30 Nov2023

Astronomers have discovered an extremely large and faint stream of stars in the Coma galaxy cluster. This is the largest stellar stream detected to date and the first of its kind found in a galaxy cluster. This discovery, published in the journal Astronomy & Astrophysics, has been possible thanks to observations made using the William Herschel Telescope (WHT).

The Coma Cluster is a large galaxy cluster comprising thousands of galaxies located at a distance of about 300 million light years from Earth, in the direction of the northern constellation of Coma Berenice. It is one of the best studied clusters since some of its brightest galaxies can be observed using relatively small telescopes.

A team of astronomers, led by the University of Groningen in The Netherlands and the University of La Laguna in Spain, has being conducting detailed observations in a region of the Coma Cluster using the PF-QHY camera on the WHT. After processing the images, the team found an extremely faint stream of stars more than 10 times the length of the Milky Way, located in the intergalactic medium and not associated with any particular galaxy.

"Initially, our objective was to carry out a study of haloes in the Coma galaxies. Surprisingly, the data revealed an astonishing giant stream of stars, until now unknown," explains Javier Román, researcher at the University of Groningen and the Universidad de La Laguna, who led this study. “Subsequent observations using the WHT allowed us to confirm that this structure is real and to characterise its extreme properties in detail".

According to the scientists, who have named this object the 'Giant Coma Stream', it is an unusual finding since the observations show an extremely fragile structure in the middle of a hostile environment of galaxies that attract and interact with each other.

Román adds "this type of extremely faint stellar stream with fine morphology has been detected so far only in environments of the Local Group, that is, in our Galaxy and in nearby galaxies, but it is the first time that something like this has been detected in a galaxy cluster, and it is the largest stream detected to date. Our study also suggests, after analysing cosmological simulations, that this type of structure could be relatively frequent in galaxy clusters".

In the coming years, researchers expect to discover new giant streams thanks to the new generation of telescopes. "We hope to find more of this type of object with the 39-metre Extremely Large Telescope (ELT) and when the Euclid space mission begins to provide data", says Reynier Peletier, a researcher at the University of Groningen and co-author of this study.

It is also a priority for this team of astronomers to continue studying this particular giant stream of stars located in the Coma cluster, one of the first places where the observed gravitational anomalies were considered to be indicative of the presence of a non-visible mass called dark matter.

"It is difficult not to be amazed by this newly discovered structure; it is so enormous that literally if we could put it between our galaxy and the Andromeda galaxy, it would bridge both galaxies", highlights Ignacio Trujillo, an IAC researcher and co-author. "Such a structure provides a unique opportunity to study whether dark matter is grouped into haloes, as predicted by the theory of cold dark matter, since such haloes, if they exist, would generate gaps when passing through the stream of stars. At the moment we have not seen them, but it is the next objective to investigate", he concludes.

[Image]

Images obtained using the Jeanne Rich Telescope (HERON Survey) and the WHT show the so-called 'Giant Coma Stream', a stellar stream (dark line) ten times bigger than the Milky Way. Galaxies are shown in colour.