A new study by University of Michigan astronomer Cody Carr has found that supernovas may not have driven the Epoch of Reionization as previously thought, and may have dampened the process. The research points to the intense light of newborn stars as the most viable source of the radiation that cleared the cosmic fog.
The findings, published October 15 in the journal Nature Astronomy, are based on observations of six compact starburst galaxies about 3.5 billion light-years away using the Hubble Space Telescope. These galaxies, which are generally smaller than 6,500 light-years in diameter and form around 10 solar masses of stars per year, serve as analogues for the ancient universe.
Around 400,000 years after the Big Bang, the universe cooled sufficiently to form neutral hydrogen atoms, shrouding the cosmos in a fog opaque to ultraviolet light. The Epoch of Reionization refers to the period when this fog was cleared by ionizing radiation, allowing light to pass freely through space.
Carr and colleagues identified a two-phase process in the nearby galaxies. In the first phase, during the first 3 million years of star formation, stellar winds from newly formed massive stars carve through neutral gas and dust at nearly 2,000 miles per second, allowing Lyman continuum (LyC) photons of ionizing radiation to escape the galaxy. In the second phase, after about 3 million years, the most massive stars die in supernovas. Their expelled stellar material adds to the opaque galactic gas and dust, churning it and loading it with mass, creating a trap for LyC light rather than enabling its escape.
The strongest LyC emitters in the study lacked fast galactic winds. "This challenges the notion that galaxies require fast supernova-driven winds to enable LyC escape," Carr told Live Science in an email. The oldest galaxies studied, which had the fastest supernova-generated winds, allowed the least ionizing radiation to escape. "In general, our results are in disagreement with most cosmological simulations, which typically show the LyC escape fraction peaking after the launch of supernova-driven winds," Carr added.
Analysis
Why This Matters
- The study overturns a key assumption about how the universe became transparent to light, shifting the likely driver from supernovas to the first stars.
- It reshapes understanding of early galaxy evolution: supernova-driven winds may have trapped ionizing radiation rather than releasing it.
- Cosmological simulations of the early universe may need to incorporate these findings to accurately model the Epoch of Reionization.
Background
The Epoch of Reionization was the last major phase transition in the universe, when neutral hydrogen was ionized, clearing the cosmic fog. For decades, astrophysicists believed that supernovas from massive stars provided the necessary ionizing radiation to drive this process.
Key Perspectives
Astrophysicists (previous consensus): believed that supernovas and the fast winds they drove were responsible for scattering ionizing radiation, enabling it to escape galaxies and reionize the universe.
Cody Carr and colleagues (University of Michigan): argue that supernovas had the opposite effect, trapping Lyman continuum photons and dampening reionization, and that the intense light of newborn stars is a more likely source.
What to Watch
- Further observational studies seeking additional galactic samples and higher-resolution observations across other wavelengths to confirm the findings.
- Updates to cosmological simulations that currently show LyC escape fraction peaking after supernova-driven winds.
- Potential refinements to the timeline and processes of reionization as new data emerges.
Checked
12 claims in this story checked against 1 source: 12 supported.
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- SupportedThe study challenges the theory that supernovas drove cosmic reionization.Latest from Live Science: “In other words, supernovas didn't drive the Epoch of Reionization, but they may have dampened it.”
- SupportedThe research suggests light from newborn stars, rather than supernovas, was the most viable source of radiation that cleared the cosmic fog.Latest from Live Science: “The intense light of newborn stars now seems like the most viable source of reionization.”
- SupportedUniversity of Michigan astronomer Cody Carr’s study found that supernovas may not have driven reionization as previously thought and may have dampened it.Latest from Live Science: “In other words, supernovas didn't drive the Epoch of Reionization, but they may have dampened it.”
- SupportedCarr said, “This challenges the notion that galaxies require fast supernova-driven winds to enable LyC escape,” in an email to Live Science.Latest from Live Science: “"This challenges the notion that galaxies require fast supernova-driven winds to enable LyC escape."”
- SupportedThe findings were published October 15 in Nature Astronomy and are based on Hubble observations of six compact starburst galaxies about 3.5 billion light-years away.Latest from Live Science: “The researchers used the Hubble Space Telescope to observe an analogue of the ancient universe in the local cosmos: six star-forming, wind-launching galaxies around 3.5 billion light-years away.”
- SupportedThe galaxies are generally smaller than 6,500 light-years in diameter, form around 10 solar masses of stars per year, and serve as analogues for the ancient universe.Latest from Live Science: “These compact starburst galaxies are generally smaller than 6,500 light-years in diameter and form around 10 solar masses' worth of stars per year.”
- SupportedAround 400,000 years after the Big Bang, the universe cooled enough to form neutral hydrogen atoms that made the cosmos opaque to ultraviolet light.Latest from Live Science: “Around 400,000 years after the Big Bang, the fledgling universe finally cooled enough to form atoms — mostly hydrogen. But these neutral, non-electrically charged hydrogen atoms shrouded the cosmos in a fog that made it opaque to ultraviolet (UV) light.”
- SupportedDuring the first 3 million years of star formation, stellar winds carved through neutral gas and dust at nearly 2,000 miles per second, allowing LyC light to escape the galaxy.Latest from Live Science: “In the initial phase, which occurred during the first 3 million years of star formation, the newly formed massive stars' stellar winds carved through their galaxies' neutral gas and dust at nearly 2,000 miles per second (3,000 kilometers per second). This allowed LyC light to escape the galaxy”
- SupportedAfter about 3 million years, material expelled by massive stars dying in supernovas added to opaque gas and dust, creating a trap for LyC light.Latest from Live Science: “As O-type stars — the hottest, bluest and most massive stars in the cosmos — lived fast and died young, their expelled stellar guts added to the opaque galactic gas and dust, churning it and loading it with mass, which created a trap for LyC light.”
- SupportedThe strongest LyC emitters in the study lacked fast galactic winds.Latest from Live Science: “The most interesting thing about our paper is the observation that the strongest LyC emitters do not have fast galactic winds,”
- SupportedCarr said, “In general, our results are in disagreement with most cosmological simulations, which typically show the LyC escape fraction peaking after the launch of supernova-driven winds.”Latest from Live Science: “"In general, our results are in disagreement with most cosmological simulations, which typically show the LyC escape fraction peaking after the launch of supernova-driven winds,"”
- SupportedThe oldest galaxies studied had the fastest supernova-generated winds and allowed the least ionizing radiation to escape.Latest from Live Science: “The oldest galaxies in this study — and, therefore, the ones with the fastest supernova-generated galactic winds — allowed the least amount of ionizing radiation to escape their environs.”
