A fossilised remnant of the early Milky Way harbouring stars of hugely different ages has been discovered by an international team of astronomers. This stellar system located in the Galactic Bulge has the appearance of a globular cluster, but it is like no other cluster known. It contains stars remarkably similar to the most ancient stars in the Milky Way but also a significant population of young stars, thus bridging the gap in understanding between our galaxy’s past and its present. The research presents a possible route for astronomers to unravel the mysteries of galaxy formation, and offers an unrivalled view into the complicated history of the Milky Way.
The system, called Terzan 5, has been classified as a globular cluster since its discovery 40 years ago. Now, an Italian-led team of astronomers has discovered that Terzan 5, which is 19 000 light years from Earth, is like no other globular cluster known.
To make the discovery, the team scoured data from the Advanced Camera for Surveys and Wide Field Camera 3 on board Hubble; the second generation Near Infrared Camera (NIRC2) on the W. M. Keck Observatory located on Mauna Kea, Hawaii; and the Multi-conjugate Adaptive Optics Demonstrator at ESO’s Very Large Telescope in Chile. The team found compelling evidence there are two distinct kinds of stars in Terzan 5, which not only vary in the elements they contain, but have an age-gap of roughly 7 billion years.
“The finding was so surprising, we decided to double check it by using the 10-metre telescope at Keck Observatory and ESO’s Very Large Telescope,” says Francesco Ferraro from the University of Bologna, Italy. “Thus in August 2010, thanks to the long-standing collaboration with Prof. Rich from UCLA, we secured ultra-deep K-band images of another portion of the cluster. These data have been crucial to solidly determine the age of the two stellar populations.”
“Back in 2009 we discovered that Terzan 5 harboured two sub-populations of stars with different chemical abundances: after seven years of research we finally succeeded in dating these populations,” he says.
The ages of the two populations indicate that the star formation process in Terzan 5 was not continuous, but 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 and to be quite massive. At least 100 million times the mass of the Sun,” says Davide Massarii from INAF, Italy.
Its unusual properties make Terzan 5 the ideal candidate for a living fossil from the early days of the Milky Way: current theories on the bulge galaxy formation assume that vast clumps of gas and stars interacted to form the primordial bulge of the Milky Way, merging and dissolving in the process.
“We think that some remnants of these gaseous clumps could remain relatively undisrupted and keep existing embedded within the galaxy,” Ferraro says. “Such galactic fossils allow astronomers to reconstruct an important piece of 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 fossilized relic of galaxy formation, representing one of the earliest building blocks of the bulge 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.
“Indeed a few characteristics of Terzan 5 resemble those detected in the giant clumps observed in star-forming galaxies at high-redshift, thus suggesting that similar assembling processes occurred in the local and in the distant Universe at the epoch of the galaxy formation,” Ferraro says.
Somehow Terzan 5 has managed to survive being disrupted for billions of years, and has been preserved as a remnant of the distant past of the Milky Way.
This discovery paves the way for a better and more complete understanding of galaxy assembly.
“Terzan 5 could represent an intriguing link between the local and the distant Universe, a survived witness of the Galactic Bulge assembly process,” Ferraro says.
“The discovery raises a number of intriguing question,” says R. Michael Rich from the University of California, Los Angeles, who coordinated the observations at Keck Observatory. “How did Terzan 5 survive the destruction? Which event triggered its second burst of star formation? We are currently searching for the answers to these questions.”
The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.
NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II telescope’s adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the centre of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.