Over the course of billions of years, the universe has steadily been evolving.
Thanks to the expansion of the universe, we are able to “see” back in time to watch that evolution, almost from the beginning.
But every once in a while we see something that doesn’t fit into our current understanding of how the universe should operate.
That’s the case for a galaxy described in a new paper by PhD student Sijia Cai of Tsinghua University’s Department of Astronomy and their colleagues.
They found a galaxy formed around 11 billion years ago that appears to be “metal-free”, indicating that it might contain a set of elusive first generation (Pop III) stars.
Before we get into the discovery itself, some context is necessary. Population III (Pop III) stars are considered to be the first generation of stars that formed early in the universe’s history.
Importantly, they have essentially no “metal”, which cosmological terms means any element other than helium and hydrogen.
Since those heavier elements can only be formed in stars themselves (or in the supernovae they create), by definition the first generation of stars can’t contain them.
Cosmologists have been searching for examples of these Pop III stars for decades, but so far have ben unable to find them.
Typically, they search a time of the universe’s history known as the Epoch of Reionization, which took place up to 1 billion years after the Big Bang, when the universe was very young and we believe the first stars themselves were starting to form.
So imagine the author’s surprise when they found a galaxy that appeared about 2 billion years later than the Epoch of Reionization.
By that point plenty of stars should have lived and died, with their remnants “infecting” any nearby gas and dust clouds, or other stars themselves, with the metal they created. That was the theory at least.
But, using data gathered by the James Webb Space Telescope (JWST), the Very Large Telescope (VLT), and the Subaru Telescope, the authors identified a galaxy they called MPG-CR3 (or CR3 for short). The spectral signature of this galaxy was unique amongst all other galaxies of that era.
It had very clean hydrogen and helium lines, and, notably, almost a complete lack of “metals” like oxygen in its spectral signature. In fact, the upper limit of the metallicity of the stars in the galaxy puts them at .7% of the metallicity of our Sun.
Even more interestingly, the galaxy itself only appears to be about 2 million years old – making it relatively young by galactic standards. We are able to see it at such a young age, despite it being formed billions of years ago, because of the expansion of space-time.
CR3 also appears to be relatively “dust-free”, and have relatively small stars, especially for such an ancient galaxy. Most galaxies during Cosmic Noon have supermassive stars compared to our own.
There is an important feature that is missing from the data for CR3 that is typically considered a key part of the detection of any Pop III stars – the Helium II (He II) emission line. While this critical line isn’t visible in the VLT spectral data that would otherwise be able to detect it, the authors offer up two explanations for why.
First, there’s already a strong “OH” emission line coming from another source in that part of the data, which cancels out the He II signal. Alternatively, the He II signal itself could have petered out, as its amplitude drops significantly only a few million years after star formation.
So the question remains, if the early generation of stars were expected to form billions of years earlier, how did this one particular galaxy evade the “pollution” of the metals from those stars formed billions of years earlier and end up with “pristine” stars so late in the universe’s evolution.
The authors believe it has to do with spacing. Essentially CR3 is sitting in an empty pocket in space. In technical terms this is called an “underdense region”, and the logic goes that, by the time the gas cloud that CR3 was previously made out of collapsed and started forming stars, no metals from surrounding areas with active stars had yet reached it.
CR3 was off on its own, far away from its neighbors, and its isolation helped it form its own first generation of stars, separately from what was going on in the rest of the universe.
Further data is still needed to confirm this as the first Pop III galaxy ever discovered.
But if it is confirmed, it would mark a major boon to scientists, as having a galaxy full of these interesting stars so much closer than they were originally expected to be found would make them much easier to study.
If further research can confirm the He II line, or provide a fool-proof explanation of why it’s not noticeable – cosmologists will be spending a lot of time looking at this young, yet also really old, galaxy.
Written by Andy Tomaswick/Universe Today.
