In Search of the First Stars: An Ultra-Compact and Very Low Metallicity Lyman-$\alpha$ Emitter Deep Within the Epoch of Reionization

Abstract

We present {\it JWST} observations of a gravitationally-lensed, extremely metal-poor galaxy at redshift $z=8.203\pm 0.001$ from the CANUCS survey. Based on the low oxygen to Balmer line ratios we infer a gas-phase metallicity of $12+{\rm log(O/H)}=6.85$ (1.4\% solar), making CANUCS-A370-z8-LAE one of the most metal-poor galaxies known at $z>7$. With a high H$\beta$ equivalent width of $225\pm50$\,\AA\ and a half-light radius of only $r_{\rm hl} = 38 ^{+3}_{-19} $\,pc, the galaxy has a high star-formation-rate density of $50 - 100\,M_{\odot}$\,yr$^{-1}$\,kpc$^{-2}$. The galaxy shows high equivalent width Lyman-$\alpha$ emission with an inferred Lyman-$\alpha$ escape fraction of $0.21 \pm 0.05$. The high escape fraction of Lyman-$\alpha$ is likely due to the compact starbursting nature of the galaxy combined with its location in an overdensity traced by at least two other galaxies spectroscopically confirmed to lie within $\delta z = 0.01$ that have helped to reionize the environment. The low metallicity of CANUCS-A370-z8-LAE is best explained by a model where infalling metal-poor gas dilutes the interstellar medium, rather than being a young galaxy forming its first stellar populations.