The qualities and behavior of dark matter, the invisible "glue" of the universe, continue to be shrouded in mystery. Though galaxies are mostly made of dark matter, understanding how it is distributed within a galaxy offers clues to what this substance is, and how it's relevant to a galaxy's evolution.
While computer simulations suggest dark matter should pile up in a
galaxy's center, called a density cusp, many previous telescopic observations
have indicated that it is instead more evenly dispersed throughout a galaxy.
The reason for this tension between model and observation continues to puzzle
astronomers, reinforcing the mystery of dark matter.
A team of astronomers has turned toward NASA's Hubble Space Telescope to try and clarify this debate by measuring the dynamic motions of
stars within the Draco dwarf galaxy, a system located roughly 250,000
light-years from Earth. Using observations that spanned 18 years, they
succeeded in building the most accurate three-dimensional understanding of
stars' movements within the diminutive galaxy. This required scouring nearly
two decades of Hubble archival observations of the Draco galaxy.
A team of astronomers analyzed observations by NASA's
Hubble Space Telescope taken over a span of 18 years to measure the dynamic
motions of stars within the Draco dwarf galaxy. The telescope's extensive
baseline and data archive enabled the team to build the most accurate
three-dimensional map of the stars' movements within the system. These improved
measurements are helping to shed "light" on the mysterious qualities
and behavior of dark matter, the universe's invisible "glue." The
left image is from the Digitized Sky Survey (DSS). It presents a wider view of
the region. The two right-side images are Hubble views.
NASA, ESA, Eduardo Vitral, Roeland van der Marel, and
Sangmo Tony Sohn (STScI), DSS; Image processing: Joseph DePasquale (STScI)
"Our models tend to agree more
with a cusp-like structure, which aligns with cosmological models," said
Eduardo Vitral of the Space Telescope Science Institute (STScI) in Baltimore
and lead author of the study. "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."
Charting the Movements of
Stars
To learn about dark matter within a
galaxy, scientists can look to its stars and their movements that are dominated
by the pull of dark matter. A common approach to measure the speed of objects
moving in space is by the Doppler Effect – an observed change of the wavelength
of light if a star is approaching or receding from Earth. Although this
line-of-sight velocity can provide valuable insight, only so much can be
gleaned from this one-dimensional source of information.
Besides moving closer or further
away from us, stars also move across the sky, measured as their proper motion.
By combining line-of-sight velocity with proper motions, the team created an
unprecedented analysis of the stars’ 3D movements.
"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. 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."
A Scientific Marathon (Not a
Sprint)
Since dwarf galaxies are known to
have a higher proportion of dark matter content than other types of galaxies,
the team honed in on the Draco dwarf galaxy, which is a relatively small and
spheroidal nearby satellite of the Milky Way galaxy.
"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."
The team analyzed a series of
epochs spanning from 2004 to 2022, an extensive baseline that only Hubble could
offer, due to the combination of its sharp stable vision and record time in
operation. The telescope's rich data archive helped decrease the level of
uncertainty in the measurement of the stars' proper motions. The precision is
equivalent to measuring an annual shift a little less than the width of a golf
ball as 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
distribution of its stars and dark matter – in their own modeling efforts.
An Exciting Future
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 entails collaboration
across different space telescope missions. For example, NASA's upcoming Nancy Grace Roman Space Telescope will help 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 we've
collected are the result of a larger group of researchers that has been working
on these things for many years."
These results are accepted for publication in The Astrophysical Journal.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation 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 science operations for NASA.
Source: NASA's Hubble Traces Dark Matter in Dwarf Galaxy Using Stellar Motions - NASA Science
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