In Search of the First Stars: An Ultra-compact and Very-low-metallicity Ly<i>α</i> Emitter Deep within the Epoch of Reionization
Chris J. Willott, Yoshihisa Asada, Kartheik G. Iyer, Jon Judež, Gregor Rihtaršič, Nicholas S. Martis, Ghassan T. E. Sarrouh, G. Desprez, Anishya Harshan, Lamiya Mowla, Gaël Noirot, Giordano Felicioni, Maruša Bradač, Gabriel Brammer, Adam Muzzin, Marcin Sawicki, Jacqueline Antwi-Danso, Vladan Markov, Roberta Tripodi
Abstract
Abstract We present JWST observations of a gravitationally lensed, extremely metal-poor galaxy at redshift z = 8.203 ± 0.001 from the CANUCS survey. Based on the low oxygen to Balmer line ratios, we infer a gas-phase metallicity of 12 + log(O/H) = 6.85 (1.4% solar), making CANUCS-A370-z8-LAE one of the most metal-poor galaxies known at z > 7. The galaxy has a high H β equivalent width of 225 ± 50 Å, small half-light radius of only <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>r</mml:mi> <mml:mi>hl</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>3</mml:mn> <mml:msubsup> <mml:mn>8</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> pc, and high star formation rate density of 50–100 M ⊙ yr −1 kpc −2 . The galaxy shows high-equivalent-width Ly α emission with an inferred Ly α escape fraction of 0.21 ± 0.05. The high escape fraction of Ly α 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 δz = 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.