Astronomers are still puzzled by ultra-diffuse galaxies (UDGs).
These galaxies are smaller in terms of their number of stars but they're still scattered over large distances making them difficult to spot. It is not known how they formed, or if something about the dark matter halos helps them form.
Recent research may be able answer some questions about UDGs, and in particular "quenched," UDGs that aren’t forming new stars. Astronomers were able to identify and analyze new galaxies matching this description through a series simulations.
Modeling and observations revealed that these quenched UDGs were created in what is known as a backsplash orbit. It extends beyond the borders of a host galaxies, but it's still connected. They were once part of a larger system, before being isolated, and they share some characteristics with that system.
"What we found is not consistent with the theories of galaxy formation," said Laura Sales, an astronomer at the University of California in Riverside.
"But we only found a few quenched UDGs." We were able identify a few of these quenched UDGs and track their evolution backwards to prove they came from backsplash orbits.
TNG50, the simulation used by this team, was capable of successfully predicting UDG systems that were similar to those observed. It could also be used as a time machine to roll back these galaxies and see where they came form billions of years before.
TNG50 also indicated that the percentage of UDGs quenched in an ultra-diffuse galaxy population could be as high as 25%, which is much higher than what was based on field observation. This could indicate that our telescopes are not able to detect many of these galaxies.
There are many possible causes for UDGs' existence. However, it has been suggested that there is no one-size-fits all explanation.
Above: The fall and subsequent ejection of a blue-diffuse galaxies into a galaxy-system.
"One popular theory to explain this is that UDGs were failed Milky Ways. This means they were destined for galaxies similar to our Milky Way, but somehow failed to form stars," Jos Benavides (Institute of Theoretical and Experimental Astronomy, Argentina) says.
We now know that this scenario is not able to explain all UDGs. These theoretical models show that more than one mechanism might be capable of forming these ultra-diffuse items.
UDGs look like dwarf galaxies, but they have a fraction of the 200 to 400 billion stars in the Milky Way. Yet they are comparable in size to the Milky Way. Stars are more evenly distributed.
UDGs have dark matter halos that break with the rules. They used to have the same amount of dark matter as a dwarf galaxy garden variety, but it shrank over time and covered a larger area.
This will allow us to get a better understanding of the dark matter characteristics and help us pinpoint the source of the changes. The next goal for researchers is to analyze the dark matter content in UDGs from the Virgo galaxy cluster, which is closest to us.
We will be able answer more questions about UDGs as more powerful telescopes become available. With the help of simulations like TNG50, more faint galaxies will be visible thanks to the improved optics of telescopes such as the Vera C. Rubin Observatory or the Nancy Grace Roman Space Telescope.
Sales says, "We hope that our results will inspire us to find new ways of surveying the low-luminosity universe. This would allow for complete censuses of this population."
Nature Astronomy published the research.