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How do black holes in the centers of galaxies form and grow over time? To answer this question, scientists need to detect and study supermassive black holes at great distances, which existed much earlier in the history of the universe. New research suggests that NASA’s Nancy Grace Roman Space Telescope, which is on track to launch on August 30, 2026, will be able to detect these ancient, distant black holes that existed up to 11 billion years ago.

The best way to study black holes is to look for the light emitted by their accretion disk: the matter that rotates around them before being consumed. Lighter supermassive black holes are difficult to observe because they tend to be less luminous due to less accretion. But occasionally, they destroy and consume an entire star, shining until it eclipses its entire host galaxy, known as a tidal disruption event (TDE). By characterizing that population of early supermassive black holes and how they evolved and grew over billions of years, Roman will provide clues to the ultimate origin of these giants.

“The Roman Space Telescope is going to be transformative for transient science,” said lead author Mitchell Karmen of Johns Hopkins University, a graduate student and National Science Foundation researcher. “Thanks to Roman’s high sensitivity, we can find multiple tidal disturbance events at greater distances and earlier cosmic times than ever before.”

An article about this research published Tuesday in The Astrophysical Journal.

Roman’s high-latitude Time-Doman survey, one of the three major community surveys, is particularly well-suited for finding and studying TDE in the early universe. This survey will cover about 18 square degrees of sky, an area equivalent to 90 full moons, at a regular cadence. By revisiting the same regions repeatedly, astronomers can find a large number of transient events like TDEs.

Tidal disruption events are phenomena unique to lighter supermassive black holes. The largest black holes, weighing more than a billion suns, will swallow incoming stars whole. But lighter black holes, between 100,000 and 100 million suns in size, can tear apart a star before consuming it, creating a beacon that illuminates for a couple of weeks before gradually fading.

The TDE rate fluctuates over cosmic time. Previous work predicted that the TDE rate would decrease with increasing distance because most young black holes were too light to generate a TDE. However, this new research takes into account numerous factors that evolve over time, such as the frequency of galaxy mergers (and therefore black holes), as well as the number of stars within the core of each galaxy and their density.

Karmen and his colleagues modeled these and other effects to predict how many tidal disruption events Roman could observe, as well as other observatories such as the ground-based National Science Foundation Department of Energy Vera C. Rubin Observatory and NASA’s James Webb Space Telescope. The team predicts that astronomers will see the rate of TDE increase as Roman explores greater distances and earlier times until “cosmic noon,” about 11 to 12 billion years ago, when star formation peaked across the universe, before slowing again.

Roman will observe near-infrared wavelengths of light. Light from distant TDEs is stretched to longer wavelengths due to the expansion of the universe, a phenomenon known as cosmological redshift. As a result, Roman is inherently optimized to detect TDE whose light traveled between 8 billion and 11 billion years to reach us.

The Rubin Observatory will also scan large swaths of the sky and detect many new TDEs. However, it will observe visible light, which limits it to closer TDEs than Roman.

Karmen’s team’s research finds that Rubin will detect between thousands and tens of thousands of TDEs per year. While Roman is expected to find up to 100 TDEs per year, those black holes will be much more distant, within the realm of cosmic history that is most important for distinguishing between black hole origin scenarios.

“Simply counting the number of TDEs as a function of redshift can place significant constraints on the population of black holes of millions of solar masses,” said co-author Suvi Gezari, an associate professor of astronomy at the University of Maryland. “Roman will be transformative in the sense that it can investigate tidal disturbance events at greater distances, so that the EDT rate can be observed to evolve over time.”

Astronomers have observed truly gigantic black holes very early in the history of the universe, so early that theories struggle to explain how they could have gotten so big so quickly. They must have started smaller and grown over time, but how much smaller?

One theory, known as “light seeds,” begins with black holes being created from the death of massive stars. These black holes could weigh up to a few hundred times our Sun. These black holes would then merge over time and consume the surrounding gas at an astonishing rate. In this scenario, every young galaxy would be expected to have a massive black hole at its center.

A second theory, known as “heavy seeds,” suggests that a black hole with a much larger mass, up to a million times that of the Sun, could be born through a process such as the direct collapse of a gas cloud. However, this process should be less common, resulting in supermassive black holes being much rarer in early galaxies.

“Tidal disturbance events help us probe the population of light supermassive black holes, which can help us discriminate between these models,” Karmen said.

Ultimately, Roman’s account of tidal disturbance events will help researchers track the global effects affecting the black hole population over time.

Once Roman and Rubin begin regular science operations, the team hopes to compare their forecasts with the actual detections those observatories make.

“Just as Webb has transformed our understanding of distant high-redshift galaxies, Roman is poised to transform our understanding of high-redshift transients,” Gezari said.

The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation from NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Baltimore Space Telescope Science Institute; and a scientific team made up of scientists from various research institutions. Primary industrial partners are BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

By Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland.

Media Contact:

Claire Andreoli
NASA Goddard Space Flight CenterGreenbelt, Maryland.
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