Though the majority of the mass in the universe lies in dark matter, many mysteries remain about its nature. A new study suggests that the smallest and faintest galaxies hold the key to unlocking how dark matter interacts within the universe.
Core-Cusp Conundrum
Surrounding the Milky Way are tiny faint galaxies — known as satellite galaxies — millions of times smaller than the massive galaxy they orbit. Despite their small sizes, these satellites are of huge importance in understanding the behavior of dark matter. In the leading cosmological model, called lambda cold dark matter (ΛCDM), dark matter is cold, collisionless, and interacts with normal matter only by way of gravity.
From simulations, the expected shape (cusp) of a galaxy’s dark matter density profile is shown with the red curve, while the observed shape (core) is shown with the blue curve. As we approach the galactic center, a cuspy profile peaks at high density, and the cored profile levels off at a lower density. Click to enlarge. [Lexi Gault]
However, according to ΛCDM, there should exist very low-mass galaxies — like the faintest satellite galaxies that orbit the Milky Way — that do not have enough mass to change an originally cuspy dark matter distribution into a cored one. If observations reveal that even these small galaxies have cored dark matter distributions, that may mean that dark matter may not behave as expected in the ΛCDM paradigm.
Dark Matter Distributions
In order to investigate dark matter distributions at the smallest scales, Jorge Sánchez Almeida (Institute of Astrophysics of the Canary Islands; University of La Laguna) and collaborators analyze the stellar count distribution of six ultra-faint dwarf satellites of the Milky Way and the Large Magellanic Cloud. These galaxies have cored stellar surface density distributions, suggesting that their underlying dark matter distributions may be cored as well.
Histograms showing how each considered dark matter profile compares to the observations, where NFW corresponds to a cuspy distribution, Plummer corresponds to a cored distribution, and the ρ230 is in the middle of the two. The the histograms plot the distributions of the innermost slopes for the fits where a slope close to 0 means the profile flattens, and a more negative slope means a steeper rise in the center. The cored (Plummer) fit aligns best with the observed galaxies. Click to enlarge. [Modified from Sánchez Almeida et al 2024]
Beyond Standard
Given that a cored dark matter distribution seems to fit best for these ultra-faint galaxies, the authors suggest that dark matter may not behave as the standard cold dark matter model anticipates. Instead, dark matter may be warm, self-interacting, or fuzzy. However, could other factors be driving the cored distributions of these galaxies?
The author’s analysis hinges on several assumptions that could blur their results. To address these concerns, they carefully consider properties like stellar feedback contributions, tidal interactions, velocity distributions, and other properties important to their analysis. When changing these assumptions slightly, in ways still physically reasonable for very low-mass galaxies, the ultra-faint dwarfs still appear to reside in cored dark matter distributions.
What does this mean? While the standard ΛCDM model explains large-scale structures of the universe well, it does not adequately characterize what is observed on smaller scales — like the Milky Way’s satellites. Though the exact nature of dark matter is still unknown, cold and collisionless dark matter does not likely produce the ultra-faint galaxies’ cored profiles. Thus, further studying the lowest-mass galaxies in the universe may reveal new characteristics of dark matter beyond the standard model.
Citation
“The Stellar Distribution in Ultrafaint Dwarf Galaxies Suggests Deviations from the Collisionless Cold Dark Matter Paradigm,” Jorge Sánchez Almeida et al 2024 ApJL 973 L15. doi:10.3847/2041-8213/ad66bc