Hubble study overturns supernova winds theory for cosmic reionization

An international team led by University of Michigan researchers used Hubble to study nearby starburst galaxies and found supernova winds prevent ionizing radiation from escaping, directly contradicting the prevailing theory that these winds cleared pathways for radiation to leave early galaxies. The findings, published October 15 in Nature Astronomy, suggest radiation from young stars escaped before supernova explosions generated powerful winds.

The team measured wind speeds by analyzing spectral dips caused when outflowing gas absorbs ultraviolet light at specific wavelengths. Higher wind speeds correlated with lower fractions of escaping ionizing radiation, indicating supernova energy increases neutral gas and dust coverage that blocks rather than facilitates radiation escape. This challenges the assumption that supernova-driven outflows helped ionizing photons reach intergalactic space.

The results support an alternative model where young stars release ionizing photons before supernovae create winds strong enough to reshape galaxies. Cody Carr, a University of Michigan postdoctoral researcher who led the study while at Zhejiang University, stated that the data show ionizing photons leave galaxies before galactic winds develop. This reverses the long-held assumption about how radiation escaped early cosmic environments.

Carr emphasized that astronomers still do not know which early galaxies drove reionization or how the process evolved over time. While the study confirms ionizing radiation escaped early galaxies, it does not pinpoint the exact population responsible for clearing the universe’s opaque fog during its first billion years.

The research involved nearly 30 collaborators from over a dozen institutions, with University of Michigan professor Sally Oey among the contributors. Stephan McCandliss, a senior author from Johns Hopkins University, explained that supernova energy disrupts surrounding gas, increasing the density of neutral material that suppresses radiation escape. Renyue Cen, another senior author from Zhejiang University, noted that supernovae remain central to simulations of radiation escape, but the findings highlight the need to study young-star winds before the first supernovae occurred.

Hubble, a collaboration between NASA and the European Space Agency, enabled the study through observations managed by the Space Telescope Science Institute. The telescope’s unique sensitivity to ultraviolet light allowed researchers to measure escaping radiation and wind speeds in nearby starburst galaxies, which serve as analogues for early-universe systems.

The study’s targets were compact starburst galaxies forming stars at exceptionally high rates, resembling some of the earliest galaxies believed to have contributed to reionization. Spectral measurements revealed how outflowing gas interacts with escaping light, providing insights into the physical conditions that regulate radiation escape. Because the team studied nearby galaxies rather than direct observations of the reionization epoch, the findings test theoretical mechanisms using modern analogues.

Reionization began approximately 13 billion years ago when radiation from early galaxies ionized the neutral hydrogen that filled the cosmos during its first billion years. This process split hydrogen atoms into protons and electrons, transforming the universe from an opaque fog into a transparent medium. Cody Carr described reionization as the last major phase transition in cosmic history, marking the point when the universe became transparent to light.

The study narrows key uncertainties about reionization but does not identify the specific early-galaxy population responsible for the transition or establish a definitive timeline for radiation escape. Researchers plan to request additional Hubble observing time to study more galaxies and determine whether ionizing radiation preferentially escapes before supernova-driven winds fully develop. Smaller-scale winds near young stellar regions may still influence the early escape phase, though further observations are required to confirm this possibility.

Separate Hubble observations published June 23 in The Astrophysical Journal detected ionizing ultraviolet radiation escaping from galaxy MXDFz4.4, which existed 1.4 billion years after the Big Bang. This measurement pushed the observational frontier 200 million years closer to reionization than previous records, placing MXDFz4.4 just 250 million years after the era ended. During the actual reionization period—between 400 million and 1.1 billion years after the Big Bang—the intergalactic medium was thick with neutral hydrogen, absorbing ionizing photons almost instantly and making direct measurements impossible.

MXDFz4.4 spans roughly 100 times less area than the Milky Way but produces stars at a rate about 10 times faster. Its compact core hosts massive young stars generating intense ultraviolet radiation. After just a few million years, these stars exploded as supernovae, creating cavities that allowed ionizing photons to stream into intergalactic space. Scientists estimate that 50 to 100 percent of the galaxy’s ionizing light escaped during the study period, with the escape fraction remaining the critical unknown in understanding reionization.

Hubble detected the galaxy’s redshifted ultraviolet light—originally shorter than 91.2 nanometers—after it traveled 12.37 billion light-years and was stretched by cosmic expansion into visible wavelengths around 490–500 nanometers. The telescope’s Advanced Camera for Surveys captured the signal using stacked deep exposures from long-term surveys of the same sky region. The galaxy’s redshift, precisely measured at z = 4.442 by the European Southern Observatory’s Very Large Telescope, confirmed its placement 1.4 billion years after the Big Bang.

Ilias Goovaerts, lead author of the MXDFz4.4 study, noted that observing such a galaxy was previously considered impossible due to the expectation that neutral hydrogen fog would block ionizing light. However, by the time the universe reached 1.4 billion years of age, the intergalactic medium had thinned enough to allow these photons to pass through. Hubble’s optical sensitivity was uniquely suited to detect this redshifted radiation, providing the first direct evidence of ionizing light escaping from a galaxy so close to the reionization era.

Marc Rafelski, Hubble’s deputy mission head, stated that the telescope’s observations of MXDFz4.4 allowed researchers to test reionization theories closer to the era than ever before. He highlighted that without complementary data from the James Webb Space Telescope—which characterized the galaxy’s older stellar populations and confirmed its bursty star formation—the team could not have drawn their conclusions. Rafelski added that discovering more galaxies like MXDFz4.4 at slightly later cosmic times would help refine measurements of what cleared the universe’s fog as reionization concluded.

Two primary candidates have competed to explain reionization: rare, luminous objects like quasars and active galactic nuclei, which are individually powerful but scarce, and the vast population of small, faint, intensely star-forming galaxies. MXDFz4.4 fits the second category, suggesting that if such galaxies were common, their collective output could have driven the phase transition. The galaxy’s high escape fraction—between 50 and 100 percent—supports the idea that these compact systems played a significant role in clearing the cosmic fog.

Future campaigns using Hubble and the James Webb Space Telescope aim to identify more compact, bursty galaxies with high escape fractions at similar and slightly later cosmic times. A sufficiently large sample could determine whether small galaxies alone powered reionization or if larger, rarer sources contributed significantly. Hubble’s continued observations remain essential, as it is the only observatory capable of directly detecting redshifted Lyman continuum photons from this epoch.

The Carr-led study and the MXDFz4.4 observation address complementary aspects of the same question: how ionizing radiation escaped early galaxies and cleared the universe’s fog. While the nearby galaxy study tests physical mechanisms using modern analogues, MXDFz4.4 provides a direct measurement from a galaxy just 250 million years after reionization ended. Together, these findings reduce uncertainty about the process but leave unresolved which specific populations drove the transition.