PREVIEW

BY MARK WORGAN

Astronomers have confirmed the existence of a new class of object within the Milky Way, identifying a rare stellar survivor from the early days of our galaxy known as a "bulge fossil fragment".

The object, called Terzan 5, is believed to be a remnant of the primordial building blocks that helped form the central bulge of our galaxy billions of years ago.

Unlike similar ancient structures that merged and dispersed during the Milky Way's formation, Terzan 5 remained largely intact - hence being named as a ‘cosmic fossil’.

The findings come from a study combining observations from the NASA/ESA/CSA James Webb Space Telescope with data collected over 12 years by the NASA/ESA Hubble Space Telescope.

Researchers say the results show definitively that Terzan 5 is not a globular cluster, as previously thought, but a much rarer and more complex object.

Globular clusters typically contain a single ancient population of stars. However, astronomers found evidence that Terzan 5 underwent as many as four distinct episodes of star formation over its history.

Located within the densely populated central bulge of the Milky Way, Terzan 5 retained its separate identity while other stellar systems mixed together to form the galaxy's core.

"It's like a lump in an otherwise well-mixed cake batter," the researchers said.

"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," added Giorgia Zullo, who led the research and is a PhD student at the University of Bologna in Italy.

Terzan 5 was discovered in 1968 by astronomer Azop Terzan and initially appeared similar to a conventional globular cluster.

However, in 2009 astronomers discovered that it contained two distinct populations of stars. Follow-up observations by Hubble in 2016 suggested one population formed around 12 billion years ago, during the assembly of the Milky Way, while another emerged roughly five billion years ago.

The system's location within a crowded and dust-obscured region of the galaxy has made it difficult to study. Webb's infrared capabilities enabled astronomers to peer through the dust and observe many more stars than had previously been possible.

By combining Webb's observations with Hubble's long-term measurements of stellar motions, the team was able to distinguish stars belonging to Terzan 5 from those in the surrounding galactic bulge.

The analysis revealed evidence for two additional stellar populations, formed approximately 3.8 billion and 2.5 billion years ago. Researchers also refined the ages of the previously identified populations, determining that they formed 12.5 billion and 4.7 billion years ago.

The discovery of four separate generations of stars rules out earlier theories that Terzan 5's unusual composition resulted from interactions with another cluster or a giant molecular cloud.
Observations from the W. M. Keck Observatory in Hawaii and the European Southern Observatory's Very Large Telescope further indicated that the stellar populations possess distinct chemical signatures.

"Along with the ages of these populations, the cluster preserves a fossil record of progressive enrichment of heavy elements by supernovae," said co-author R. Michael Rich, a research astronomer at the University of California, Los Angeles.

Scientists believe Terzan 5 was massive enough to retain gas and material expelled by exploding stars, allowing new generations of stars to form over billions of years.

The researchers conclude that Terzan 5 is likely the surviving remnant of a much larger stellar system that first formed around 12.5 billion years ago.

Its significance lies in the fact that it survived the formation of the Milky Way's bulge without being fully absorbed into it.

"For some reason, this peculiar clump of stars formed separately from the bulge and was not destroyed as the bulge itself formed," said Francesco R. Ferraro, a professor at the University of Bologna and principal investigator of the Webb observations.

"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."

Only one other known object, Liller 1, has so far been reclassified as a bulge fossil fragment. Like Terzan 5, it contains multiple generations of stars.

Researchers now plan to examine between 40 and 50 additional globular clusters within the Milky Way's bulge to determine whether more examples exist.

Scientists say the discovery could shed light on how galactic bulges form across the Universe.

"Based on observations and in-depth simulations, we think that galaxies in the early Universe had huge disks of gas that fragmented into clumps and formed stars. These clumps migrated to the centre of the galaxies, and many merged to form their bulges," said Barbara Lanzoni, a co-author and associate professor at the University of Bologna.

Recent observations by the James Webb Space Telescope have revealed several young, "clumpy" galaxies dating back to the Universe's earliest epochs, including structures seen in the Firefly Sparkle galaxy.

"Terzan 5 may provide direct evidence that can help explain how bulges formed in galaxies throughout the Universe," Lanzoni said.

The findings were presented at the 248th meeting of the American Astronomical Society and published in the journal Astronomy & Astrophysics.