The mysterious properties and behavior of dark matter, the invisible "glue" of the universe, remain shrouded in mystery. Although galaxies are mostly composed of dark matter, understanding how it is distributed within a galaxy offers clues about what this substance is and how it is relevant to the galaxy's evolution.

While computer simulations suggest that dark matter should clump in the center of a galaxy, called the density peak, many previous telescope observations indicate that it is instead more evenly distributed within the galaxy. The reason for this discrepancy between models and observations continues to puzzle astronomers, further deepening the mystery of dark matter.

A team of astronomers turned to NASA's Hubble Space Telescope to try to clarify this debate by measuring the dynamic movements of stars within the dwarf galaxy Draco, a system located about 250,000 light-years from Earth. Using observations spanning 18 years, they were able to build the most accurate three-dimensional understanding of star movements within the small galaxy. This required thoroughly searching nearly two decades of Hubble's archival observations of the Draco galaxy.

"Our models are more consistent with a density peak structure, which aligns with cosmological models," said Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore and the study's lead author. "While we cannot definitively say that all galaxies contain a density peak distribution of dark matter, it's exciting to have such well-measured data that surpasses anything we've had before."

Mapping Star Movements
To learn more about dark matter within a galaxy, scientists can study its stars and their movements, which are dominated by the attraction of dark matter. A common approach for measuring the speed of objects moving in space is the Doppler effect – the change in the wavelength of light if a star is approaching or moving away from Earth. While this line-of-sight velocity can provide valuable insight, only so much can be learned from this one-dimensional source of information.

Besides approaching or moving away from us, stars also move across the sky, measured as their proper motion. By combining line-of-sight velocity with proper motion, the team created an unprecedented analysis of the three-dimensional movements of stars.

"Improvements in data and improvements in modeling usually go hand in hand," explained Roeland van der Marel of STScI, a co-author of the paper who initiated the study more than 10 years ago. "If you don't have very sophisticated data or only one-dimensional data, then relatively simple models can often fit. The more dimensions and complexity of data you collect, the more complex your models need to be to truly capture all the subtleties of the data."

A Scientific Marathon, Not a Sprint
Since dwarf galaxies are known for having a higher proportion of dark matter compared to other types of galaxies, the team focused on the dwarf galaxy Draco, a relatively small and spheroidal neighbor of the Milky Way.

"When you measure proper motion, you record the position of a star at one epoch and then measure the position of that same star many years later. You measure the shift to determine how much it has moved," explained Sangmo Tony Sohn of STScI, another co-author of the paper and the principal investigator of the latest observational program. "For this type of observation, the longer you wait, the better you can measure the shift in stars."

The team analyzed a range of epochs from 2004 to 2022, an extensive database that only Hubble could offer, thanks to its combination of sharp stable vision and record operational time. The telescope's rich archive of data helped reduce the level of uncertainty in measuring the proper motions of stars. The precision is equivalent to measuring an annual shift slightly smaller than the width of a golf ball seen on the Moon from Earth.

With three dimensions of data, the team reduced the amount of assumptions applied in previous studies and considered characteristics specific to the galaxy – such as its rotation and the distribution of stars and dark matter – in their models.

An 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 galaxies Sculptor and Ursa Minor.

Studying dark matter requires observing different galactic environments and also involves collaboration between various space telescope missions. For example, the upcoming NASA Nancy Grace Roman Space Telescope will help uncover 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," reflected Vitral. "We can conduct this science thanks to all the planning that has been done over the years to actually collect these data. The insights we have gathered are the result of the work of a broader group of researchers who have been working on these things for years."

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

Source: National Aeronautics and Space Administration