“Cosmic Lighthouses” – Webb reveals the secrets of the universe's first light

Pioneering James Webb Space Telescope observations reveal the pivotal role of low-mass galaxies in the reionization of the early universe, challenging existing theories of cosmic evolution.

Scientists working with data from NASA's James Webb Space Telescope (JWST) have obtained the first complete spectra of some of the oldest starlight in the universe. The images provide the clearest picture yet of very low-mass newborn galaxies, which formed less than a billion years after the Big Bang, and suggest that young galaxies have a central role in the cosmic origin story.

The international team of researchers, including two Penn State astrophysicists, recently published their findings in the journal nature. The spectra reveal some of the first visible light from a period in the universe known as reionization, which was fueled by the arrival of the first stars and galaxies.

Deep-field images taken by NASA's James Webb Space Telescope have provided the first glimpses of extremely faint galaxies that researchers have identified as strong candidates for objects that sparked the reionization of the universe. Image source: Hakim Atiq/Sorbonne University/JWST

The Primordial Universe: The Transition from Darkness to Light

Normal matter in the universe began as a hot, dense fog composed almost entirely of hydrogen and helium nuclei, explained Joel Lyga, an assistant professor of astronomy and astrophysics at Penn State and an author on the paper. As it expanded and cooled, individual protons and electrons began to bond, forming neutral hydrogen for the first time. About 500 to 900 million years later the great explosionNeutral hydrogen – which prevailed in the early universe – began to separate again into ionized gas, stimulating the formation of stars and galaxies and lifting the primordial fog so that light could travel unhindered through the universe for the first time.

“Something was triggered and started pumping very high-energy photons into the void between galaxies,” Lyga said. “These sources acted like cosmic beacons that burned up the neutral hydrogen fog. Whatever this was, it was so energetic and persistent that the entire universe became ionized again.”

Galaxy pioneers: the role of low-mass galaxies

By analyzing the spectra of small, low-mass galaxies, scientists demonstrated that small galaxies were strong candidates for the “thing” that reionized the universe by heating the dense primordial gas around them and ionizing the previously neutral hydrogen.

“If other low-mass galaxies in the universe are as common and vibrant as these, then we think we've finally arrived at the beacons that burned off the cosmic fog,” Lyga said. “They were incredibly active stars in many, many small galaxies.”

Leija added that the majority of galaxies in the early universe are expected to be relatively small, which makes studying their frequency and properties very difficult. Thanks to a technological breakthrough made possible by the unique combination of the sensitivity of the James Webb Space Telescope and the gravitational lensing effect of the Abell cluster 2744 — nearby galaxies that act like cosmic magnifiers, distorting space and amplifying the light of background galaxies — it is now possible to determine the abundance of small galaxies and their ionizing properties over a billion years. The first time in the universe.

“We found that small galaxies outnumber massive galaxies by about a hundred to one during this epoch of reionization of the universe,” Hakim Atiq, an astrophysicist at the Sorbonne University, a researcher at the Paris Institute of Astrophysics and first author of the paper, said in a press release. “These new observations also reveal that these small galaxies produced a large amount of ionizing photons, exceeding four times the fundamental values ​​normally assumed for distant galaxies. This means that the total flux of ionizing photons emitted by these galaxies far exceeds the threshold required for reionization.”

Charting cosmic evolution: future directions

The Penn State team led the modeling for the UNCOVER survey, which targeted the large foreground galaxy cluster that imaged smaller, more distant galaxies. Researchers at Penn State analyzed all the tiny points of light in the scan to understand the object's properties as well as its mass and likely distances. This analysis was then used to guide the more detailed JWST observations that led to this discovery, Lija explained.

Prior to these results, there were a number of hypotheses that identified other sources responsible for cosmic reionization, such as supermassive black holes; Large galaxies with a mass greater than a billion solar masses; And small galaxies with a mass of less than a billion solar masses. The researchers said that confirming the hypothesis regarding low-mass galaxies has proven particularly difficult, given their low brightness, but the new results provide the clearest evidence yet that low-mass galaxies played a central role in reionizing the universe.

The researchers now want to expand the study to a larger scale to ensure that the specific location they analyzed represents the average distribution of galaxies in the universe. Beyond the reionization process, their observations provide insight into the process of early star formation, how galaxies arose from primordial gas, and how they evolved into the universe we know today.

Reference: “Most of the photons that reionized the universe came from dwarf galaxies” by Hakim Atiq, Ivo Lappé, Lukas J. Sedona H. Price, Pratika Dayal, Adi Zitrin, Vasily Kokorev, John R. Weaver, Gabriel Brammer, Peter van Dokkum, Christina C. Williams, Sam E. Cutler, Robert Feldman, Yoshinobu Fudamoto, Jenny E. Green, Joel Leija, Michael V. Maceda, Adam Muzin, Richard Pan, Casey Papovich, and Erika J. Nelson, Themia Nanayakkara, and Daniel B. Stark, Mauro Stefanone, and Katherine A. Suss, Bingjie Wang, and Catherine E. Whitaker, February 28, 2024, nature.
doi: 10.1038/s41586-024-07043-6

Bingyi Wang, a postdoctoral researcher in astrophysics, is the other co-author of the study from Penn State. A full list of authors and their institutions is available on the published paper. The researchers acknowledge funding and support from the French National Center for Space Studies, the National Program for Cosmology and Galaxies, the CEA, the Cosmic Dawn Centre, the Danish National Research Foundation, the Australian Research Council, NOW, the Rosalind Franklin Program of the European Commission Common Fund and the University of Groningen. US-Israel National Science Foundation, US National Science Foundation (NSF), Ministry of Science and Technology, Israel and NOIRLabIt is managed by the Consortium of Universities for Research in Astronomy under a cooperative agreement with NSF.