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Ultra-diffuse galaxies (UDG) are a mystery to astronomers.
They are smaller galaxies in terms of star count, but they are still scattered over great distances, making them faint and difficult to spot. It is not known how they form or if there is something special about the dark matter halos that helps them form.
Recently published research may be able to answer some outstanding questions about UDGs and, in particular, “extinct” UDGs – those that do not form new stars. Thanks to a series of simulations, astronomers have been able to locate and analyze new galaxies corresponding to this description.
Observations and modeling revealed that these extinct UDGs were born in what is called a backsplash orbit, well beyond the edges of a host galaxy but still weakly connected. In other words, they were part of a larger system before isolating themselves and sharing certain characteristics with that original system.
“What we have detected is at odds with the theories of galaxy formation, as extinct dwarfs must be in clusters or group environments in order to remove their gas and stop forming stars,” explains the astronomer Laura Sales of the University of California, Riverside.
“But the soaked UDGs that we detected are isolated. We were able to identify a few of these soaked UDGs in the field and trace their evolution over time to show that they came from backsplash orbits.”
The simulation used by the team, called TNG50, successfully predicted UDG systems similar to those observed. It could also act like a time machine, moving these galaxies back to see where they came from billions of years ago.
TNG50 also suggested that the percentage of extinct UDGs in an ultra-diffuse population of galaxies could be as high as 25 percent, far more than previously thought based on field observations. This could mean that there are still many of these galaxies to be detected by our telescopes.
A variety of internal processes and external forces have already been explored as possible reasons for the existence of UDGs, but so far there has been no single explanation that explains all of these galaxies.
(Vanina Rodriguez)
Above: The fall of a blue ultra-diffuse galaxy into a galactic system and its subsequent ejection as a red ultra-diffuse galaxy.
“One of the popular theories to explain this was that UDGs are failed Milky Ways, meaning they were meant to be galaxies like our own Milky Way but somehow failed to form stars. “, explains astronomer José Benavides, of the Institute for Theoretical and Experimental Astronomy. in Argentina.
“We now know that this scenario cannot explain all UDGs. Thus, theoretical models appear where more than one formation mechanism may be able to form these ultra-diffuse objects.”
UDGs are like dwarf galaxies in terms of the number of stars they contain – just a fraction of the 200 billion to 400 billion Milky Way – and yet they are comparable in size to the Milky Way. The stars are spread out much finer.
UDG’s dark matter halos break normal rules. Where they could have had the same concentration of dark matter as a dwarf garden variety galaxy, at one point it shrunk and expanded to cover a much larger area, possibly stripped and marked by interactions with its host.
Better insight into the characteristics of dark matter could help determine the cause of these changes. This is the next target for the researchers: they will use the Keck telescope in Hawaii to analyze the dark matter content of UDGs in the Virgo cluster, the cluster of galaxies closest to us.
As more powerful telescopes come online, we should be able to answer more questions about these UDGs. Improved optics from telescopes like the Vera C. Rubin Observatory and the Roman Nancy Grace Space Telescope will allow more of these faint galaxies to be detected using simulations such as TNG50.
“We hope our results will inspire new strategies to study the low-light universe, which would allow a full census of this population of dwarf galaxies,” said Sales.
The research was published in Nature astronomy.
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