Dark matter study of dwarf galaxies hints at a ‘signal’

• Astronomers think there’s about 6 times more dark matter in our universe than ordinary matter. But dark matter has never been observed directly. We only believe it exists due to its gravitational pull.
• So astronomers are observing nearby, dark-matter-rich dwarf galaxies, searching for a “signal” from dark matter. They believe that a “signal” should exist in the form of gamma rays, resulting from dark matter annihilation or decay.
• Their results place new constraints on the properties of dark matter and hint that a signal might be confirmed in the future.

Clemson University published this original article by Cindy Landrum on March 19, 2024. Edits by EarthSky.

The scientists published their detailed findings in the peer-reviewed journal Physical Review D on March 19, 2024.

New research in dark matter

Dark matter is one of science’s greatest mysteries. It doesn’t absorb, reflect or emit light, so we can’t see it. However, its presence is implied by the gravitational effects it appears to have on galaxies. Although dark matter – combined with dark energy – makes up about 85% of the cosmos, scientists know very little about its fundamental nature. Now, new research by Clemson University postdoctoral fellow Alex McDaniel provides some of the most stringent constraints on the nature of dark matter yet.

In addition, his research also reveals a small hint of a signal that, if real, could be confirmed sometime in the next decade or so. McDaniel said:

With data collection and new discoveries in the future, this small hint could potentially turn into a very concrete detection of a dark matter model.

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The mystery of dark matter

Indeed, detecting dark matter would be groundbreaking. Marco Ajello, an associate professor in the Clemson Department of Physics and Astronomy and McDaniel’s adviser, said:

Dark matter is one of the most important things in astrophysics, and we know next to nothing about it. Discovering it will be a tremendous breakthrough. Whoever discovers may win a Nobel Prize. It’s that big.

Dwarf galaxies

In their work, McDaniel and collaborators were searching dwarf galaxies for dark matter that self-annihilates into ordinary matter and gamma rays. Gamma rays are a form of light at the highest energy levels. Moreover, dwarf galaxies are ideal for study because they are small, rich in dark matter and mostly lack other astrophysics phenomena such as gas, dust and supernova that could contaminate the findings. McDaniel explained:

We look for these because, ideally, they give us a clean signal or allow us to rule out certain particle theories.

Some models predict that dark matter has a certain mass or cross section, which is the probability of a specific event occurring due to the interaction of particles. In essence, that would determine what researchers would expect to see in gamma rays. And, if they don’t see that, they can rule out those masses and cross sections, he said.

A hint of a signal

Chris Karwin, a former postdoc at Clemson, was also a co-author of the study. Karwin is now a postdoctoral fellow at the NASA Goddard Space Flight Center. Karwin said:

In this paper, we do more ruling out, saying that dark matter can’t have those masses or cross section. But compared to previous studies, we do start to see a hint of something that might be a signal from these systems.

McDaniel’s study uses the larger samples that include additional discovered dwarf galaxies and larger amounts of data than previous studies. Overall, he included about 50 dwarf galaxies in his study. But McDaniel said that with new, more powerful telescopes coming online in the near future, he expects that number to increase to 150-200. McDaniel said:

The new telescopes are basically dwarf galaxy detectors. We’re projecting with those improvements it’s possible that instead of having just a little hint of a signal, we can have something that’s a bona fide detection.

Ajello added:

If it (the signal) is real, eventually it will be confirmed.

More about the signal

EarthSky asked McDaniel to elaborate on the signal. McDaniel replied:

By ‘hint of a signal,’ we mean that in the data we have found evidence that there is more emission than would be expected without the dark matter, however it is not bright enough for us to distinguish entirely from background noise. For that we’ll need more data.

Think of it as like looking at a very faint star in an area with a lot of light pollution. It can be difficult to distinguish the signal (e.g. the star) from the background. With more data, the signal should become more pronounced, allowing us to claim a concrete detection.

Typically in similar dark matter studies there is no evidence for a signal at all, so the fact that we are starting to see something in the data is quite exciting!

Bottom line: Researchers studied nearby, dark-matter dominated dwarf galaxies. They were looking for gamma rays resulting from dark matter annihilation or decay.

Source: Legacy analysis of dark matter annihilation from the Milky Way dwarf spheroidal galaxies with 14 years of Fermi-LAT data

Via Clemson University