Euclid captured a panoramic image of the globular cluster NGC 6397, which is located in the disk of the Milky Way and contains stars that provide insight into the history of the galaxy. Observing the entire cluster, especially the faint stars in its outer regions, has been a challenge for existing telescopes. However, Euclid’s capabilities allow it to distinguish between these faint stars, enabling the search for “tidal tails” that can indicate past interactions and help map the dark matter within the Milky Way. Image source: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Colander (CEA Paris-Saclay), c. Anselmi, CC BY-SA 3.0 IGO
Euclid’s detailed imaging of globular cluster NGC 6397 may reveal tidal tails and provide new insights into the role of dark matter in… milky wayIn addition to the evolution of one of the oldest stellar structures in the galaxy.
Capturing the brightness of NGC 6397
This bright image shows a Euclid view of a globular cluster called NGC 6397. Globular clusters are groups of hundreds of thousands of stars held together by gravity.
Located about 7,800 light-years from Earth, NGC 6397 is the second closest globular cluster to us. It rotates along with other globular clusters in Milky Way discWhere the majority of stars are located.
Uncover history through the stars
Globular clusters are among the oldest objects in the universe. That’s why they contain a lot of clues about the history and evolution of their host galaxies, such as those of the Milky Way.
The challenge is that it is usually difficult to observe an entire spherical mass in just one session. Their centers contain so many stars that the brightest ones “obliterate” the fainter ones. Its outer regions extend far and contain mostly faint, low-mass stars. It is the faint stars that can tell us about past interactions with the Milky Way.
Euclid’s unique abilities
“Currently, there is no telescope other than Euclid that can observe the entire globular cluster and at the same time distinguish its faint stellar members in the outer regions from other cosmic sources,” explains Euclid Consortium scientist Davide Massari of the National Institute of Astrophysics in Italy.
This ancient stellar jewel box, a globular cluster called NGC 6397, sparkles with light from hundreds of thousands of stars. Astronomers used NASA and the European Space Agency’s Hubble Space Telescope to measure the cluster’s distance at 7,800 light-years away. Image source: NASA, ESA, T. Brown and S. Casertano (STScI), Acknowledgments: NASA, ESA, and J. Anderson (STScI)
For example, Hubble Space Telescope The core of NGC 6397 has been observed in detail (see image above), but it would take a lot of observing time with Hubble to map the cluster’s outskirts, something Euclid can do in just an hour. The European Space Agency’s Gaia mission can track the movement of globular clusters, but it cannot know what is happening with very faint stars. Ground-based telescopes can cover a larger field, but with less depth and resolution, so they can’t quite distinguish the fainter fringes.
Search for tidal tails
David and his colleagues will use Euclid to look for “tidal tails” in globular clusters: a tidal tail is a trail of stars extending beyond the cluster due to a previous interaction with a galaxy.
This cropped image of Euclid’s full view of NGC 6397 is at high resolution of the VIS instrument. This is nine times better than the NISP definition chosen for full width; This was done for the practical reason of limiting the full image format to a manageable size for downloading. The clip fully demonstrates Euclid’s power to obtain extremely sharp images over a large area of sky in a single signal. Although this image represents only a small portion of the full color display, the same quality shown here is available in full field. Image source: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Colander (CEA Paris-Saclay), c. Anselmi, CC BY-SA 3.0 IGO
“We expect all globular clusters in the Milky Way to have them, but so far we’ve only seen them in a few,” says David. “If there were no tidal tails, there could be a dark matter halo around the globular cluster, preventing outer stars from escaping. But we don’t expect dark matter halos to appear around smaller objects like globular clusters, only around larger structures.” Such as dwarf galaxies or the Milky Way itself.
A new understanding of stellar evolution
If David and his team find tidal tails for NGC 6397 and other globular clusters in the Milky Way, that will allow them to make a very precise calculation of how the clusters orbit our galaxy. “This will tell us how dark matter is distributed in the Milky Way,” David adds.
From Euclid’s observations, the team also wants to determine the age of globular clusters, investigate the chemical properties of their star clusters, and study ultracool dwarf stars – the least massive members of the cluster.
See more of Euclid’s first pictures.
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