MACS 1423-z7p64 is a galaxy listed in the MAssive Cluster Survey (MACS), and announced on 10 April 2017 in the journal Nature Astronomy,[1] as being the most distant source of reionization known at this time, with a redshift z = 7.640 ± 0.001 (lookback time >13.1 Gyr).
To identify the galaxy, the astronomers used the slitless grism spectrograph of the Wide Field Infrared Survey Telescope on the Hubble Space Telescope, and to determine its distance the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) detector of the Keck Observatory.
[1][2] Nature received the paper on 27 October 2016.
)[1] With a redshift z = 7.640 ± 0.001, and being an order of magnitude lower in intensity than the four other Lyman-α emitters currently known at z > 7.5, it is probably the most distant representative source of reionization found to date.
[1] This is from a time when the universe was around 700 million years old.