Hubble Telescope tracks a dwarf galaxy's stars to map out dark matter

The Hubble Space Telescope has shown that dark matter is concentrated within the core of a nearby dwarf galaxy, a finding that rushes to the rescue of the Standard Model of cosmology. This model basically predicts dark matter to be "cold," but recent findings have started to hint at the substance being "warm." These new observations, however, are on the Standard Model's side.

Dark matter is the invisible substance purported to make up 85% of the mass of the universe, but nobody knows what dark matter actually is, or exactly how it behaves. Our best idea is that it is "cold," which, in other words, means it is predicted to consist of a low-energy particle that's not zipping about hither and thither, but is rather slow-moving and capable of clumping together to form huge haloes inside which galaxies grow. The concept of cold dark matter (CDM) and its influence on structure formation in the universe is a critical part of our current Standard Model of cosmology. That part is known as Lambda–CDM (the lambda refers to dark energy).

In the cold dark matter paradigm, dark matter should especially pile up in the core of a dark matter halo, hence dark matter should be densest in the core of a galaxy that grows inside that halo. Astronomers call this the dark matter "cusp" because of the shape it makes on a graph of dark-matter density relative to its radius from the center of a galaxy.

However, astronomers have been stumped by some recent observations of dwarf galaxies that have hinted that dark matter might behave differently than they thought. Instead of acting like cold dark matter and clumping most densely in the core of the dark matter halo, these observations imply that dark matter may be more evenly distributed throughout a galaxy instead. This would be a sign that dark matter is "warm," or have  enough energy to not clump together so much. If true, this would have significant repercussions for our cosmological models that rely on dark matter being able to clump in certain ways.

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Now, astronomers led by Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, have put this to the test. 

Dwarf galaxies are the best place to study dark matter because, proportionally, they have the highest abundance of dark matter of any galaxy type. The Draco dwarf galaxy, which was chosen for this study, orbits our Milky Way galaxy at a distance of 250,000 light years from Earth. In Hubble's archives, there is data describing the motions of stars in the Draco dwarf spanning 18 years, between 2004 and 2022. Vitral's team were able to use these motions to calculate an accurate measurement of the Draco dwarf’s gravitational field, and hence the distribution of its mass, including the part dedicated to dark matter.

By combining the "proper motions" of the stars — that is, their motion across the sky — with their radial motions toward or away from us that is detectable as either a blueshift or a redshift in light, Vitral's team was able to track the movements of the stars in the Draco dwarf in 3D. 

"When measuring proper motions, you note the position of a star at one epoch and then many years later measure the position of that same star. You measure the displacement to determine how much it moved," said team-member Sangmo Tony Sohn of STScI in a statement. "For this kind of observation, the longer you wait, the better you can measure the stars shifting."

Certainly, Hubble's longevity in space is an advantage here, as is its powerful resolution from its vantage point high above Earth's turbulent atmosphere. The proper motion of the Draco dwarf's stars over the course of 18 years at a distance of a quarter of a million light years is tiny, equivalent to less than the width of a golf ball on the moon as seen from Earth. Hubble's results are therefore the most detailed measurements of stellar motions in another galaxy ever made.

Using these stellar motions, Vitral's team was able to conclude that the total mass of the Draco dwarf's dark-matter halo, out to a radius of nearly 3,000 light years, is 120 million times the mass of our sun. Furthermore, the results strongly indicate that the Draco dwarf's dark-matter density profile does have a cusp in the core and that therefore dark matter is probably cold. As the researchers write in their research paper, "The results lessen the tension around the ‘cusp-core’ problem and give further credence to standard lambda-CDM cosmology."

"Our models tend to agree more with a cusp-like structure, which aligns with cosmological models," said Vitral in the statement. "While we cannot definitively say all galaxies contain a cusp-like dark-matter distribution, it’s exciting to have such well measured data that surpasses anything we've had before."

The next step, therefore, is to repeat the analysis for other dwarf galaxies, and Vitral's team are currently working on studies of the Sculptor and Ursa Minor dwarf galaxies, which also orbit our Milky Way galaxy.

If the findings can be repeated in those and other galaxies, it would effectively rule out some dark matter candidates such as sterile neutrinos and gravitinos, the latter being a hypothetical particle predicted by the theory of supersymmetry as being the massive partner to the equally hypothetical (but probably real) graviton. The results therefore strengthen the possible models of cold dark matter, principally weakly interacting massive particles (WIMPs), primordial black holes and axions.

The results from the Draco dwarf were published on July 11 in The Astrophysical Journal.