Images taken 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 spheres of old stellar bodies, called globular clusters, could be influenced by clumps of dark matter.
Until now, astronomers have only been able to study these dangling streams of stars in the Milky Way, meaning our understanding of them is limited. Roman, which will be launched in 2027, should be sensitive enough to see these structures in our neighboring galaxy Andromeda – and with such detail it would be possible to see distortions caused by dark matter, which would give astronomers clues about this elusive substance.
“There are star streams in our own galaxy, where we see gaps that may be due to dark matter,” Tjitske Starkenburg, team member and scientist at Northwestern University, said in a statement. “But these holes can also be formed in other ways.”
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The team claims that as Roman observes holes in galaxies other than the Milky Way, scientists will get a better picture of these holes as a whole. This could ultimately help establish the existence and properties of clumps of dark matter.
Reading between the lines (or rather: the stars)
Dark matter is troubling to scientists because, despite the fact that it makes up an estimated 85% of the matter in the universe, they have very little idea what it is.
Dark matter does not interact with light, meaning it is effectively invisible to our eyes and cannot be made up of atoms made up of the electrons, protons and neutrons that make up the ‘everyday’ matter we are used to. Think of stars, planets, flowers, books. Everything we see with the naked eye – including our bodies – consists of such ‘normal’ matter.
However, dark matter does interact with gravity, and that means the only way scientists can infer its presence is to see how its influence on gravity then affects all that everyday matter and light.
The fact that dark matter interacts with gravity is actually fortunate for the evolution of the universe. For example, some galaxies rotate so quickly that the gravity of their visible matter – stars, gas, dust and planets – would not be enough to keep them from virtually flying apart.
“We are seeing the effect of dark matter on galaxies,” said Christian Aganze, team member and postdoctoral researcher at Stanford University, in the statement. “For example, when we model how galaxies rotate, we need extra mass to explain their rotation. Dark matter can provide that missing mass.”
Clues about dark matter may dangle from globular star clusters, which often contain millions of stars, along with loose streams of stars. That’s because scientists think dark matter can ‘punch holes’ in these stellar streams, creating gaps that can be used to assess the nature of this mysterious form of matter.
“The reason these streams are so interesting to see the effects of these clumps of dark matter is twofold,” said Starkenburg. ‘First, these streams ‘live’ in the very outer regions of a galaxy, where there is otherwise very little structure.
“And second, these streams are intrinsically very thin because they are formed from dense star clusters, which means you can see holes or distortions much more easily.”
This is not a new idea, but it is one that has not yet been fully exploited to get to the bottom of the dark matter problem. Current space telescopes and ground-based instruments are limited to looking for dark matter gaps in a small number of stellar streams hanging from globular clusters in the Milky Way.
From its position about 1.6 million kilometers from Earth, Roman will be able to examine such features in neighboring galaxies, especially Andromeda, for the first time. The wide-angle instrument will produce images 200 times larger than those from the Hubble Space Telescope.
To test this, this team simulated streams of stars and allowed them to interact with clumps of dark matter, creating holes as predicted. The scientists then created fake Roman observations of these dark matter-pierced spaces in star streams. They concluded that Roman will indeed be able to discover these holes when it finally opens its eyes to the cosmos.
Novel will shed more light on dark matter
The study of streams of stars hanging from globular clusters won’t be the only hunt for dark matter that Roman tackles as he studies the universe.
Scientists believe that most, if not all, galaxies are shrouded in halos of this mysterious matter. And these haloes are thought to extend far beyond the visible matter content of the galaxies. The latter will also be studied by the space telescope named after NASA’s first head of astronomy, Nancy Grace Roman, affectionately known as the “mother of Hubble.”
Novel will be used not only to investigate the dark matter halo around Andromeda, but also the possible existence of smaller dark matter ‘sub-halos’ around the nearby galaxy.
“We expect that smaller dark matter sub-halos will interact with globular cluster currents,” said Starkenburg. ‘If these sub-halos are present in other galaxies, we predict that we will see gaps in the streams of globular clusters that are likely caused by these sub-halos.
“That will give us new information about dark matter, including what types of dark matter haloes are present and what their masses are.”
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In addition to his involvement in this study, Starkenburg is already helping lay the groundwork for Roman’s dark matter detective work with funding through NASA’s Nancy Grace Roman Space Telescope Research and Support Participation Opportunities program.
“That team plans to model how globular clusters form into stellar streams by developing a much more detailed theoretical framework,” she said. “We continue to predict the origins of current-forming globular clusters and whether these currents will be observable with Roman.”
The team’s research is detailed in a preprint paper in the paper repository arXiv and has been accepted for publication in The Astrophysical Journal.