According to predominant theories of galaxy formation, the earliest galaxies in the Universe were born from the merger of globular clusters, which were in turn created by the first stars coming together. Today, these spherical clusters of stars are found orbiting around the a galactic core of every observable galaxy and are a boon for astronomers seeking to study galaxy formation and some of the oldest stars in the Universe.
Interestingly enough, it appears that some of these globular clusters may not have survived the merger process. According to a new study by an international team of astronomers, a cluster was torn apart by our very own galaxy about two billion years ago. This is evidenced by the presence of a metal-poor debris ring that they observed wrapped around the entire Milky Way, a remnant from this ancient collision.
The study, which recently appeared in the journal Nature, was led by Zhen Wan and Geraint Lewis (a Ph.D. astrophysics student and his professor at the University of Sydney, respectively) and included members from the Macquarie University Research Centre for Astronomy, the Observatories of the Carnegie Institution for Science, the ASTRO 3D center, the McWilliams Center for Cosmology, and multiple universities.
Their study was part of the Southern Stellar Stream Spectroscopic Survey (S5), an international collaboration dedicated to observing stellar streams in the Milky Way. Using the Anglo-Australian Telescope at the Siding Spring Observatory in New South Wales, Australia, the collaboration measured the speeds of the Phoenix Stream (a stream of stars in the Phoenix constellation) that appeared to be the remnants of a globular cluster.
“Once we knew which stars belonged to the stream, we measured their abundance of elements heavier than hydrogen and helium; something astronomers refer to as metallicity,” explained Wan in a recent Lowell Observatory press release.
To break it down, the oldest stars in the Universe are metal-poor because heavier elements – like calcium, oxygen, phosphorous, iron, etc – did not exist in abundance. Unlike hydrogen and helium (which were extremely plentiful in the early Universe) these elements formed in the interiors of stars sand were dispersed only after the earliest generation of stars collapsed and dispersed these elements when they exploded in supernovae.
In this respect, astronomers are able to discern the age of stars based on how metal-rich they are. Previous observations of globular clusters have found that their stars are enriched with heavier elements, which they obtained from previous generations of stars. As a result, astronomers have established a “metallicity floor” for globular clusters, a value that none of them can theoretically fall below.
However, the S5 collaboration noted that the metallicity of the Phoenix Stream (specifically, its iron-to-hydrogen content) sits well below this floor. In short, the Phoenix Stream represents the debris of the most metal-poor globular clusters discovered to date, making it distinct from the roughly 150 globular clusters that form a tenuous halo envelop the Milky Way today.
As Lowell Observatory astronomer Kyler Kuehn, one of the founders of the S5 collaboration and a co-author of the article, remarked:
“We can trace the lineage of stars by measuring the different types of chemical elements we detect in them, much like we can trace a person’s connection to their ancestors through their DNA. The most interesting thing about the remains of this cluster is that its stars have much lower abundance of these elements than any others we have seen. It’s almost like finding someone with DNA that doesn’t match any other person, living or dead. That leads to some very interesting questions about the cluster’s history that we’re missing.”
“We were really surprised to find that the Phoenix Stream is distinctly different to all of the other globular clusters in the Milky Way,” added Wan. “Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early universe.”
In short, the Phoenix Stream’s very existence indicates the existence of globular clusters that were below the metallicity floor. As to why none have been discovered so far, the answer may lie in the debris disk itself: they were destroyed in the early Universe as they were still merging with galaxies and galaxies merged with each other.
Of course, this is not yet a conclusive explanation for the origins of the Phoenix Stream’s progenitor cluster or where it sits in the evolutionary timeline of galaxies. What’s needed at this point is more observations and more evidence gathering to see if other progenitor clusters show the same levels of low metallicity.
“There is plenty of theoretical work left to do,” said co-author Geraint Lewis of the University of Sydney (and a co-author on the study). “There are now many new questions for us to explore about how galaxies and globular clusters form, which is incredibly exciting.”
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