Astronomers discover rare fossil relic of early Milky Way

Terzan 5, 19 000 light-years from Earth in the constellation of Sagittarius (the Archer) in the direction of the galactic centre, has been classified as a globular cluster for the forty-odd years since its detection. Now, an Italian-led team of astronomers have discovered that Terzan 5 is like no other globular cluster known. The team scoured data from the Multi-conjugate Adaptive Optics Demonstrator [1], installed at the Very Large Telescope, as well as from a suite of other ground-based space telescopes [2]. They found compelling evidence that there are two distinct kinds of stars in Terzan 5 which not only differ in the elements they contain, but have an age-gap of roughly 7 billion years [3].

The ages of the two populations indicate that the star formation process in Terzan 5 was not continuous, but was dominated by two distinct bursts of star formation. “This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars to be quite massive. At least 100 million times the mass of the Sun,” explains Davide Massari, co-author of the study, from INAF, Italy, the University of Groningen, Netherlands.

Its unusual properties make Terzan 5 the ideal candidate for a living fossil from the early days of the Milky Way. Current theories on galaxy formation assume that vast clumps of gas stars interacted to form the primordial bulge of the Milky Way, merging dissolving in the process.

“We think that some remnants of these gaseous clumps could remain relatively undisrupted keep existing embedded within the galaxy,” explains Francesco Ferraro from the University of Bologna, Italy, lead author of the study. “Such galactic fossils allow astronomers to reconstruct an important piece of the history of our Milky Way.”

While the properties of Terzan 5 are uncommon for a globular cluster, they are very similar to the stellar population which can be found in the galactic bulge, the tightly packed central region of the Milky Way. These similarities could make Terzan 5 a fossilised relic of galaxy formation, representing one of the earliest building blocks of the Milky Way.

This assumption is strengthened by the original mass of Terzan 5 necessary to create two stellar populations: a mass similar to the huge clumps which are assumed to have formed the bulge during galaxy assembly around 12 billion years ago. Somehow Terzan 5 has managed to survive being disrupted for billions of years, has been preserved as a remnant of the distant past of the Milky Way.

“Some characteristics of Terzan 5 resemble those detected in the giant clumps we see in star-forming galaxies at high-redshift, suggesting that similar assembling processes occurred in the local in the distant Universe at the epoch of galaxy formation,” continues Ferraro.

Hence, this discovery paves the way for a better more complete understanding of galaxy assembly. “Terzan 5 could represent an intriguing link between the local the distant Universe, a surviving witness of the Galactic bulge assembly process,” explains Ferraro while commenting on the importance of the discovery. The research presents a possible route for astronomers to unravel the mysteries of galaxy formation, offers an unrivaled view into the complicated history of the Milky Way.

Source: ESO