Astronomers have discovered something surprising about the universe’s smallest and faintest class of galaxies: Ultra-Diffuse Galaxies (UDGs).
A research team studying these galaxies found that around half of the ones they investigated showed signs of motion that defy previous theories about the formation and evolution of such realms. In particular, the team found an unexpected rotational motion of stars within many of these dwarf galaxies.
The scientists reached these findings while studying stellar motion in 30 UDGs in the Hydra galaxy cluster located over 160 million light-years away from us. The findings could change our understanding of how UDGs form and evolve.
“The results we obtained were doubly satisfying,” Chiara Buttitta, a researcher at the National Institute for Astrophysics and co-author of a paper on these results, said in a statement. “Not only were we able to deduce the stellar motions in these extremely faint galaxies, but we found something we didn’t expect to observe.”
The team utilized the “Looking into the faintest With MUSE,” or LEWIS, observing program, conducted by the MUSE integral field spectrograph that’s installed on the Very Large Telescope (VLT). The VLT is the world’s most advanced visible-light astronomical observatory, and is situated in Chile.
The origin of faint galaxies
UDGs were first discovered in 2015; the formation and evolution of these ultrafaint, strangely elongated galaxies immediately presented a puzzle for astronomers.
The LEWIS findings allowed the new study’s team to determine that UDGs dwell in environments that greatly vary in terms of their physical properties, the amount of dark matter they contain and the motions and compositions of their stars.
Specifically, the scientists were able to conduct a detailed investigation of the UDG designated “UDG32.” This dwarf galaxy is located at the tail end of a filament of gas attached to the spiral galaxy dubbed “NGC 3314A.”
One possible theory regarding the formation of UDGs suggests they form when filaments of gas are dragged from larger galaxies via gravitational interactions.
If gas clouds remain in these filaments, these clouds can become overly dense and collapse, forming stars that become the foundation of a UDG.
The data from LEWIS confirmed that UDG32’s association with the filament tail of NGC3314A isn’t the result of a coincidental alignment. There’s something more that makes UDG32 appear to be situated at the tip of NGC3314A’s tidal tail.
Additionally, UDG32 is more enriched in elements heavier than hydrogen and helium, which astronomers collectively call “metals,” than other UDGs in the Hydra cluster.
Metals are forged by the nuclear processes occurring at the hearts of stars and are dispersed when these stars explode at the ends of their lives to become the building blocks of the next generation of stars.
This is interesting because, despite the stars in UDG32 being younger than the stars in other Hydra cluster UDGs, they are richer in metals. This suggests they formed in the pre-metal-enriched gas and dust shed by a larger and more ancient galaxy, supporting the idea that this UDG was dragged from its spiral galaxy neighbor.
The team’s results are important validation for the LEWIS project, which has thus far doubled the number of UDGs that have been analyzed spectroscopically. Additionally, LEWIS has provided the first “global” view of these faint galaxies within a galaxy cluster that is still forming.
“The LEWIS project was a challenge. When this program was accepted by ESO we realized that it was a goldmine of data to be explored. And that is what it turned out to be,” Enrichetta Iodice, the LEWIS scientific director, said in the statement.
“The ‘strength’ of LEWIS, thanks to the integral spectroscopy of the instrument used, lies in being able to study simultaneously, for each individual galaxy, not only the motions of the stars, but also the average stellar population,” Iodice added, “and, therefore, have indications on the formation age and the properties of globular clusters, fundamental tracers also for the dark matter content.
“By putting together the individual results, like in a puzzle, we reconstruct the formation history of these systems.”
The team’s research was detailed across two papers published in the journal Astronomy & Astrophysics.
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