Astrophysicists at Caltech say they have detected the oldest, most distant galaxy known so far. It's 13.2 billion years old — just over half a billion years younger than the universe itself — and the discovery may change what astrophysicists know about the early history of the universe.
Adi Zitrin and Richard Ellis first spotted EGS8p7 with the Hubble Space Telescope and the Spitzer Space Telescope, and based on those initial observations, they decided it was worth a closer look. To learn more about EGS8p7, they needed different instruments, so they turned to the Multi-Object Spectrometer For InfraRed Exploration (MOSFIRE) on the Keck Observatory in Hawai'i.
Spectrographic analysis is a way of looking at the whole spectrum of electromagnetic radiation emitted by an object. In particular, they wanted to know the galaxy's redshift. When an object in space moves away from us, the light waves get stretched out. Longer wavelengths of light appear redder, so the spectra of more distant objects will be shifted towards red. That's called redshift, and astrophysicists often use it as a measure of the distance and age of galaxies.
EGS8p7 had a redshift of 8.68. Prior to its discovery, the oldest, most distant galaxy that scientists knew about had a redshift of 7.73. But its age and distance weren't the only remarkable thing about EGS8p7.
Zitrin, Ellis, and their colleagues were surprised by what they saw in EGS8p7's spectra. Hot hydrogen gas, heated by the ultraviolent emissions of new stars, produces a spectral its signature called the Lyman-alpha line. When astrophysicists see a Lyman-alpha line, it's usually a sign of star formation in a young, newly-forming galaxy. That made sense for EGS8p7, except that 13.2 billion years ago, its Lyman-alpha line should have been absorbed by clouds of hydrogen gas.
"The surprising aspect about the present discovery is that we have detected this Lyman-alpha line in apparently faint galaxy at a redshift of 8.68, corresponding to a time when the universe should be full of absorbing hydrogen clouds," said Ellis in a statement. It turns out that EGS8p7's out-of-place spectrographic signature may shed some light on the early history of our universe. The researchers published their findings in the Astrophysical Journal Letters.
Clouds in the Early Universe
Right after the Big Bang, matter existed only as charged particles — protons and electrons — because the universe was still too hot for the particles to come together and form atoms. After about 380,000 years, according to astrophysicists, the universe had cooled enough that the free electrons and protons could join into neutral hydrogen atoms, without a positive or negative charge. Thanks to gravity, those atoms eventually started to condense together into huge clouds of gas that happened to absorb radiation in the wavelengths emitted by young galaxies, including the Lyman-alpha line. This is about when EGS8p7 was born.
"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 Zitrin in a statement. "We expect that most of the radiation from this galaxy would be absorbed by the hydrogen in the intervening space. Yet we still see Lyman-alpha from this galaxy."
So why does EGS8p7's spectrographic signature show up when our current models of the early universe say it shouldn't have?
Ionizing the Universe
As the first galaxies — including EGS8p7 — formed, between half a billion and 1 billion years after the Big Bang, the radiation they emitted swept through the hydrogen clouds and ionized, or charged, the gas by either adding electrons or sweeping them away. Ionized hydrogen absorbs different wavelengths of radiation than neutral hydrogen, so after about 1 billion years, galaxies have visible Lyman-alpha lines again.
Researchers say that reionization didn't happen uniformly, at the same time across the whole universe. When EGS8p7 formed, the galaxy's emissions may have been hot and bright enough to ionize the hydrogen clouds around it, making it possible for the radiation on the Lyman-alpha line to shine through. That may be because EGS8p7 is special, like a giant galactic snowflake.
"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," explained Caltech graduate student Sirio Belli, who was involved with the project, in a statement.
Zitrin and Ellis say they are now re-examining the timeline of re-ionization, in light of what they're learning from EGS8p7. In the long run, that could improve our understanding of the early evolution of our universe.
Top image: NASA/JPL-Caltech