Left, the simulation results for one of the super-diffuse galaxies. The image of the DF2 galaxie, which is nearly transparent, can be seen on the right. Credit: ESA/Hubble.
Ultradiffuse galaxies (or UDGs) are dwarf galaxies with stars scattered over large areas. This results in very low surface brightness and makes them difficult to spot. There are many questions regarding UDGs that remain unanswered. Are their dark matter halosthe halosof invisible matter surrounding galaxiesspecial
An international team of astronomers led by Laura Sales, an Astronomer at the University of California Riverside, has reported in Nature Astronomy that they have used sophisticated simulations to detect "quenched UDGs" in low-density areas in the universe. A quaked galaxy is one that doesn't form stars.
Sales, an associate professor in physics and astronomy, stated that the evidence we found is not consistent with theories about galaxy formation. "Quenched dwarfs must be in clusters or groups in order to get their gas removed so they can stop forming stars." The quenched UDGs that we found are not clustered or group environments, however, they can be identified as isolated. We were able identify a few of these quenched UDGs and track their evolution backwards to prove they came from backsplash orbits.
The term "in the field" is used to refer to galaxies that are isolated in quieter environments, and not in a cluster or group environment. Sales explained that while a backsplash galaxy looks like an isolated object today, it was once a satellite of an even more massive system, similar to a comet. It visits our sun occasionally but spends most of its journey alone, far away from the rest of the solar system.
She stated that satellite galaxies as well as isolated galaxies possess different properties due to the different physics of their evolution. These backsplash galaxies share many properties with satellites, but they are now seen to be isolated from their former home.
Image depicts the transformation of a blue-colored galaxy into a galaxie system, and then its subsequent ejection to become a red-colored galaxy after it has lost its gas. Credit: Vanina Rodrigo.
Dwarf galaxies, which are small galaxies with a few billion to 100 million stars, are smaller than the Milky Way. The Milky Way, however, has between 200 billion and 400 billion stars. All UDGs are dwarf galaxies. However, not all UDGs can be considered dwarf galaxies. At similar luminosity, dwarfs can show a wide range of sizes from small to large, even if they are compact. UDGs are the tail ends of most extended objects with a given luminosity. UDGs have the stellar content of a dwarf galaxie, which is 10-100 times smaller than that of the Milky Way. Its size is similar to that of the Milky Way, which gives it its unique surface brightness.
Sales explained that the mass of the dark matter halo in a dwarf galaxy is at least 10x smaller than the Milky Way's, and the scales are similar. UDGs however break this rule, and exhibit a radial expansion comparable to those of larger galaxies.
"One popular theory to explain this is that UDGs were 'failed Milky Ways', meaning they were destined galaxies just like our Milky Way, but somehow failed to form stars," Jos A. Benavides (a graduate student at the Institute of Theoretical and Experimental Astronomy, Argentina) was the first author of the research paper. 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 ultradiffuse items.
Sales says the work has twofold value. The simulation, TNG50, was used to predict UDGs that had characteristics similar to those observed. The researchers also discovered a few rare quaked UDGs that have no formation mechanism.
She said that she used TNG50 as a "time machine" to determine how the UDGs arrived at their current location.
Animation depicts a rotation of one of the simulated superdiffuse galaxies from TNG50. The red color indicates stars and the blue/white the gas content. Credit: Jose Benavides.
According to the researchers, 25% of the ultradiffuse population can be made up of quenched UDGs according to simulations. However, observations show that this percentage is much lower.
Sales stated that this means there may be many dwarf galaxies hidden in the darkness that have not been detected by our telescopes. We hope that our results will inspire new strategies to survey the low-luminosity universe. This would allow us to complete a census of this population.
This study is the first to identify the many environments, from isolated dwarfs to clusters of dwarfs. It also provides high resolution data to examine their structure and morphology.
The research team will then continue their study of UDGs using TNG50 simulations to better understand the reasons these galaxies seem so far away from other dwarf galaxies that have the same stellar content. To measure the dark matter content in UDGs within the Virgo galaxy cluster, which is the nearest to Earth, the researchers will use Hawaii's Keck Telescope. It is one of the largest telescopes in the country.
Sales stated that future telescopes such as the Large Synoptic Satellite Telescope and the Roman Space Telescope will be available online within the next five to ten years, with the capability of detecting many other UDGs.
The paper is entitled "Quiescent ultradiffuse galaxies in a field originating from backsplash orbits."
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More information: Quiescent super-diffuse galaxies found in the field that originates from backsplash orbits, Nature Astronomy (2021). www.nature.com/articles/s41550-021-01458-1 Journal information: Nature Astronomy Quiescent ultra-diffuse galaxies in the field originating from backsplash orbits,(2021). DOI: 10.1038/s41550-021-01458-1
