The mysteries of ultradiffuse galaxies

An international team of astronomers have shed light on a mysterious galactic phenomenon: elusive ultradiffuse galaxies.

Ultradiffuse galaxies (UDGs) are dwarf galaxies whose stars are spread out over a vast region, resulting in extremely low surface brightness, making them very difficult to detect.

An international team of astronomers have used simulations to detect a few “quenched” UDGs – ones that don’t form stars – in low-density environments in the universe.

Their report is published in Nature Astronomy.

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“What we have detected is at odds with theories of galaxy formation since quenched dwarfs are required to be in clusters or group environments in order to get their gas removed and stop forming stars,” says Laura Sales, a researcher at the University of California, US, and co-author on the paper.

The quenched UDGs the team detected are not clusters – they’re isolated. The researchers were able to trace their evolution backward in time to show they originated in “backsplash orbits” – an object that looks like an isolated galaxy today but in the past was a satellite of a more massive system.

A rotation around one of the simulated ultradiffuse galaxies in TNG50. Red indicates stars, and the blue/white colors show the gas content. Credit: Jose Benavides.

“Isolated galaxies and satellite galaxies have different properties because the physics of their evolution is quite different,” Sales says. “These backsplash galaxies are intriguing because they share properties with the population of satellites in the system to which they once belonged, but today they are observed to be isolated from the system.”

A UDG has the stellar content of a dwarf galaxy: 10–100 times fewer stars than the Milky Way. But its size is comparable to the Milky Way, giving it the extremely low surface brightness that makes it special.

Trajectory of a blue dot through a galaxy cluster, coming out as a red dot
The fall of a blue ultradiffuse galaxy into a galaxy system and its subsequent ejection as a red ultradiffuse galaxy (having already lost its gas). Credit: Vanina Rodriguez.

Sales explains that the dark matter halo of a dwarf galaxy has a mass at least 10 times smaller than the Milky Way, and the size scales similarly. UDGs, however, break this rule and show a radial extension comparable to that of much larger galaxies.

“One of the popular theories to explain this was that UDGs are ‘failed Milky Ways,’ meaning they were destined to be galaxies like our own Milky Way but somehow failed to form stars,” says José A Benavides, a graduate student at the Institute of Theoretical and Experimental Astronomy, Argentina, and the first author of the research paper. “We now know that this scenario cannot explain all UDGs. So theoretical models are arising where more than one formation mechanism may be able to form these ultradiffuse objects.”

According to Sales, the value of the new work is twofold. First, the simulation used by the researchers, called TNG50, successfully predicted UDGs with characteristics similar to real observations. Second, the researchers found a few rare quenched UDGs for which they have no formation mechanism.

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“Using TNG50 as a ‘time machine’ to see how the UDGs got to where they are, we found these objects were satellites several billion years before but got expelled into a very elliptical orbit and look isolated today,” she says.

The researchers also report that according to their simulations, quenched UDGs can commonly make up 25% of an ultradiffuse population of galaxies. In observations, however, this percentage is much smaller.

“This means a lot of dwarf galaxies lurking in the dark may have remained undetected to our telescopes,” Sales says. “We hope our results will inspire new strategies for surveying the low-luminosity universe, which would allow for a complete census of this population of dwarf galaxies.”

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