11 Jul. () –
Observations with the Hubble Space Telescope spanning 18 years have allowed us to trace dark matter in the Draco dwarf galaxy, 250,000 light years from Earth, using star moves.
The qualities and behavior of dark matter, the invisible “glue” of the universeremain shrouded in mystery.
Although galaxies are mostly made of dark matter, understanding how it is distributed within a galaxy offers clues about what this substance is and its relevance to the evolution of a galaxy.
Although computer simulations suggest that dark matter should accumulate at the center of a galaxy, called the density cusp, many previous telescopic observations have indicated that it is instead more evenly dispersed throughout the galaxy. The reason for this tension between model and observation continues to puzzle astronomers, which reinforces the mystery of dark matter.
“Our models tend to agree more with a cusp-like structure, which aligns with cosmological models,” he said. it’s a statement Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore and lead author of the new study: “While we can’t say definitively that all galaxies contain a cusp-like distribution of dark matter, it’s exciting to have such well-measured data that surpasses anything we’ve had before.”
To learn about dark matter inside a galaxy, scientists can observe its stars and their motions which are dominated by the attraction of dark matter. A common approach to measuring the speed of objects moving through space is by using the Doppler Effect, an observed shift in the wavelength of light if a star is moving toward or away from Earth. While this line-of-sight speed can provide valuable information, Only certain information can be obtained from this one-dimensional source of information.
In addition to moving toward or away from us, stars also move across the sky, measured as their proper motion. By combining line-of-sight speed with proper motions, The team created an unprecedented analysis of the three-dimensional motions of stars.
“Improvements in data and improvements in modeling often go hand in hand,” explained STScI’s Roeland van der Marel, co-author of the paper that initiated the study more than 10 years ago. “If you don’t have very sophisticated data or only one-dimensional data, you can often use relatively simple models. The more dimensions and complexity of the data you collect, the more complex the models are.” “the more complex the models need to be to truly capture all the subtleties of the data.”
Since dwarf galaxies are known to have a higher proportion of dark matter content than other types of galaxies, the team focused on the Draco dwarf galaxy, which is a relatively small, spheroidal satellite close to the Milky Way.
“When you measure proper motions, you note the position of a star at one time and then many years later you measure the position of that same star. You measure the shift to determine how much it moved,” explained Sangmo Tony Sohn of STScI, another co-author on the paper and principal investigator of the latest observing program. “For this type of observation, “The longer you wait, the better you can measure the motion of the stars.”
The team looked at a series of epochs spanning from 2004 to 2022 — a long baseline that only Hubble could offer, thanks to its combination of sharp, stable vision and record-breaking time in operation. The telescope’s rich archive of data helped reduce the level of uncertainty in measuring the stars’ proper motions. The precision is equivalent to measuring an annual displacement a little smaller than the width of a golf ball as seen on the Moon from Earth.
With three dimensions of data, the team reduced the number of assumptions applied in previous studies and considered specific features of the galaxy, such as its rotation and the distribution of its stars and dark matter, in their own modeling efforts.
The methodologies and models developed for the Draco dwarf galaxy can be applied to other galaxies in the future. The team is already analyzing Hubble observations of the Sculptor dwarf galaxy and the Ursa Minor dwarf galaxy.
Studying dark matter requires observing different galactic environments and also involves collaboration between different space telescope missions. For example, NASA’s upcoming Nancy Grace Roman Space Telescope will help reveal new details of the properties of dark matter between different galaxies thanks to its ability to study large swaths of the sky.
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