The Hubble Space Telescope has recently provided critical insights into dark matter’s distribution within a nearby dwarf galaxy, offering strong support for the Standard Model of cosmology.
This model predicts that it is predominantly “cold,” meaning it consists of low-energy particles that move slowly and cluster together to form large halos where galaxies develop. This contrasts with emerging theories suggesting it might be “warm,” or have higher energy, causing it to spread more evenly.
What is Dark Matter?
Dark matter, an invisible substance thought to account for about 85% of the universe’s mass, remains one of the biggest mysteries in modern astronomy. According to the cold dark matter (CDM) theory, it is expected to form dense clusters in the core of its halos, which in turn affects galaxy formation. This concentration in the galaxy’s core is often referred to as the “cusp.”
However, recent observations of dwarf galaxies have hinted that it might not behave as expected. Instead of concentrating densely at the core, these studies suggested that it could be distributed more evenly, indicating it might be “warm.” This shift in understanding could have profound implications for our cosmological models.
New Findings from Hubble
Astronomers, led by Eduardo Vitral of the Space Telescope Science Institute (STScI), aimed to test this hypothesis using the Draco dwarf galaxy. Situated approximately 250,000 light years from Earth and orbiting our Milky Way, Draco provides an ideal laboratory for studying dark matter due to its high its density.
The team analyzed data from Hubble spanning 18 years (2004-2022) to track the motion of stars within Draco. By measuring both the “proper motions” (stars’ movements across the sky) and their radial motions (changes in light’s color due to movement toward or away from Earth), they created a detailed 3D map of the galaxy’s gravitational field and dark matter distribution.
Precision Measurements
Sangmo Tony Sohn of STScI highlighted the significance of these observations: “For this kind of study, the longer you wait, the more accurately you can measure stellar movements.” Over 18 years, Hubble captured incredibly subtle changes in star positions, equivalent to less than the width of a golf ball on the Moon as seen from Earth.
These precise measurements revealed that Draco’s dark-matter halo, extending nearly 3,000 light years, has a mass about 120 million times that of the Sun. Importantly, the data confirmed a “cusp” in the dark matter density profile, reinforcing the idea that dark matter is likely cold. This finding aligns well with the Lambda-CDM cosmological model, reducing previous uncertainties about dark matter distribution.
Future Research Directions
Vitral expressed enthusiasm about these findings, noting that “the results align closely with cosmological models, although we can’t yet confirm if all galaxies have a cusp-like dark matter structure.” The team plans to extend their analysis to other dwarf galaxies, such as Sculptor and Ursa Minor, to further test these conclusions.
If these results are consistent across various galaxies, they could challenge some dark matter theories, like those involving sterile neutrinos and gravitinos. Instead, the findings would bolster support for cold dark matter candidates, such as weakly interacting massive particles (WIMPs), primordial black holes, and axions.
