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 Msun yr-1, it contains 108.3-8.7 Msun of stars 50-80 Myr old, with stellar metallicity ˜0.03-0.12 Zsun. 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.