Abstract
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 zphot = 2.41$^{+0.04}_{-0.21}$ has previously been available. By visually inspecting an archival Very Large Telescope (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 [O iii] 5007 Å at zspec = 2.035. Guided by this proposal, we were able to detect the faint C iii] 1907,1909 Å emission doublet in a deep VLT Multi Unit Spectroscopic Explorer datacube. The C iii] 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 ± 0.20. 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.
Highlights
In double-source-plane gravitational lenses, a foreground galaxy forms multiple images of two background objects, at different redshifts
The photometric redshift estimate has been used to derive constraints in the ( m, w) cosmological parameter plane by Collett & Auger (2014, hereafter CA14), and has been used to test the massdensity profile of the main lens by Sonnenfeld et al (2012). Since these analyses rely critically on the accuracy of the photometric redshift, it is of interest to corroborate the value spectroscopically. (A precise spectroscopic redshift will improve the precision of these analyses, the photometric redshift uncertainty is a sub-dominant component of the total error budget for both applications.) With this goal, in CS20, we searched the deep optical datacube for spectral features from s2, but found no convincing lines or continuum breaks
We take into account the known separation from the lens centre, and the expected spatial and spectral line profile. Through this method we identified a single emission line in the H-band, at 15 197.5 Å, offset south along the slit by ∼2 arcsec
Summary
In double-source-plane gravitational lenses, a foreground galaxy forms multiple images of two background objects, at different redshifts. The photometric redshift estimate has been used to derive constraints in the ( m, w) cosmological parameter plane by Collett & Auger (2014, hereafter CA14), and has been used to test the massdensity profile of the main lens by Sonnenfeld et al (2012) Since these analyses rely critically on the accuracy of the photometric redshift, it is of interest to corroborate the value spectroscopically. If the published photometric redshift is broadly correct, the MUSE data cover rest wavelengths 1400–2800 Å, where no strong features are expected Weaker lines, such as [C III] at 1907 Å and C III] at 1909 Å (collectively C III] hereafter) might be present, but could not be identified when working ‘blind’, i.e. without advance clues as to the precise redshift. C 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society
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