NASA's Roman Space Telescope to Investigate Galactic Fossils

Artist’s concept of spiral galaxy at the center of a much larger spherical halo of stars, with ground-based photograph pulled out to show details of the central core and spiral arms. View of the galaxy is halfway between face-on and edge-on, with disk oriented 45 degrees counter-clockwise from horizontal. Artist’s Concept: Galaxy core is whitish yellow, circled by a brownish purple disk. Grainy white halo enveloping the disk is dense and bright near the center, becoming more diffuse and fading out with distance from the main disk. Halo is mottled with a wispy cloud-like appearance, suggesting variations in density of stars. Halo covers an area of sky about 250 times greater than main disk. Photo: Telescope zoom into galaxy core and disk, outlined in yellow, shows brown dust lanes tracing spiral arms that wrap clockwise around fuzzy core; cloud-like patches of blue and red; star-like points of light of various size and brightness. Galaxy is 45 times larger in zoom than in illustration. — Halo of the Andromeda Galaxy (Illustration) The stellar halo is illustrated here with exaggerated brightness and density to show how far it extends. In reality, the full stellar halo is too dim and sparse to examine without painstaking hours of observation time on the sharpest resolution telescopes available. When the Nancy Grace Roman Space Telescope launches, it will be able to use its wide field of view to comprehensively image many more stellar halos of more distant galaxies. Illustration: NASA, Joseph Olmsted (STScI)

The universe is a dynamic, ever-changing place where galaxies are dancing, merging together, and shifting appearance. Unfortunately, because these changes take millions or billions of years, telescopes can only provide snapshots, squeezed into a human lifetime.

However, galaxies leave behind clues to their history and how they came to be. NASA’s upcoming Nancy Grace Roman Space Telescope will have the capacity to look for these fossils of galaxy formation with high-resolution imaging of galaxies in the nearby universe.

Astronomers, through a grant from NASA, are designing a set of possible observations called RINGS (the Roman Infrared Nearby Galaxies Survey) that would collect these remarkable images, and the team is producing publicly available tools that the astronomy community can use once Roman launches and starts taking data. The RINGS survey is a preliminary concept that may or may not be implemented during Roman’s science mission.

Roman is uniquely prepared for RINGS due to its resolution akin to NASA’s Hubble Space Telescope and its wide field of view – 200 times that of Hubble in the infrared – making it a sky survey telescope that complements Hubble’s narrow-field capabilities.

Galactic Archaeologists

Scientists can only look at brief instances in the lives of evolving galaxies that eventually lead to the fully formed galaxies around us today. As a result, galaxy formation can be difficult to track.

Luckily, galaxies leave behind hints of their evolution in their stellar structures, almost like how organisms on Earth can leave behind imprints in rock. These galactic “fossils” are groups of ancient stars that hold the history of the galaxy’s formation and evolution, including the chemistry of the galaxy when those stars formed.

Such cosmic fossils are of particular interest to Robyn Sanderson, the deputy principal investigator of RINGS at the University of Pennsylvania in Philadelphia. She describes the process of analyzing stellar structures in galaxies as “like going through an excavation and trying to sort out bones and put them back together.”

Roman’s high resolution will allow scientists to pick out these galactic fossils, using structures ranging from long tidal tails on a galaxy’s outskirts to stellar streams within the galaxy. These large-scale structures, which Roman is uniquely capable of capturing, can give clues to a galaxy’s merger history. The goal, says Sanderson, is to “reassemble these fossils in order to look back in time and understand how these galaxies came to be.”

Shedding Light on Dark Matter

RINGS will also enable further investigations of one of the most mysterious substances in the universe: dark matter, an invisible form of matter that makes up most of a galaxy’s mass. A particularly useful class of objects for testing dark matter theories are ultra-faint dwarf galaxies. According to Raja GuhaThakurta of the University of California, Santa Cruz, “Ultra faint dwarf galaxies are so dark matter-dominated that they have very little normal matter for star formation. With so few stars being created, ultra-faint galaxies can essentially be seen as pure blobs of dark matter to study.”

Roman, thanks to its large field of view and high resolution, will observe these ultra-faint galaxies to help test multiple theories of dark matter. With these new data, the astronomical community will come closer to finding the truth about this unobservable dark matter that vastly outweighs visible matter: dark matter makes up about 80% of the universe’s matter while normal matter comprises the remaining 20%.