A huge, shining bauble of stars called Terzan 5 could be a clump of our galaxy’s central bulge that hasn’t been smoothed out into the mix, and has instead survived as a fossil relic leftover from the birth of the Milky Way galaxy.
“Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the universe,” said Barbara Lanzoni of the University of Bologna in a statement. Lanzoni is a member of a team of astronomers, led by Bologna colleagues Giorgia Zullo and Francesco Ferraro, who tackled Terzan 5 with the James Webb Space Telescope (JWST).
Terzan 5 is a globular cluster — a huge sphere of stars with a total mass two million times greater than our sun’s and a total luminosity 800,000 times greater. The problem is, Terzan 5 lies about 18,800 light-years away in the bulge of the Milky Way galaxy. This means dense lanes of intervening galactic dust block our view, significantly dimming Terzan 5’s apparent brightness. That’s why it wasn’t discovered until 1968 by the Turkish–French–Armenian astronomer Agop Terzan.
Globular clusters tend to be ancient. They also tend to have formed all their stars in one giant burst. As such, all their stars should be the same age, 12 to 13 billion years old. Yet, a select few globular clusters show evidence of having more than one generation of stars. These include Omega Centauri, NGC 2808 and NGC 1783 in the Milky Way galaxy, as well as NGC 411 in the Small Magellanic Cloud and NGC 1696 in the Large Magellanic Cloud. Several explanations have been put forward, including the possibility that they are the core remnants of dwarf galaxies that have been stripped of most of their stars by gravitational tidal forces emanating from the Milky Way. Or perhaps these clusters were simply massive enough to retain some molecular gas for future stellar generations.
When the Hubble Space Telescope took a look at Terzan 5 in 2009 and then again in 2016, it found that it too was among the ranks of weird globular clusters with two generations of stars, dating back 12.5 and 4.7 billion years. However, because it is behind so much galactic dust, not even Hubble has the clearest of views.
The JWST, however, does. Its near-infrared vision can see through the dust.
“Webb’s new near-infrared observations, cross-referenced with Hubble’s archival observations, have given us a much clearer picture of the history of Terzan 5,” said study leader Giorgia Zullo, who is a Ph.D. student at Bologna.
The JWST detected two further generations of stars, one generation born 3.8 billion years ago and another 2.5 billion years ago. Four generations of stars is hard to explain for any globular cluster, which is why the team think that Terzan 5 could be something more primordial: a leftover building block of the Milky Way’s bulge that was never quite assimilated by our galaxy.
“For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed,” said Ferraro. “Terzan 5 is what we now call a bulge fossil fragment because it resembles the primordial clumps that contributed to the formation of the bulge.”
Disk galaxies sport two main components: a relatively narrow disk formed from spiral arms, and a bulbous core called the bulge. Galactic bulges tend to be the oldest parts of galaxies, forming billions of years before the disks, at least in the Milky Way’s case. The JWST is seeing this process occurring in the early universe, revealing clumpy, young galaxies, but given the great expanse of space and time that JWST is looking across, the observations of the building blocks that go into making these galaxies are still not totally clear. With Terzan 5, we could be looking at one of the building blocks of the Milky Way’s bulge relatively close-up, and it could provide new insights into the birth of our galaxy.
Terzan 5 is probably not the only bulge fossil fragment either. As well as those other aforementioned globular clusters, some of which might be fossil fragments and others might be the cores of dwarf galaxies, the globular cluster Liller 1 close to the center of our galaxy shares many of Terzan 5’s properties, including its high abundance of heavy elements produced by multiple generations of stars that have died either in supernova explosions.
The team are now looking to chase up another 40 or 50 globular clusters in the bulge to see if they could also be bulge fossil fragments, or whether they are just regular globular clusters.
The findings were presented at the 248th meeting of the American Astronomical Society in Pasadena, California which took place between June 14 and June 18. A paper describing the JWST observations has also been published in the journal Astronomy & Astrophysics.