"Galactic fossil" could connect Milky Way's past and present

The Terzan 5 stellar system might be a kind of cosmic fossil from the Milky Way's past(Credit: Hubble Space Telescope)

Using data from the Hubble Space Telescope's Advanced Camera for Surveys and Wide Field Camera 3, a team of scientists has discovered what might be an ancient fossilized remnant of the Milky Way's early years that could connect the past and present of our galaxy's formation.

The fossil comes in the form of a stellar system called Terzan 5 located 19,000 light-years away from the Earth in the Sagittarius constellation. In the 40 years since its discovery, scientists have regarded this system as a globular cluster – a tightly packed grouping of ancient stars – like any other. However, the new findings reveal that it is different from all currently known globular clusters due to the presence of two distinct kinds of stars in the system with unique characteristics.

With different elemental compositions and an age difference of approximately seven billion years, these two star types suggest that Terzan 5 did not experience a continuous formation process like other clusters, and instead grew over time through two major star formation events.

"This requires the Terzan 5 ancestor to have large amounts of gas for a second generation of stars and to be quite massive," says Davide Massari of the University of Gröningen and co-author of the study. "At least 100 million times the mass of the Sun."

That mass has the researchers thinking that Terzan 5 is leftover from the formation of the Milky Way, as our galaxy's central bulge also formed from the interactions of large masses of gas and clumps of stars. It's a though a chunk of the cosmic cloud that formed the Milky Way's bulge tore off and remained in tact after 12 billion years.

If Terzan 5 is indeed a relic of the Milky Way's past, its gaseous clumps could be examined in order to get a clearer picture of our galaxy's earliest years and determine how it changed and evolved into the spiral home it is today.

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

In addition to its implications for the history of the Milky Way, Terzan 5 also possesses similarities to distant galaxies, which could be used to shed light on the differences between local and distant galaxy formation throughout the history of the universe.

"Terzan 5 could represent an intriguing link between the local and the distant universe, a surviving witness of the galactic bulge assembly process," adds Ferraro.

The findings are set to be released in The Astrophysical Journal.

Watch Hubble zoom in on the Terzan 5 stellar system below.
Video credit: Nick Risinger (skysurvey.org)/DSS/Hubble

Source: Hubble Space Telescope

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