Selections from 2015: EGSY8p7, the Galaxy Far, Far Away

Editor’s Note: In these last two weeks of 2015, we’ll be looking at a few selections from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

Lyα Emission from a Luminous z = 8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization

Published August 2015


Main takeaway:

A team led by Adi Zitrin (Hubble Fellow at California Institute of Technology) detected Lyα emission in the bright galaxy EGSY8p7 using the MOSFIRE spectrograph at Keck Observatory. From this emission line, they calculated that the galaxy has an astonishing redshift of z=8.68.

Why it’s interesting:

This spectroscopic confirmation crowned EGSY8p7 as the record-holder for the farthest-known (and therefore oldest) galaxy. Its redshift shattered the previous record, a galaxy at z=7.73.

Why it’s even more interesting than that:

spectroscopic confirmation

Spectroscopic detection of emission in EGSY8p7 with MOSFIRE. The black line is the raw data; the red line shows the best-fit model to the data. [Zitrin et al. 2015]

Based on our understanding of how the universe evolved, the detection of Lyα emission from this galaxy came as a surprise. At EGSY8p7’s redshift of 8.68, the universe was still full of clouds of neutral hydrogen that should have absorbed the galaxy’s Lyα emission long before it reached us. So what does it mean that we do see Lyα emission from EGSY8p7? The reionization of the universe — through which the neutral hydrogen clouds were made transparent — may have been a patchy process. In particular, EGSY8p7 might have emitted an unusual amount of ionizing radiation, creating an early ionized bubble around it that allowed the Lyα emission to escape.


Adi Zitrin et al 2015 ApJ 810 L12. doi:10.1088/2041-8205/810/1/L12