New Roman Space Telescope Survey to Map Cosmic Dawn and Galaxy Growth

The launch of NASA’s Nancy Grace Roman Space Telescope on Aug. 30 has opened a new chapter in observational cosmology, and one of its flagship early programs will be co-directed from the University of Kansas. Gregory Rudnick, professor of physics and astronomy at KU, will serve as co-leader of GRACE, the Grism Reionization and Cosmic Evolution Survey, one of five general astrophysics surveys selected by NASA from a pool of fifty competing proposals. The project represents the largest of the accepted surveys and will be supported by 1.2 million dollars in NASA funding, resources that will sustain data analysis, a new postdoctoral researcher, and undergraduate involvement at Kansas for years to come.

Roman, as astronomers call the observatory, is designed from the ground up for speed and breadth. Rudnick described it as a thousand times faster than the Hubble Space Telescope at taking data over large portions of the sky. That advantage comes from Roman’s combination of a sensitive wide-field camera and a spectrographic mode that can capture the light of many objects at once. Where Hubble painstakingly stares at narrow patches of sky, Roman can sweep vast regions in a fraction of the time, making it the ideal instrument for a survey that aims to chart the distant universe on a scale never attempted before.

The GRACE team, assembled with colleagues at NASA’s Goddard Space Flight Center and a large group of international researchers, proposed to observe for 420 hours with Roman, covering an area of sky ten times the size of the full moon. Within that enormous footprint, the survey will take spectra of every galaxy by exposing for 40 hours at each location. Spectroscopy splits the light from each galaxy into its individual colors, and from those split spectra astronomers can extract a remarkable range of physical information: how fast the galaxy is receding, what chemical elements it contains, how vigorously it is forming new stars, and how its stars are arranged into a coherent shape.

The payoff will be a detailed three-dimensional map of the universe covering hundreds of thousands of galaxies. For each of them, GRACE team members plan to measure precise properties including distances, the mix of elements within them, the rate at which they form new stars, and their detailed morphologies. Rudnick emphasized that this combination of ultrasensitive spectroscopy over such a large area of sky is unprecedented and is expected to revolutionize knowledge of the distant universe. Because light from remote galaxies takes billions of years to reach Earth, the map will function as a time machine, showing astronomers how galaxies looked when the universe was far younger than it is today.

For Sangeeta Malhotra, a supervisory research astrophysicist at Goddard and co-principal investigator of GRACE, the mission is the culmination of a dream more than two decades in the making. She recalled that the team started imagining a project of this kind in 2005, as soon as they saw what Hubble could do for spectra of the earliest galaxies. From that point forward, she said, the group did not merely dream but worked with colleagues to build the scientific case for the wide-field space telescope that was ultimately realized as Roman. Now that the mission has launched, the team is looking forward to seeing the first data from GRACE arrive.

The survey’s observations are aimed at two pivotal epochs in cosmic history that astronomers refer to as cosmic dawn and cosmic noon. Rudnick leads the portion of the program focused on cosmic noon, a name he acknowledged can be misleading. The universe is 13.8 billion years old, and cosmic noon actually spans the interval from roughly 2 to 6 billion years after the Big Bang, nowhere near the midpoint of cosmic time. The label refers instead to what happened during that era: it was the period when galaxies across the universe were forming stars most rapidly. In that sense, cosmic noon marks the peak epoch of galaxy formation, the moment when the stellar output of the cosmos reached its zenith.

Among his multiple science goals with the GRACE data, Rudnick will investigate protoclusters, the earliest large assemblies of galaxies in the universe. In the present-day cosmos, galaxies are distributed across a hierarchy of structures. Galaxy clusters contain hundreds to thousands of galaxies bound together by their own gravity, moving around within a common gravitational well. Galaxy groups hold tens to hundreds of members. Beyond the groups and clusters, galaxies line filaments, long threads of matter that connect these systems into the vast network cosmologists call the cosmic web. The clusters and groups sit at the nodes, the intersections of that web, while still other galaxies drift in complete isolation, far from any neighbors.

A central question for Rudnick is how galaxies are shaped by the environments in which they live. Protoclusters contain all the galaxies that will eventually end up in a present-day galaxy cluster, but at these distant epochs they are spread out over very large areas of the sky. Observing a nearby cluster is comparatively easy because its member galaxies sit close together, but catching a cluster in the act of assembling requires the ability to study galaxies in exquisite detail across a wide field, which is precisely what Roman provides. Rudnick also hopes to learn how these structures connect with one another and how filaments tie the densest regions of the universe together. By his assessment, GRACE will be by far the biggest systematic survey to assess the cosmic web in the distant universe.

The environmental question extends to the striking differences between galaxies in crowded and empty regions today. In sparse, almost rural environments, galaxies tend to be spirals: rich in gas, actively forming stars, and rotating in an orderly fashion like a Frisbee. In dense regions such as clusters and groups, the picture changes dramatically. There, galaxies are no longer forming new stars, have exhausted much of their gas, and appear as conglomerations of stars moving in random orbits, like bees in a swarm. Astronomers call these elliptical galaxies because they look like fuzzy elliptical blobs on the sky, and their reddish color betrays a population dominated by old stars. What remains unknown is why galaxies in these different settings look so different, and by probing the earliest times at which galaxies could have been influenced by their surroundings, GRACE is positioned to make fundamental advances on that problem.

The practical timeline is already taking shape. Rudnick expects the first data from the Nancy Grace Roman Space Telescope to arrive a few months after commissioning, with the first GRACE-specific observations coming within the first year of operations. The impact will reach well beyond the professional community. NASA’s support will fund a postdoctoral researcher at Kansas, undergraduates at the university will work directly with data from the cutting-edge observatory, and Rudnick plans to incorporate the survey into a long-running outreach program at Lawrence High School. He noted that the program could define his scientific direction for the next five to ten years, and he highlighted what he considers one of the most meaningful aspects of GRACE: bringing data from NASA’s next flagship observatory to Kansas, giving researchers in the state first access to game-changing research on how the universe built its galaxies.

Subject of Research: Galaxy evolution and the cosmic web during cosmic noon, studied with the Nancy Grace Roman Space Telescope's GRACE survey

Article Title: Researcher will examine galactic evolution with new ‘Roman’ space telescope

Article References: Researcher will examine galactic evolution with new ‘Roman’ space telescope. (n.d.). Original publication

Image Credits: AI Generated

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Keywords: Nancy Grace Roman Space Telescope, GRACE survey, galaxy evolution, cosmic noon, cosmic dawn, protoclusters, cosmic web, spectroscopy, NASA, Goddard Space Flight Center, University of Kansas, 3D galaxy map