Dark matter, the mysterious invisible component of the universe, continues to pose many scientific puzzles.

Although it makes up most of the matter in galaxies, the distribution of dark matter within them could reveal key information about its nature and role in the evolution of galaxies.

According to computer simulations, dark matter should concentrate in the centers of galaxies, creating a so-called cusp density. However, many telescope observational studies suggest that dark matter is more evenly distributed. This contradiction between models and observations presents a major challenge for astronomers, increasing the mystery surrounding dark matter.

An astronomical team, using NASA's Hubble Space Telescope, is trying to clarify this dilemma by measuring the dynamic movements of stars within the dwarf galaxy Draco, located about 250,000 light-years from Earth. Thanks to observations spanning 18 years, they managed to create the most precise three-dimensional model of star movements in this tiny galaxy. This required a detailed study of almost two decades of Hubble archival data on the Draco galaxy.

Dark Matter Models
"Our models lean more towards a cusp-like structure, which is consistent with cosmological models," said Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore, the lead author of the study. "While we can't definitively claim that all galaxies contain dark matter distributed in a cusp shape, it's exciting to have such precise data that surpasses everything we've had so far."

Star Movements
To study dark matter in the galaxy, scientists analyze the movements of stars influenced by the gravity of dark matter. A common method for measuring the velocity of objects in space is the Doppler effect – a change in the wavelength of light from a star approaching or moving away from Earth. While this method provides valuable information, it is limited to one-dimensional data.

Besides the movement towards and away from us, stars also move across the sky, which is measured as their proper motion. By combining line-of-sight motion with proper motion, the team created an unprecedented analysis of the three-dimensional movement of stars.

"Advancement of data and models usually goes hand in hand," explained Roeland van der Marel of STScI, a co-author of the paper who started this study more than 10 years ago. "If you don't have very sophisticated data or only one-dimensional data, simple models can often suffice. The more dimensions and complexity of data you collect, the more complex your models must be to truly capture all the subtleties of the data."

Long-term Commitment
Since dwarf galaxies are known for having a higher proportion of dark matter than other types of galaxies, the team focused on the dwarf galaxy Draco, which is a relatively small and spheroidal satellite galaxy of the Milky Way.

"When measuring proper motions, you record the position of a star in one epoch, and then many years later measure the position of the same star. You measure the displacement to determine how much the star has moved," explained Sangmo Tony Sohn of STScI, co-author of the paper and principal investigator of the latest observational program. "For such observations, the longer you wait, the better you can measure the movement of stars."

The team analyzed a series of epochs from 2004 to 2022, a comprehensive database that only Hubble could provide, thanks to the combination of its sharp stable vision and long-term operation. The rich archival data of the telescope helped reduce the level of uncertainty in measuring the proper motions of stars. The precision is equivalent to measuring an annual displacement slightly less than the width of a golf ball viewed from the Moon from Earth.

With three-dimensional data, the team reduced the number of assumptions used in previous studies and took into account the characteristics specific to the galaxy – such as its rotation, distribution of stars, and dark matter – in their models.

Exciting Future
The methodologies and models developed for the dwarf galaxy Draco can be applied to other galaxies in the future. The team is already analyzing Hubble's observations of the dwarf galaxy Sculptor and the dwarf galaxy Ursa Minor.

Studying dark matter requires observing different galactic environments and the cooperation of various space telescope missions. For example, the upcoming NASA Nancy Grace Roman Space Telescope will help reveal new details about the properties of dark matter among different galaxies, thanks to its ability to survey large portions of the sky.

"This type of research is a long-term investment and requires a lot of patience," said Vitral. "We can conduct such science thanks to planning that lasted for years to actually collect this data. The insights we've gathered are the result of the work of many researchers who have been working on these things for years."

These results have been accepted for publication in The Astrophysical Journal.

The Hubble Space Telescope has been operating for more than three decades and continues to bring revolutionary discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (the European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space in Denver, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, operated by the Association of Universities for Research in Astronomy, conducts Hubble's science operations for NASA.

Source: National Aeronautics and Space Administration