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 solar masses/yr, it contains ~ 10^{8.3-8.7} solar
mass of stars 50-80 Myr old, with stellar metallicity ~ 0.03-0.12 of the solar
value. 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 <= 0.2 and
extra ionizing radiation from sources clustered around UDFy-38135539 is
necessary. Finally, we predict that there is a 70% (15%) probability of
detecting at least 1 such source from JWST (HST/WFC3) observations in a
physical radius ~ 0.4 Mpc around UDFy-38135539.
