One of the few galaxies with a well-studied stellar halo is our neighbor, Andromeda, depicted here in this artistic concept. The stellar halo is a common but not well-studied feature of galaxies. This loose collection of stars extends 15 to 20 times beyond the radius of the brightest part of the galaxy, which is what we’re used to seeing in telescope images. The stars comprising a halo are some of the oldest in a galaxy.NASA/Ralf Crawford
The universe is a dynamic place where galaxies are dancing, merging and shifting appearance. Unfortunately, because these changes take millions or billions of years, telescopes can only provide snapshots, squeezed into a human lifetime.
Luckily, galaxies leave behind clues to their histories and origins. NASA’s upcoming will have the capacity to look for these “fossils” of galaxy formation by conducting high-resolution imaging of galaxies in the nearby universe.
Through a grant from NASA, astronomers are designing a set of possible observations called RINGS – the Roman Infrared Nearby Galaxies Survey – that would collect these images, and the team is producing publicly available tools that the astronomy community can use once Roman launches and starts collecting data.
“Roman is the next flagship NASA mission, and it will provide a treasure trove of new data for unraveling the evolutionary histories of galaxies,” said RINGS principal investigator , a University of Washington research associate professor of astronomy.
Roman is uniquely prepared for RINGS due to its resolution, which is akin to NASA’s Hubble Space Telescope, as well as 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.
Galaxies leave behind “hints” about how they evolved, embedded in their stellar structures – similar to how living 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 complex chemical makeup of the galaxy when those stars formed.
Such cosmic fossils are of particular interest to , the deputy principal investigator of RINGS and an assistant professor at the University of Pennsylvania. 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 it. These large-scale structures, which Roman is uniquely capable of capturing, can give clues to a galaxy’s merger history. The goal is to “reassemble these fossils in order to look back in time and understand how these galaxies came to be,” said Sanderson.
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. Scientist cannot currently directly detect dark matter and do not know what it consists of. Yet there exists a particularly useful class of objects for testing dark matter theories: ultra-faint dwarf galaxies.
“Ultra faint dwarf galaxies are so dark matter-dominated that they have very little normal matter for star formation,” said , professor at the University of California, Santa Cruz. “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%.
Ultra-faint galaxies are far from the only test of dark matter. Often, just looking in an average-sized galaxy’s backyard is enough. Structures in the halo of stars surrounding a galaxy often give hints to the amount of dark matter present. But, due to the sheer size of galactic halos – they are often 15-20 times as big as the galaxy itself – current telescopes are deeply inefficient at observing them.
At the moment, the only fully resolved galactic halos scientists have to go on are our own Milky Way and Andromeda, our neighbor galaxy.
“We only have reliable measurements of the Milky Way and Andromeda, because those are close enough that we can get measurements of a large number of stars distributed across their stellar halos,” said Williams. “So, with Roman, all of a sudden we’ll have 100 or more of these fully resolved galaxies.”
When Roman launches by May 2027, it is expected to fundamentally alter how scientists understand galaxies. In the process, it will shed some light on our own home galaxy. The Milky Way is easy to study up close, but we do not have a large enough selfie stick to take a photo of our entire galaxy and its surrounding halo. RINGS shows what Roman is capable of should such a survey be approved. By studying the nearby universe, RINGS can examine galaxies similar in size and age to the Milky Way, and shed light on how we came to be here.
