On Wednesday, Charlie Bonkowsky attended “Decoding Dark Matter with Stellar Streams from Beyond the Milky Way,” a colloquium presented by Dr. Sarah Pearson, a Hubble Fellow at NYU.

Last week, we learned about how dwarf galaxies have provided insight into the formation of chemical elements in the universe. But it turns out dwarf galaxies are surprisingly valuable in a range of astrophysical research, including, as Dr. Sarah Pearson spoke about, discovering what dark matter is made of.

Dark matter makes up about 80% of the mass in the universe. We know it’s there because of its gravitational effect on galaxies and other large structures in the universe—but nobody’s sure what it could be composed of, what strange new particle might have such properties. The particle physicists are searching, from direct-detection experiments like DAMA in Italy to attempts at particle colliders to spot the telltale energy signals of potential new particles—but they haven’t gotten anywhere so far.

Astrophysicists can place constraints on the amount of dark matter in the universe by observing its gravitational effects. But detailed observations can also potentially reveal what dark matter might be made of, Pearson said. Dark matter forms different kinds of “clumps” or “halos” around galaxies and star clusters depending on what kind of particle it’s composed of: large, cold particles (known as WIMPs) or smaller, warmer particles. But the problem is—we can’t see these haloes, nor the complex gravitational substructures and sub-haloes within that would more precisely tell us their properties.

Enter stellar streams. Streams are formed from dwarf galaxies or globular clusters which have been pulled apart by tidal forces and stretched into long trails millions of light-years in length.

Animation of a globular cluster becoming disrupted and forming a stellar stream. Note the long leading (red) and trailing (blue) arms.

Stellar streams are useful because they tell us a lot about stellar motion around galaxies. It takes our Sun 230 million years to orbit once around the center of the Milky Way—not something that can be observed in any human lifespan. But stellar streams offer a snapshot of an entire orbit, wrapped around a galaxy.

There are two types of stellar streams: those formed from dwarf galaxies, pulled apart when they fall into the gravitational radius of a larger galaxy; and those formed from globular clusters, star groupings that form within a galaxy and then are torn apart by it. Dwarf-galaxy streams tend to be larger, but also more energetic, meaning they don’t remain intact as long; globular-cluster streams, on the other hand, can remain intact for billions of years. They hold their structure so well, in fact, that it’s possible to trace the effect of small gravitational disruptions, such as that of dark matter subhalos.

Great news, right? Well, not so fast. We’ve only spotted these globular-cluster streams in the Milky Way so far, since they’re faint and often require clever data handling to reconstruct, picking out stars that all have the same motion and momentum. But the Milky Way isn’t the perfect laboratory to analyze these streams for signs of dark matter disruptions: the spiral arms of the galaxy or scattered molecular clouds can also form gaps in the stream.

So we should start looking beyond the Milky Way. And with an era of new space telescopes, especially the Nancy G. Roman telescope, set to launch in 2026, Pearson said, we’re “about to enter a very exciting era in stellar stream science.”

The Roman telescope will see just as far as the Hubble Space Telescope and with 100 times its field of view—perfect for spotting the long stretch of stellar streams. To prepare for it, Pearson and her colleagues have been developing techniques that will allow them to spot and analyze stellar streams in distant galaxies. She talked about the Hough Stream Spotter, a systematic way to look through thousands of galaxies and find stellar streams. It’s a mathematical technique in which the (comparatively) straight line of stellar streams can be recovered by analyzing the position data of stars and finding at what angles stars are most concentrated. [The specific formulas are also above my paygrade—check out Pearson’s paper on it for more info.]

One of the more interesting stellar streams in the Milky Way is the Palomar 5 stream, which shows clear signs of having been disrupted throughout its history (though it’s still unknown whether dark matter sub-haloes, the Milky Way’s spiral arms, or molecular clouds caused it). Pearson’s simulations showed that the Roman telescope should be able to see similarly-sized streams in almost 500 nearby galaxies, greatly extending the potential of research.

For brighter streams, like dwarf-galaxy streams, work is already being done. In 2022, Pearson published research on an extended stellar stream surrounding the Centaurus A galaxy. By carefully measuring its orbit and the radial velocity of the stars, she and her team were able to place a lower limit on the mass of the dark matter halo at 4.7 * 10^12 stellar masses.

Stellar streams around Centaurus A.

As with any research, there are caveats. The data from more distant galaxies will be extremely valuable, but not quite as precise as that from near the Milky Way. Most of the nearby galaxies these telescopes allow us to study are dwarf galaxies, which may have different dark matter haloes and substructures than those around spiral galaxies like the Milky Way or Andromeda. But Pearson was optimistic: “the thousands of streams we’ll find with these three upcoming telescopes,” she said, “will revolutionize the field.”

Stellar Streams and Stream Formation via Adrian Price-Whelan

Centarus A Stream via Sarah Pearson

Header via Adrian Price-Whelan