Star Cluster Terzan 5

Star Cluster Terzan 5.

Astronomers using ESO’s Very Large Telescope, the NASA/ESA Hubble Space Telescope and the Keck Telescope, have revealed the unusual mix of stars in the star cluster known as Terzan 5. This object resembles a globular cluster, but is like no other cluster known, and is in fact one of the Galactic bulge’s primordial building blocks, most likely the relic of the early Milky Way.

Terzan 5, also known as ESO 520-27 and 2MASX J17480455-2446441, is a star cluster located approximately 19,000 light-years away in the constellation of Sagittarius. This grouping of very old stars has been classified as a globular cluster for the forty-odd years since its detection. Now, an international team of astronomers led by Dr. Francesco Ferraro from the University of Bologna, Italy, has discovered that Terzan 5 is like no other globular cluster known. The team 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 7.5 billion years. The ages of the two populations – 12 billion and 4.5 billion years – indicate that the star formation process in this cluster 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 and to be quite massive — at least 100 million times the mass of the Sun,” said team member Dr. Davide Massari, from INAF in Italy and the University of Groningen in the Netherlands.

Current theories on 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,” Dr. Ferraro said. “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 central region of the Milky Way Galaxy. These similarities could make Terzan 5 a fossilized 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, and has been preserved as a remnant of the distant past of our Galaxy. “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 and in the distant Universe at the epoch of galaxy formation,” Dr. Ferraro said. Hence, this discovery paves the way for a better and more complete understanding of galaxy assembly.

The team’s findings were published in the current issue of the Astrophysical Journal.

Credit: Sci News 7 September 2016