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Hubble discovers dark matter 'clump' in center of another galaxy

If you’ve ever wondered what keeps the universe together, you are not alone. Scientists around the globe have been engrossed in one question: what on earth (or should we say, in space) is dark matter? Despite the fact that it forms the majority of galaxies, this elusive matter is invisible and wrapped in mystery. It’s like the invisible ‘glue’ that binds the universe together.

Dark matter galaxy distribution

Eduardo Vitral, lead author of the study from the Space Telescope Science Institute (STScI), explains the complexity of understanding dark matter.

Although computer simulations predict dark matter should clump up in the center of a galaxy, most observations tell a different story.

Instead of an accumulation in the center (a density cusp), it seems the dark matter is spread more evenly across a galaxy. This discrepancy between theory and observation creates a layer of intrigue around dark matter.

Draco Dwarf galaxy where Hubble spotted a likely dark matter clump. Credit: Hubble/NASA
Draco Dwarf galaxy where Hubble spotted a likely dark matter clump. Credit: Hubble/NASA

In an attempt to add clarity to this debate, a team of astronomers decided to peek through the lens of NASA’s Hubble Space Telescope.

They meticulously observed the dynamic motions of stars within the Draco dwarf galaxy, a system located around 250,000 light-years from Earth.

“Our models tend to agree more with a cusp-like structure, which aligns with cosmological models,” said Vitral.

“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.”

Studying galaxies to understand dark matter

The astronomers employed a common approach to understanding dark matter distributions: studying the stars and their movements.

By observing the stars and interpreting the Doppler Effect, they could gain valuable insights into the invisible matter’s influence.

However, there’s a limit to what one can understand from this one-dimensional source of information. This is where a new perspective came into play.

In addition to observing the stars’ shifting away from or close to us, the astronomers took the stars’ lateral movements into account (known as ‘proper motion’).

By blending line-of-sight velocity with proper motions, the team created an unprecedented analysis of the stars’ 3D movements.

Getting the most out of the data

“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 straightforward models can often fit.”

He explained that the more dimensions and complexity of data you gather, the more complex your models need to be to truly capture all the subtleties of the data.

Focusing on the Draco Dwarf Galaxy

With its relatively small size and spheroidal shape, the Draco dwarf galaxy, a satellite of the Milky Way, became the perfect candidate for their study.

Dwarf galaxies have a higher proportion of dark matter content, making them ideal for such research.

The team meticulously measured shifting stars over an 18-year period, thus reducing uncertainties and providing precise measurements.

“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,” explained Sangmo Tony Sohn of STScI, another co-author of the paper and the principal investigator of the latest observational program.

“For this kind of observation, the longer you wait, the better you can measure the stars shifting.”

Future of dark matter studies

The methodologies and models developed for the Draco dwarf galaxy will serve as a blueprint for future studies on other galaxies. The team is already analyzing Hubble observations of the Sculptor and Ursa Minor dwarf galaxies.

Moreover, the upcoming NASA’s Nancy Grace Roman Space Telescope will reveal new details of dark matter’s properties among different galaxies, thanks to its ability to survey large swaths of the sky.

“This kind of study is a long-term investment and requires a lot of patience,” reflected Vitral. “We’re able to do this science because of all the planning that was done throughout the years to actually gather these data.

The insights this brilliant team has collected are the result of a larger group of researchers that has been working on these things for many years.

So, the next time you look up at the stars, remember that astronomers are tirelessly working to understand the invisible ‘glue’ that binds the cosmos together. Their patience and perseverance might just provide the answer to the riddle of the invisible universe.

The full study was published in the Astrophysical Journal.


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