{+0.04}_{-0.21}$ has previously been available. By visually inspecting an archival VLT X-Shooter observation, we located a single emission line from the source in the H-band. Among the possible options we find that this line is most likely to be [OIII] 5007 Ang at $z_{\rm spec} $ = 2.035. Guided by this proposal, we were able to detect the faint CIII] 1907,1909 Ang emission doublet in a deep VLT MUSE datacube. The CIII] emission is spatially coincident with the brightest parts of the second Einstein ring, and strongly supports the redshift identification. The spectroscopic redshift is only marginally consistent with the photometric estimate. Re-examining the cosmological constraints from J0946+1006, the revised measurement favours less negative values of the dark energy equation-of-state parameter $w$; when combined with a cosmic microwave background prior, we infer $w$ = $-1.04\pm0.20$. The revised redshift does not significantly help to reconcile the small discrepancy in the image positions for the even more distant third source in J0946+1006.