Eduardo Vitral

Dark matter in dwarf galaxies

Dark matter laboratories

Fraction of the Universe large-scale structure known as the cosmic web, as viewed in a cosmological simulation by the TNG Collaboration. Our main cosmological models predict that dark matter composes about 85% of all the mass in the Universe. Most galaxeis are thus believed to be embedded in a large and massive dark matter halo of their own. Image credits: TNG Collaboration. The Universe harbors a profound puzzle: the vast majority of its matter is invisible, known as dark matter. Despite its elusive nature, evidence suggests that dark matter is some sort of binding component that holds galaxies together, yet it defies direct detection, challenging our understanding of the physical world. Dwarf galaxies, orbiting the Milky Way, offer a unique window into this dark universe, as large dark matter reservoirs. Their stars move under the influence of dark matter's gravitational pull, providing clues about its distribution and properties.

Thus, a large observational effort has been invested in obtaining line-of-sight (LOS) velocities of these dwarf galaxies, but results from analyzing these data have been inconclusive about some predictions from our best current cosmological theories. A conspicuous example of this is the so-called “core-cusp problem”: the tension around the predicted and observed dark matter mass-density profiles of galaxies. Dark matter halos in collisionless cosmological N-body simulations follow a nearly universal mass-density profile that increases and diverges toward the center, forming a ‘cusp’. In contrast, observations of some dwarfs favor shallower density profile slopes, consistent with a constant density ‘core’ at the center.

The need for 3D velocities

Significant uncertainties are introduced by the fact that most observational studies are based solely on LOS velocity measurements, which constrain only one component of motion. When only LOS velocities are used, there is a strong degeneracy between the mass density profile and the velocity anisotropy profile, which quantifies differences in velocity dispersions in orthogonal directions. Some models mitigate this degeneracy by restricting parameter space or using higher-order moments, but having only the LOS component of motion fundamentally limits what can be achieved.

The key to progress is to measure the internal proper motion (PM) kinematics of stars, or in other words, their transverse motion in the plane-of-sky. The radial and tangential PM components directly measure the projected velocity dispersion anisotropy. This makes PMs crucial for dynamical modeling, with models making use of PMs performing consistently better than those based solely on LOS velocities. However, measuring PMs require long time baselines and exquisite telescope precision, which hardly come together and are hard to obtain.

Long-time baselines: To resolve the internal motions of dwarf galaxies orbiting the Milky Way in sufficient detail, even with state-of-the-art telescopes such as the Hubble Space Telescope need to follow up stellar motions for periods of at least ∼10 years. The video above, created by Tony Sohn with Hubble data from the Draco dwarf galaxy, depicts variations in stellar positions over a time baseline of 18 years, and still some motions are not even visible to the naked eye. The magenta arrows point to foreground stars with very high PM values.

HSTPROMO studies of dwarf galaxies

Given the hard work required to measure PMs, the HSTPROMO Collaboration has inversted a significant amount of team effort to monitor stellar systems for many years, and thus aqcuire the necessary data for PM studies of dwarf galaxies (see animation above). With this data, we were able to measure, for the first time, the 3D velocity dispersion profile of a dwarf galaxy, with data uncertainties smaller than dispersion itself. This allowed us to constrain the dark matter slope in the Draco dwarf galaxy to a cusp with enough confidence to give more credence to current cosmological models, such as ΛCDM.

3D kinematics: The quantities showcased are upper left: velocity dispersion in the line-of- sight; upper right: plane-of-sky velocity dispersion in radial direction; lower left: line-of-sight rotation amplitude; lower right: plane-of-sky velocity dispersion in tangential direction. The black circles and error bars represent the data. Model predictions and the adopted galaxy distance (which we use to convert mas/yr to km/s in the rightmost panel) are from axisymmetric JamPy MCMC fits. Our best fit is depicted as a black solid line, which we interpolate with respect to projected radius R from the actual data R values (this is done for visualization purposes, since there is also a dependence with the projected angle). The plot uses data from Vitral et al. 2024. The data supports the existence of a dark matter cusp in the Draco dwarf galaxy, in agreement with the ΛCDM cosmological framework.