Geometric Corroboration of the Earliest Lensed Galaxy at z~10.8 from Robust Free-Form Modelling

TL;DR

This paper uses a free-form gravitational lensing model to independently confirm that the galaxy MACS0647-JD is at a redshift of approximately 10.8, supporting its status as one of the earliest known galaxies formed before reionization.

Contribution

The study introduces a robust, lensing-based geometric redshift measurement method that independently verifies the high redshift of MACS0647-JD, enhancing confidence in early galaxy observations.

Findings

Geometric redshift of MACS0647-JD is approximately 10.8.

The lens model shows a tight relation between image brightness and magnification.

The galaxy's luminosity remains high after magnification correction.

Abstract

A multiply-lensed galaxy, MACS0647-JD, with a probable photometric redshift of $z\simeq 10.7^{+0.6}_{-0.4}$ is claimed to constitute one of the very earliest known galaxies, formed well before reionization was completed. However, spectral evidence that MACS0647-JD lies at high redshift has proven infeasible and so here we seek an independent lensing based "geometric redshift" derived from the angles between the three lensed images of MACS0647-JD, using our free-form mass model (WSLAP+) for the lensing cluster MACSJ0647.7+7015 (at $z=0.591$). Our lens model uses the 9 sets of multiple images, including those of MACS0647-JD, identified by the CLASH survey towards this cluster. We convincingly exclude the low redshift regime of $z<3$, for which convoluted critical curves are generated by our method, as the solution bends to accommodate the wide angles of MACS0647-JD for this low redshift. Instead, a best fit to all sets of lensed galaxy positions and redshifts provides a geometric redshift of $z\simeq 10.8^{+0.3}_{-0.4}$ for MACS0647-JD, strongly supporting the higher photometric redshift solution. Importantly, we find a tight linear relation between the relative brightnesses of all 9 sets of multiply lensed images and their relative magnifications as predicted by our model. This agreement provides a benchmark for the quality of the lens model, and establishes the robustness of our free-form lensing method for measuring model-independent geometric source distances and for deriving objective central cluster mass distributions. After correcting for its magnification the luminosity of MACS0647-JD remains relatively high at $M_{UV}=-19.4$, which is within a factor of a few in flux of some surprisingly luminous $z\simeq 10$--$11$ candidates discovered recently in Hubble black field surveys.