Determine the redshift of SMACS J0723 system 8

Determine the source redshift of multiple-image system 8 in SMACS J0723, whose lens-model posterior remains unconstrained at the upper end of the adopted prior range $z\in[4,15]$, through spectroscopic confirmation.

Background

The SMACS J0723 lens model treats the redshifts of 12 multiple-image systems as free parameters, while fixing five systems with spectroscopic measurements. For system 8, the inferred redshift is not constrained by the strong-lensing data within the prior interval 4≤z≤154\leq z\leq15; the model favors z>10z>10.

The unresolved redshift affects the interpretation of this image family and cannot be settled by the current lens modeling alone. The paper states that spectroscopic observations will likely be needed to determine it.

References

For system~8, the posterior remains unconstrained at the upper end of the prior range, $z\in[4,15]$; the model favors $z>10$, again consistent with citet{2022A&A...666L...9C}, who similarly found the redshift of this system to be unconstrained. Spectroscopic confirmation will likely be required to determine the redshift of this system.

— $\texttt{LensFactory.jl}$: A general-purpose strong lens modeling package  (2609.08649 - Meena, 8 Sep 2026) in Section 5, paragraph discussing the inferred redshift distributions for SMACS J0723 multiple-image families