Astronomers have discovered the remains of an ancient family of stars that was ripped apart by our galaxy, the Milky Way, some two billion years ago.
These elder stars once occupied a globular cluster, a spherical formation of stars, until they were flung into the Milky Way’s halo some 60,000 light years from Earth, forming a stellar river called the Phoenix Stream. This bygone globular cluster, known as the Phoenix progenitor, “apparently occupies a special position, which is distinctly different from the present-day globular cluster population observed in the local Universe,” according to a study published on Wednesday in Nature.
What sets the Phoenix Stream apart is its exceptionally low metal content—”metals” meaning any element heavier than helium or hydrogen—which is far below the so-called “metallicity floor” that scientists thought was necessary to form globular clusters. The discovery of this extreme outlier upends our ideas about how these rounded structures form, which in turn has implications for our understanding of galaxy formation and evolution in general.
The Phoenix progenitor was clearly born a long time ago, in an environment bereft of heavy elements, but it’s not clear how it survived the Milky Way’s tidal forces for so long. All we know is that about two billion years ago, our galaxy got a gravitational grip on this cluster and chucked its stars into the galactic halo, hinting that other weird remnants may be lurking out there, too.
“To be involved in such an exciting discovery is amazing,” said lead author Zhen Wan, a PhD student at the University of Sydney in Australia, in an email. “It took a lot of work, but realizing that the Phoenix Stream comes from a globular cluster is very surprising.
“The metallicity of Phoenix, its chemical enrichment, is significantly lower than any known globular cluster in the Milky Way,” he added. “This was a surprise as all known globular clusters seem to have been more chemically enriched than Phoenix. The big question is why!”
One important clue is the chemical evolution of stars over the 13.8-billion year history of the cosmos. The first stars in the universe contained virtually no metals, but each new stellar generation forges heavy elements such as carbon, oxygen, nickel, iron, and gold, which get incorporated into the next batch of baby stars. In this way, stars tend to get more chemically complex and metal-rich over time, and their metallicity reflects the properties of the gas from which they were born.
Enter: The Phoenix Stream. This ribbon of stars was discovered in 2016 by the Southern Stellar Stream Spectroscopic Survey Collaboration at the Anglo-Australian Telescope, a project dedicated to finding and mapping star streams in the Milky Way’s halo.
Wan’s team was able to calculate the metal content of the Phoenix stars from the patterns in their light. That led to the unexpected finding that the chemical (metal) abundances of the stream were under 0.3 to 0.4 percent of the Sun, which is the lower limit of what scientists expect to see in the globular cluster stars.
“Our team was absolutely excited when we first found the metallicity of Phoenix Stream was below the metallicity floor,” said Ting Li, an astrophysicist at Carnegie Observatories who co-authored the study, in an email. “I could not believe it at the beginning and thought we did something wrong, but several people checked this independently and we finally were confident that it was real.”
But as titillating as the discovery was, Li also pointed out that “the metallicities of stellar streams have not been studied much in the past due to lack of observations” which is why the team hopes to find more of these weird metal-poor streams in the future.
“I think we need efforts from two ends,” Li explained. “One is on the observational side: we should check if there are more globular clusters in our Milky Way (or disrupted globular clusters like the Phoenix Stream) that show such extremely low metallicities.”
“The other is on the theoretical models on the globular cluster formation, that theorists can improve their model to explain this discovery and also predict how odd such systems exist,” she added.
To that point, sophisticated observatories such as NASA’s James Webb Space Telescope or the European Space Agency’s Gaia spacecraft will be able to track down more of these metal-poor cluster graveyards.
“It is clear that the progenitor of the Phoenix Stream is different,” said Wan. “It was born in a place, or time, different to all of the other globular clusters, and so is telling us something important about the formation of galaxies.”
“With enough data on those ancient structures, we will be able to comprehensively understand the properties of the globular clusters and the host galaxies in the early universe, and then reveal the origin of the progenitor of Phoenix Stream,” he concluded.