A Multiply-Imaged z $\sim$ 6.3 Lyman Alpha Emitter candidate behind Abell 2261

TL;DR

This paper reports the discovery of a gravitationally-lensed candidate Ly$ extalpha$ emitter at z~6.3 behind Abell 2261, with potential Population III characteristics, using Hubble imaging and Keck spectroscopy.

Contribution

It presents the first spectroscopic redshift of a multiply-imaged galaxy behind Abell 2261 and identifies a high-redshift Ly$ extalpha$ emitter candidate with possible Population III features.

Findings

Identified three lensed images of a z~6.3 LAE candidate.

Measured spectroscopic redshift of a galaxy at z=3.337 behind Abell 2261.

Estimated intrinsic Ly$ extalpha$ equivalent width of ~160 Å for the candidate.

Abstract

While the Lyman-$\alpha$ ($\mathrm{Ly}\alpha$) emission line serves as an important tool in the study of galaxies at $z\lesssim 6$, finding Ly$\alpha$ emitters (LAE) at significantly higher redshifts has been more challenging, probably because of the increasing neutrality of the intergalactic medium above $z\sim6$. Galaxies with extremely high rest-frame Ly$\alpha$ equivalent widths, EW(Ly$\alpha$) $\gtrsim 150$ \AA{}, at $z>6$ are good candidates for Ly$\alpha$ follow-up observations, and can stand out in multiband imaging surveys because of their unusual colors. We have conducted a photometric search for such objects in the Cluster Lensing And Supernova survey with Hubble (CLASH), and report here the identification of three likely gravitationally-lensed images of a single LAE candidate at $z\sim6.3$, behind the galaxy cluster Abell 2261($z = 0.225$). In the process, we also measured with Keck/MOSFIRE the first spectroscopic redshift of a multiply-imaged galaxy behind Abell 2261, at $z = 3.337$. This allows us to calibrate the lensing model, which in turn is used to study the properties of the candidate LAE. Population III galaxy spectral energy distribution (SED) model fits to the CLASH broadband photometry of the possible LAE provide a slightly better fit than Population I/II models. The best fitted model suggests intrinsic EW(Ly$\alpha$) $\approx 160$ \AA{} after absorption in the interstellar and intergalactic medium. Future spectroscopic observations will examine this prediction as well as shed more light on the morphology of this object, which indicates it may be a merger of two smaller galaxies.