The first stars in the universe formed out of pristine hydrogen and helium clouds, in the first few hundred million years after the Big Bang. New James Webb Space Telescope (JWST) observations reveal that some of the first stars in the universe could have been very different from regular (nuclear fusion-powered) stars, which have been observed and catalogued by astronomers for millennia. A recent study led by Cosmin Ilie, at Colgate University, in collaboration with Shafaat Mahmud (Colgate ’26), Jillian Paulin (Colgate ’23) at UPenn, and Katherine Freese, at The University of Texas at Austin, identifies four extremely distant objects which are consistent, both from the point of view of their observed spectra and morphology, with being supermassive dark stars.
“Supermassive dark stars are extremely bright, giant, yet puffy clouds made primarily out of hydrogen and helium, which are supported against gravitational collapse by the minute amounts of self-annihilating dark matter inside them,” Ilie said. Supermassive dark stars and their black hole remnants could be key to solving two recent astronomical puzzles: i. the larger than expected extremely bright, yet compact, very distant galaxies observed with JWST, and ii. the origin of the supermassive black holes powering the most distant quasars observed.
Freese developed the original theory behind dark stars with Doug Spolyar and Paolo Gondolo. They published their first peer-reviewed paper on this theory in the journal Physical Review Letters in 2008. In that paper, they envisioned how such dark stars might have led to supermassive black holes in the early universe. In a 2010 Astrophysical Journal publication, Freese, Ilie, Spolyar, and collaborators identified two mechanisms via which dark stars can grow to become supermassive, and predicted that they could seed the supermassive black holes powering many of the most distant quasars in the Universe.
Although dark matter makes up about 25% of the universe, its nature has eluded scientists. It is now widely believed that dark matter consists of a new type of elementary particle, yet to be observed or detected. While the hunt to detect such particles has been on for a few decades, no conclusive evidence has been found yet. Among the leading candidates for dark matter are Weakly Interacting Massive Particles. When they collide, these particles would theoretically annihilate themselves, depositing heat into collapsing clouds of hydrogen and converting them into brightly shining dark stars.
The conditions for the formation of dark stars were just right a few hundred million years after the Big Bang, and at the center of dark matter halos. This is when, and where the first stars in the universe are expected to have formed.
“For the first time we have identified spectroscopic supermassive dark star candidates in JWST, including the earliest objects at redshift 14, only 300 Myr after the Big Bang,” said Freese, the Jeff and Gail Kodosky Endowed Chair in Physics and director of the Weinberg Institute and Texas Center for Cosmology and Astroparticle Physics at UT Austin. “Weighing a million times as much as the Sun, such early dark stars are important not only in teaching us about dark matter but also as precursors to the early supermassive black holes seen in JWST that are otherwise so difficult to explain.”
In a 2023 PNAS study by Ilie, Paulin, and Freese, the first supermassive dark star candidates (JADES-GS-z13-0, JADES-GS-z12-0, and JADES-GS-z11-0) were identified using photometric data from JWST’s NIRCam instrument. Since then, spectra from JWST’s NIRSpec instrument became available for those, and a few other extremely distant objects. The team, which now also includes Shafaat Mahmud analyzed the spectra and morphology of four of the most distant objects ever observed (including two candidates from the 2023 study): JADES-GS-z14-0, JADES-GS-z14-1, JADES-GS-13-0, and JADES-GS-z11-0 and found that each of them is consistent with a supermassive dark star interpretation.
JADES-GS-z14-1 is not resolved, meaning it is consistent with a point source, such as a very distant supermassive star would be. The other three are extremely compact, and can be modeled by supermassive dark stars powering a nebula (i.e. ionized H and He gas surrounding the star). Each of the four objects analyzed in this study is also consistent with a galaxy interpretation, as shown in the literature. Dark stars have a smoking gun signature, an absorption feature at 1640 Angstrom, due to the large amounts of singly ionized helium in their atmospheres. And in fact, one of the four objects analyzed shows signs of this feature.
“One of the most exciting moments during this research was when we found the 1640 Angstrom absorption dip in the spectrum of JADES-GS-z14-0. While the signal to noise ratio of this feature is relatively low (S/N~2), it is for the first time we found a potential smoking gun signature of a dark star. Which, in itself, is remarkable,” Ilie said.
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) measured the spectrum of the same object, revealing the presence of oxygen, via a nebular emission line. Researchers said that if both spectral features are confirmed, the object cannot be an isolated dark star, but rather may be a dark star embedded in a metal rich environment. This could be the outcome of a merger, where a dark matter halo hosting a dark star merges with a galaxy. Alternatively, dark stars and regular stars could have formed in the same host halo, as the researchers now realized it is possible.
The identification of supermassive dark stars would open up the possibility of learning about the dark matter particle based on the observed properties of those objects, and would establish a new field of astronomy: the study of dark matter-powered stars. This published PNAS research is a key step in this direction.
Funding Acknowledgements: This research was made possible by generous funding from the following agencies: Colgate University Research Council, The Picker Interdisciplinary Sciences Institute, the U.S. Department of Energy’s Office of High Energy Physics program, Swedish Research Council, LSST Discovery Alliance, the Brinson Foundation, the WoodNext Foundation, and the Research Corporation for Science Advancement Foundation.