Images created by NASA’s upcoming Nancy Grace Roman Space Telescope could allow scientists to search for dark matter among stars.
An international team of researchers believes that gaps in strings of stars hanging from tightly packed balls of ancient stellar bodies, called globular clusters, may be affected by clumps of dark matter.
So far astronomers have only been able to study these dangling streams of stars in the Milky Way, meaning our understanding of them is limited. Roman, scheduled to be launched in 2027, should be sensitive enough to see these structures in our neighboring galaxy Andromeda, and in such detail it will be possible to see distortions caused by dark matter, giving astronomers clues about this elusive substance.
“There are streams of stars in our own galaxy where we see cavities that may be caused by dark matter,” team member and scientist Tjitske Starkenburg from Northwestern University said in a statement. “But these gaps can also be created in other ways.”
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The team suggests that Roman’s observations of voids in galaxies outside the Milky Way will give scientists a better picture of these voids as a whole. This could ultimately help determine the existence and properties of dark matter clumps.
Reading between the lines (or rather the stars)
Dark matter is worrying to scientists because although it makes up an estimated 85% of the matter in the universe, they have little idea what it is.
Dark matter does not interact with light, which means it is virtually invisible to our eyes and cannot be composed of the atoms of electrons, protons, and neutrons that make up the “everyday” matter we are accustomed to. Think, stars, planets, flowers, books. Everything we see with the naked eye (including our bodies) consists of such “normal” matter.
But dark matter interacts with gravity, which means the only way scientists can understand its existence is to look at how its effect on gravity then affects all everyday matter and light.
The fact that dark matter interacts gravitationally is actually a chance for the evolution of the universe. For example, some galaxies rotate so fast that the gravity of the visible matter they contain (stars, gases, dust and planets) is not enough to prevent them from flying away from each other.
“We see the impact of dark matter on galaxies,” said Christian Aganze, a team member and postdoctoral researcher at Stanford University. “For example, when modeling how galaxies rotate, we need extra mass to explain their rotation. Dark matter could provide this missing mass.”
Hints of dark matter may be hanging from globular clusters and loose stellar streams that often contain millions of stars. This is because scientists think dark matter may “poke holes” in these stellar streams, creating voids that can be used to assess the nature of this mysterious form of matter.
“There are two reasons why it’s most interesting to see the effects of these flows, these clumps of dark matter,” Starkenburg said. “First, these streams ‘live’ in the outermost regions of the galaxy, places where little structure would otherwise exist.
“Secondly, these streams are very thin in nature because they are formed by dense clusters of stars, which means you can see gaps or any disturbances much more easily.”
This isn’t a new idea, but it’s one that hasn’t been fully used to get to the bottom of the dark matter problem. Current space telescopes and ground-based instruments are limited to searching for dark matter-punctured holes in the small stream of stars hanging from globular clusters within the Milky Way.
From its location about 1 million miles (1.6 million kilometers) from Earth, Roman will be able to investigate such features in neighboring galaxies, particularly Andromeda, for the first time. The wide-field instrument will produce images 200 times larger than those created by the Hubble Space Telescope.
To test this, this team simulated stellar streams and allowed them to interact with clumps of dark matter, creating gaps as predicted. Scientists then created pseudo-Roman observations of these dark matter-pierced areas in stellar streams. They concluded that Roman would be able to detect these gaps when he finally opened his eyes to the cosmos.
Novel will shed more light on dark matter
Examining streams of stars hanging from globular clusters won’t be the only dark matter hunt Roman undertakes as he studies the universe.
Scientists think that most, if not all, galaxies are surrounded by halos of this mysterious substance. And these halos are believed to extend far beyond the visible matter content of galaxies. The latter will also be studied by the space telescope, named after NASA’s first Chief of Astronomy, Nancy Grace Roman, affectionately known as “Hubble’s mother.”
Roman will not only be used to investigate the dark matter halo around Andromeda, but will also investigate the possible existence of smaller dark matter “subhalos” around the nearby galaxy.
“We expect smaller dark matter subhaloes to interact with globular cluster fluxes,” Starkenburg said. “If these subhaloes exist in other galaxies, we predict that we will likely see gaps in the globular cluster flows caused by these subhaloes.
“This will give us new information about dark matter, including what types of dark matter halos exist and what their masses are.”
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In addition to his involvement in this work, Starkenburg is already helping lay the groundwork for Roman’s dark matter detective work, with the help of funding provided through NASA’s Nancy Grace Roman Space Telescope Research and Support Participation Opportunities program.
“This team plans to model how globular clusters form into stellar streams by developing a much more detailed theoretical framework,” he said. “We will continue to speculate on the origins of stream-forming globular clusters and whether these flows will be observable with Rome.”
The team’s research is detailed in a preprint article on the arXiv article repository and has been accepted for publication in The Astrophysical Journal.