The Nancy Grace Roman Space Telescope of NASA will investigate wispy streams of stars that extend far beyond the visible edges of many galaxies. To see these structures around nearby galaxies in full, missions like the Hubble and James Webb space telescopes would have to stitch together hundreds of small images. Roman will accomplish this in a single snap. These observations will be used by astronomers to investigate how galaxies form and the nature of dark matter.
Stellar streams resemble ethereal strands of hair extending outward from some galaxies, drifting peacefully through space as part of the halo – a spherical region surrounding a galaxy. These stellar flyaways, on the other hand, are remnants of an ancient cosmic-scale drama that serve as fossil records of a galaxy’s past. Studying them transforms astronomers into galactic archaeologists.
Previously detected stellar streams formed when the Milky Way siphoned stars from globular star clusters, but never in other galaxies. They’re fainter because there are fewer stars in them, making them much harder to spot in other, more distant galaxies.
Roman may be able to detect them for the first time in several of our neighboring galaxies. Individual stars should be visible in these vast, dim structures thanks to the mission’s wide, sharp, and deep vision. Pearson previously led the development of an algorithm to search for stellar streams originating from globular clusters in nearby galaxies.
As individual stars leak out of the dwarf galaxy and fall into the more massive one, they form long, thin streams that remain intact for billions of years. So stellar streams hold secrets from the past and can illuminate billions of years of evolution.Sarah Pearson
Starkenburg’s new study adds to the picture by predicting that Roman will be able to detect dozens of streams that originated in dwarf galaxies in other galaxies, providing unprecedented insight into how galaxies grow.
“It’s exciting to learn more about our Milky Way, but we need a larger sample size to truly understand galaxy formation and dark matter,” Starkenburg said. “Working with Roman to study stellar streams in other galaxies will help us see the big picture.”
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA’s Jet Propulsion Laboratory and Caltech/IPAC in Southern California, the Space Telescope Science Institute in Baltimore, and a science team comprising scientists from various research institutions. The primary industrial partners are Ball Aerospace and Technologies Corporation in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.
“Halos are mostly made of stars stripped away from other galaxies,” said Tjitske Starkenburg, a postdoctoral fellow at Northwestern University in Evanston, Illinois, who investigated Roman’s potential in this area. “Roman’s wide, deep images will be sharp enough that we will be able to resolve individual stars in the halos of other galaxies, allowing us to study stellar streams in a large number of galaxies for the first time.”
The team, led by Starkenburg, will present their findings at the 240th meeting of the American Astronomical Society in Pasadena, California.
Galactic Cannibalism, Stolen Stars
Simulations support the theory that galaxies grow in part by gobbling up smaller groups of stars. A dwarf galaxy captured into orbit by a larger one becomes distorted by gravity. Its stars drizzle out, tracing arcs and loops around the larger galaxy until they ultimately become its newest members.
“As individual stars leak out of the dwarf galaxy and fall into the more massive one, they form long, thin streams that remain intact for billions of years,” said Sarah Pearson, a Hubble postdoctoral fellow at New York University in New York City and the lead author of a separate study about the mission’s projected observations in this area. “So stellar streams hold secrets from the past and can illuminate billions of years of evolution.”
Astronomers have caught this cannibalistic process in the act using telescopes like ESA’s (European Space Agency’s) Gaia satellite, which is fine-tuned to measure the positions and motions of stars in our Milky Way galaxy. Roman will extend these observations by making similar measurements of stars in both the Milky Way and other galaxies.
The Milky Way is home to at least 70 stellar streams, meaning it has likely eaten at least 70 dwarf galaxies or globular star clusters — groups of hundreds of thousands of gravitationally bound stars. Roman’s Milky Way images could allow astronomers to string together snapshots in time to show stars’ movement. That will help us learn about what dark matter — invisible matter that we can only detect via its gravitational effects on visible objects — is made of.
According to one theory, dark matter is “cold,” or composed of heavy, sluggish particles. If that’s the case, it should clump together within galaxy halos, disrupting stellar streams in ways Roman can see. Roman could narrow down the candidates for what dark matter could be made of by detecting or ruling out these distortions.
Astronomers are also excited to study stellar streams in several Milky Way neighbor galaxies. Because they are so faint and far away, they are not well studied in other galaxies. They’re also so massive that they can encompass an entire galaxy. Images that are both large and detailed enough to see require an unrivaled panoramic view like Roman’s.