The Euclid team has released a beautiful new image of NGC 6397, one of the closest globular clusters to Earth. NGC 6397, or GCl 74, is located 7,800 light-years away in the southern constellation of Ara. Because of its very dense nucleus, it is known as a core-collapsed cluster. NGC 6397 is 13.4 billion years old and so formed not long after the Big Bang.
This Euclid image shows NGC 6397, a globular cluster some 7,800 light-years away in the constellation of Ara. Image credit: ESA / NASA / Euclid / Euclid Consortium / J.-C. Cuillandre, CEA Paris-Saclay / G. Anselmi / CC BY-SA 3.0 IGO.
Globular clusters are systems of very ancient stars, gravitationally bound into a single structure about 100-200 light-years across.
They contain hundreds of thousands or perhaps a million stars. The large mass in the rich stellar center of a cluster pulls the stars inward to form a ball of stars.
Globular clusters are among the oldest known objects in the Universe and are relics of the first epochs of galaxy formation.
Of the 150 globular clusters belonging to our Milky Way Galaxy, about 70 lie within 13,000 light-years from the Galactic center where their density tends to peak.
“The challenge is that it is typically difficult to observe an entire globular cluster in just one sitting,” Euclid Consortium scientists said.
“Their centers contain lots of stars, so many that the brightest ‘drown out’ the fainter ones.”
“Their outer regions extend a long way out and contain mostly low-mass, faint stars. It is the faint stars that can tell us about previous interactions with the Milky Way.”
“Currently no other telescope than Euclid can observe the entire globular cluster and at the same time distinguish its faint stellar members in the outer regions from other cosmic sources,” added Euclid Consortium scientist Davide Massari, an astronomer at the National Institute for Astrophysics.
“For example, the NASA/ESA Hubble Space Telescope has observed the core of NGC 6397 in detail, but it would take a lot of observing time with Hubble to map the outskirts of the cluster, something Euclid can do in just one hour.”
“ESA’s Gaia mission can track the movement of globular clusters, but can’t tell what’s going on with very faint stars.”
“And telescopes from the ground can cover a larger field, but with a poorer depth and resolution, so they can’t distinguish the faint outskirts entirely.”
Dr. Massari and his colleagues will use Euclid to search for ‘tidal tails’ in globular clusters: a tidal tail is a trail of stars that extends far beyond the cluster because of a previous interaction with a galaxy.
“We expect all of the globular clusters in the Milky Way to have them, but so far we have only seen them around just a few,” Dr. Massari said.
“If there are no tidal tails, then there could be a dark matter halo around the globular cluster, preventing the outer stars from escaping.”
“But we don’t expect dark matter haloes around smaller-scale objects like globular clusters, only around bigger structures like dwarf galaxies or the Milky Way itself.”
If the astronomers find tidal tails for NGC 6397 and other globular clusters in the Milky Way, that would allow them to very precisely calculate how the clusters orbit our Galaxy.
“And this will tell us how dark matter is distributed in the Milky Way,” Dr. Massari said.
“With Euclid’s observations, we also want to determine the age of globular clusters, to investigate the chemical properties of their stellar populations, and to study ultra-cool dwarf stars — the lowest mass members of the cluster.”
