For the first few hundred million years after the Big Bang, the universe was a very cold, dark place. Basic elements like oxygen, carbon and nitrogen weren't very common until the first stars had fired up, burnt out and exploded. Now, astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected the most distant – and hence, earliest – signature of oxygen, in a galaxy 13.28 billion light-years away.
Hailing from a galaxy known as MACS1149-JD1, the oxygen signature dates back to about 500 million years after the Big Bang. But for the galaxy to have enough oxygen to be visible, it must have been creating stars for around 250 million years before that, making it one of the earliest known star-producing galaxies.
"This extremely distant, extremely young galaxy has a remarkable chemical maturity to it," says Wei Zheng, lead astronomer on the study. "It is truly remarkable that ALMA detected an emission line – the fingerprint of a particular element – at such a record-breaking distance."
As elements like oxygen are floating around out there, the light and radiation from stars ionizes the atoms, making them glow brightly in infrared. It's this light signal that ALMA has picked up some 13 billion years later, although in that time the expansion of the universe has stretched the light into the millimeter wavelength. Astronomers are able to date the signature by measuring that change in wavelength from infrared to millimeter.
After figuring out the age of the signal, the astronomers worked backwards to determine when the galaxy's first stars fired up. To have a detectable amount of oxygen, MACS1149-JD1 would have had to go through a few generations of star births and deaths. The cloud of dust and gas left over from the Big Bang would have been mostly hydrogen in the early days, but supernovae from the deaths of the first stars would have spread elements like oxygen, carbon and nitrogen.
The team determined that, if the oxygen signature was from 500 million years after the Big Bang, then the first stars in that galaxy would have been born about 250 million years post-Big Bang.
"The mature stellar population in MACS1149-JD1 implies that stars were forming back to even earlier times, beyond what we can currently see with our telescopes," says Nicolas Laporte, an astronomer on the research team. "This has very exciting implications for finding 'Cosmic Dawn' when the first galaxies emerged."
Cosmic Dawn is the name given to that time when the very first stars lit up the pitch-black universe. While this ALMA study has indirectly dated that time to 250 million years after the Big Bang, the earliest direct detection was announced earlier this year, in the form of an ionized hydrogen signature hailing from when our universe was a mere 180 million years old.
The research was published in the journal Nature, and the animation below shows how gravity would form the first stars in this early galaxy.
Source: National Radio Astronomy Observatory