Feb 10 2021
A research team headed by astronomers from the University of California, Riverside, has discovered that certain dwarf galaxies might now appear dark-matter free although they formed as galaxies dominated by dark matter previously.
Laura Sales (seated, left) with her research group of former and current students, including Jessica Doppel (seated, right). Image Credit: UCR/Stan Lim.
Galaxies that seem to have very little to zero dark matter—nonluminous material considered to make up 85% of matter in the universe—complicate astronomers’ insights into the dark matter content in the universe.
Galaxies such as these, identified recently in observations, question a cosmological model used by astronomers, known as Lambda Cold Dark Matter (LCDM), according to which a massive and extended dark matter halo surrounds all galaxies.
The astronomical community does not have a better understanding of dark-matter-free galaxies.
One method to explore the probable formation mechanisms for such elusive galaxies—examples are the ultradiffuse DF2 and DF4 galaxies—is to detect similar objects in numerical simulations and analyze their time evolution and the conditions that result in their dark matter loss.
According to Jessica Doppel, a graduate student in the UC Riverside Department of Physics and Astronomy and the first author of the study published in the Monthly Notices of the Royal Astronomical Society, in an LCDM universe, all galaxies should be dominated by dark matter.
That’s the challenge. Finding analogs in simulations of what observers see is significant and not guaranteed. Beginning to pin down the origins of these types of objects and their often-anomalous globular cluster populations allows us to further solidify our theoretical framework of dark matter and galaxy formation and confirms that no alternative forms of dark matter are needed. We found cold dark matter performs well.
Jessica Doppel, Graduate Student, Department of Physics and Astronomy, University of California, Riverside
In their study, the research team employed cosmological and hydrodynamical simulation known as Illustris, which provides a galaxy formation model including black hole growth, stellar evolution, mergers, and supernova feedback.
The team identified that two 'dwarf galaxies' in clusters—DF2 and DF4—exhibited similar stellar content, dark matter mass and globular cluster numbers. As their name indicates, a dwarf galaxy is small, including up to several billion stars.
On the other hand, the Milky Way, with over 20 known dwarf galaxies orbiting it, includes 200 to 400 billion stars. In general, globular clusters are used to predict the dark matter content of galaxies, specifically dwarfs.
The Illustris simulation was used by the team to study the origin of odd dwarf galaxies like DF2 and DF4. They identified simulated analogs to dark-matter-free dwarfs as objects that had evolved inside the galaxy clusters for a longer period and lost over 90% of their dark matter through tidal stripping—the stripping away of material through galactic tidal forces.
Interestingly, the same mechanism of tidal stripping is able to explain other properties of dwarfs like DF2 and DF4—for example, the fact that they are ‘ultradiffuse’ galaxies. Our simulations suggest a combined solution to both the structure of these dwarfs and their low dark matter content. Possibly, extreme tidal mass loss in otherwise normal dwarf galaxies is how ultradiffuse objects are formed.
Laura Sales, Study Co-Author and Associate Professor of Physics and Astronomy, University of California, Riverside
Working with researchers at the Max Planck Institute for Astrophysics in Germany, a group led by Sales, who is Doppel’s graduate advisor, is now using enhanced simulations featuring more in-depth physics and a numerical resolution around 16 times better than the Illustris simulation.
“With these data, we will be able to extend our study to even lower-mass dwarfs, which are more abundant in the universe and expected to be more dark matter dominated at their centers, making them more challenging to explain,” noted Doppel.
We will explore if tidal stripping could provide a path to deplete dwarfs of their inner dark matter content. We plan to make predictions about the dwarfs’ stellar, globular cluster, and dark matter content, which we will then compare to future observations.
Jessica Doppel, Graduate Student, Department of Physics and Astronomy, University of California, Riverside
The team of researchers has already been granted time at the W. M. Keck Observatory to help find solutions to certain questions regarding observations of dwarfs in the Virgo cluster.
Julio F. Navarro of the University of Victoria in Canada, Mario G. Abadi and Felipe Ramos-Almendares of the National University of Córdoba in Argentina, Eric W. Peng of Peking University in China, and Elisa Toloba of the University of the Pacific in California joined Sales and Doppel in the study.
Grants from NASA and the National Science Foundation supported the research.
Journal Reference:
Doppel, J. E., et al. (2020) Globular clusters as tracers of the dark matter content of dwarfs in galaxy clusters. Monthly Notices of the Royal Astronomical Society. https://doi.org/10.1093/mnras/staa3915.
Source: https://www.ucr.edu/