Ancient giants: on the farthest galaxy at z = 8.6

The observational frontiers for the detection of high-redshift galaxies have recently been pushed to unimaginable distances with the record-holding Lyman alpha emitter (LAE) UDFy-38135539 discovered at redshift z= 8.6. However, the physical nature and the implications of this discovery have yet to be assessed. By selecting galaxies with observed luminosities similar to UDFy-38135539 in state-of-the-art cosmological simulations tuned to reproduce the large-scale properties of LAEs, we bracket the physical nature of UDFy-38135539: it has a star formation rate ∼2.7–3.7 M⊙ yr−1, it contains 108.3-8.7 M⊙ of stars 50–80 Myr old, with stellar metallicity ∼0.03–0.12 Z⊙. For any of the simulated galaxies to be visible as a LAE in the observed range, the intergalactic neutral hydrogen fraction at z= 8.6 must be χH I≤ 0.2 and extra ionizing radiation from sources clustered around UDFy-38135539 is necessary. Finally, we predict that there is a 70 per cent (15 per cent) probability of detecting at least one such source from JWST (HST/WFC3) observations in a physical radius ∼0.4 Mpc around UDFy-38135539.