After spending years searching the cosmos, a research team from CalTech may have found the oldest and most distant galaxy yet. As part of its search, the team combed through data collected by the Hubble Space Telescope and the Spitzer Space telescope, and ultimately identified the galaxy EGS8p7 as a target to investigate further.
The team then performed a spectrographic analysis of EGS8p7 using the Keck Observatory’s multi-object spectrometer for infrared exploration (MOSFIRE) to calculate its redshift. Redshift is a result of the Doppler effect – like when an ambulance drives by and the siren fades as it passes. In astronomy, instead of sound waves, light from the galaxy is stretched and shifted towards the red end of the spectrum as it moves farther away. Based on the data collected, the team estimates that EGS8p7 is over 13.2 billion years old, meaning it was formed only around 600 million years after the Big Bang.
A galaxy’s redshift is commonly used to determine its age. However, when looking at very distant objects, it’s difficult to calculate. Right after the Big Bang, the universe was like a hot, soupy mixture of charged particles, unable to transmit any light. Approximately 380,000 years later, the universe cooled just enough to allow the protons and electrons making up the particle soup to combine, forming neutral hydrogen and permitting light to travel through the universe.
The first galaxies started cranking out young stars when the universe was just half a billion years old, thus reionizing the cosmos. Before the reionization started, the neutral hydrogen atoms would have absorbed certain light emitted by early galaxies. Typically, when stars form they emit blasts of ultraviolet light. This UV emission heats up surrounding hydrogen gas, producing a trackable spectral signature known as a Lyman-alpha line. Astronomers look for this signature when they are looking for new star formation. Theoretically, due to the absorption, the team should not have been able to observe a Lyman-alpha line for EGS8p7.
"If you look at the galaxies in the early universe, there is a lot of neutral hydrogen that is not transparent to this emission," said Adi Zitrin, a NASA Hubble postdoctoral scholar in a statement. "We expect that most of the radiation from this galaxy would be absorbed by the hydrogen in the intervening space. Yet still we see Lyman-alpha from this galaxy."
Keck’s MOSFIRE spectrometer records chemical signatures of stars and distant galaxies in near-infrared wavelengths. Prior to EGS8p7, the farthest galaxy on record had a redshift of 7.73. Based on the MOSFIRE observations, the team detected a Lyman-alpha line within EGS8p7 with a redshift of 8.68. One possible explanation for this surprising discovery is that the reionization process could be patchy, meaning it doesn’t happen uniformly.
"The galaxy we have observed, EGS8p7, which is unusually luminous, may be powered by a population of unusually hot stars, and it may have special properties that enabled it to create a large bubble of ionized hydrogen much earlier than is possible for more typical galaxies at these times," said Sirio Belli, a Caltech graduate student who worked on the project.
The team is currently studying this discovery in greater detail to determine exactly what the chances are for detecting early galaxies such as EGS8p7 and their emissions to better understand the reionization timeline.
The findings are published in the Astrophysical Journal Letters.
