Oldest stars ever seen give clue to the end of Big Bang dark age
Stars that shone just 250 million years after the universe began take astronomy closer to its Holy Grail. Richard A Lovett reports.
Scientists studying a galaxy more than 13 billion light years away have found stars that formed when the universe was only 250 million years old. That makes them the oldest stars ever seen, dating back nearly to the end of the universe’s “dark age,” when the first stars and galaxies coalesced to illuminate a previously dark cosmos.
The galaxy, called MACS1149-JD1, was discovered in 2012, but at the time, scientists were uncertain exactly how far away it was, and by implication how long its light had been traveling toward us.
In a study published in the journal Nature, however, a team lead by Takuya Hashimoto of Osaka Sangyo University, Japan, used a giant radio telescope in Chile, called the Atacama Large Millimetre/submillimetre Array (ALMA) to identify a spectral line from oxygen in MACS1149-JD1’s emissions.
Normally, this spectral line is in the infrared part of the spectrum, but in the case of MACS1149-JD1, it has been traveling so long that the expansion of the universe has stretched it into the microwave.
Based on this, the astronomers were able to pin down the distance to the galaxy, determining that it is 13.24 billion light years away. That means that the light we see from it was emitted when the universe was only about 550 million years old.
The researchers then looked at the colour and brightness of MACS1149-JD1’s stars, making use of the same wavelength-stretching factor to determine precisely what they were seeing from these stars. From this, they were able to use astrophysical models to determine their age and maturity.
They found that many of them were about 300 million years old at the time the light left the galaxy — meaning that they had formed only 250 million years after the Big Bang.
It’s an exciting find because it is the most accurate evidence yet for pinning down when the first stars and galaxies appeared — an event that brought to an end the prolonged dark period that began with the initial cooling of the Big Bang.
Not that the find reveals exactly when the darkness ended, says study team member Nicolas LaPorte, an astronomer from University College London, UK, and Université de Toulouse in France.
“But we know the dark ages finished when the first stars and galaxies appeared, [and] we can say that galaxies were already there 250 million years after the Big Bang,” he says.
Team member Richard Ellis, also an astronomer at University College London, agrees. “Determining when cosmic dawn occurred is akin to the Holy Grail of cosmology and galaxy formation,” he says. “With these new observations we are getting closer to directly witnessing the birth of starlight.”
Other scientists are also enthused. “Being able to answer these questions is paramount to understanding our cosmic history and the formation of structure in the universe,” says Fabian Schneider, an astrophysicist from the University of Oxford who was not part of the study team.
Rychard Bouwens, of Leiden University in the Netherlands, and author of a related paper in the same issue of Nature, adds that other galaxies similar to MACS1149-JD1 are probably out there, but observing them with ALMA will be difficult because most of the ones visible from Chile are significantly fainter.
“Astronomers will likely try,” he says, “but it will be very challenging.”
But ALMA isn’t the only instrument that can be used for such studies. Such galaxies, Bouwens adds, will also be important targets for NASA’s next-generation James Webb Space Telescope, scheduled for launch in 2020.
Renske Smit, an astronomer at the Kavli Institute of Cosmology at the University of Cambridge, UK, adds that it is interesting that MACS1149-JD1 has a relatively “old” age for a galaxy seen so early in the universe — meaning that even though it dates back more than 13 billion years, the new paper finds that it had still been around for at least 300 million.
That makes it one of a handful of other ancient galaxies recently found to be similarly mature. “These discoveries might tell us that a small fraction of the galaxy population is born very shortly after the Big Bang and that they 'grow up' very quickly,” she says.
And, of course, that makes MACS1149-JD1 itself an attractive target for the Webb space telescope, once it becomes operational.
“[The Webb] will be able to put tight constraints on the previous episodes of star-formation that have taken place in this galaxy, pinning down its formation epoch and giving us an insight into the onset of star formation after the universe's dark ages,” Smit says.