The universe is full of mysteries, and one of the greatest mysteries of all is the nature of dark matter. Scientists have been studying dark matter for decades, trying to understand its properties and how it interacts with the visible matter in the universe. One approach to studying dark matter is to look for its effects on visible matter, such as the gravitational lensing of light or the perturbations it causes in the orbits of stars and galaxies.
In a recent study, Megan Barry and colleagues used computer simulations to predict the population of dark matter subhaloes around Milky Way-mass galaxies. These subhaloes are effectively low-mass self-gravitating dark matter structures, which reside within a more massive halo. Despite lacking visible matter, subhaloes may be detectable through their effects on visible matter, such as gravitational lensing or the perturbations they cause in the orbits of stars and galaxies.
The team used data from the FIRE-2 baryonic simulations to quantify the counts and orbital fluxes for subhaloes with properties relevant to stellar stream interactions, such as masses down to 10^6 Msun, distances < 50 kpc of the galactic center, and across z = 0 – 1 (lookback time 0 – 8 Gyr). They provided fits to their results and their dependence on subhalo mass, distance, and lookback time, for use in (semi)analytic models.
The team found that a typical Milky Way-mass halo contains about 16 subhaloes with masses greater than 10^7 Msun within 50 kpc at z = 0. They also compared their results with dark-matter-only versions of the same simulations and found that the subhalo counts were overpredicted by 2-10x. This difference was primarily due to the additional tidal force from the Milky Way-mass galaxy potential in the baryonic simulations.
The team’s findings also suggest that subhalo counts around a given Milky Way-mass galaxy declined over time, being about 10x higher at z = 1 than at z = 0. They also found that subhaloes have nearly isotropic orbital velocity distributions at z = 0. They identified four analogs of Large Magellanic Cloud satellite passages across their simulations. These analogs boosted low-mass subhalo counts by 1.4-2.7 times, significantly increasing the expected subhalo population around the Milky Way today. The team’s results imply an interaction rate of about five per Gyr for a stream like GD-1, which is sufficient to make subhalo-stream interactions a promising method of measuring dark subhaloes.
These findings have several implications for our understanding of dark matter and its effects on visible matter. The study provides a more accurate prediction of the population of dark matter subhaloes around Milky Way-mass galaxies, which will help to guide future observational campaigns aimed at testing dark-matter models. The team’s findings also highlight the importance of incorporating baryonic physics in simulations of dark matter structures, as the presence of visible matter can significantly affect the subhalo counts.
Furthermore, the team’s identification of LMC satellite analogs that boost low-mass subhalo counts suggests that the Milky Way’s interaction with the LMC may have played a significant role in shaping the distribution of dark matter subhaloes around our galaxy. These findings could help us to better understand the formation and evolution of galaxies and the role of dark matter in these processes.
Megan Barry and colleagues’ study provides new insights into the population of dark matter subhaloes around Milky Way-mass galaxies and highlights the importance of incorporating baryonic physics in simulations of dark matter structures. The team’s findings could help to guide future observational studies.
Source: Barry, M., Wetzel, A.R., Chapman, S., Samuel, J., Sanderson, R.E., & Arora, A. (2023). The dark side of FIRE: predicting the population of dark matter subhaloes around Milky Way-mass galaxies. https://doi.org/10.48550/arXiv.2303.05527