Astronomers using NASA’s Hubble Space Telescope have found that powerful winds from exploding stars may make it harder—not easier—for ultraviolet radiation to escape early galaxies. The result challenges a widely accepted explanation for how the early universe became transparent during reionization, suggesting that ionizing photons may have escaped before the first supernovae reshaped their surroundings.
How the early universe emerged from cosmic fog
During the first billion years after the Big Bang, vast amounts of neutral hydrogen filled intergalactic space. This hydrogen acted as a cosmic fog, absorbing ultraviolet light and preventing it from traveling freely through the universe.
That began to change during a period known as reionization, which started about 13 billion years ago. Intense radiation from early galaxies gradually stripped electrons from hydrogen atoms, leaving positively charged protons and free electrons.
As more hydrogen became ionized, ultraviolet light was able to travel farther through intergalactic space. This transformation helped create the relatively transparent universe observed by astronomers today.
“Eventually, when most of the hydrogen that fills intergalactic space is ionized, that’s the end of reionization, and we can see the Universe as we see it today,” said Cody Carr, a postdoctoral fellow in the University of Michigan’s Department of Astronomy. “We call reionization the last major phase transition in the universe.”
Despite its importance, astronomers still do not fully understand which objects produced the radiation responsible for reionization or how enough of that radiation escaped from early galaxies.
“We don’t know exactly what objects caused reionization or how it evolved,” said Kerr, who conducted and led the study while working as a postdoctoral fellow at Zhejiang University in China before joining the University of Michigan. “We know it happened, but we don’t have complete answers to those questions.”
Hubble challenges the supernova-wind explanation
For many years, one leading explanation focused on supernovae—the powerful explosions that occur when certain stars reach the ends of their lives. These explosions can produce enormous winds that push gas and other material out of a galaxy.
Scientists had proposed that these winds might clear gas from around young stars, opening channels through which ultraviolet radiation could escape into intergalactic space. Once outside the galaxy, the radiation could help ionize the neutral hydrogen responsible for the universe’s ancient fog.
However, new Hubble observations suggest that the process may work in the opposite way. The researchers found that stronger galactic winds were associated with less ionizing radiation escaping from galaxies.
“Basically, how these galaxies leak ionizing radiation is not what we thought,” Carr said.
The findings support another possibility: radiation from young, massive stars may escape before those stars explode as supernovae and generate large-scale winds that alter their surroundings.
“Our observations show that these photons escape before the galactic wind forms,” Kerr said.
Photons are individual packets of light energy. In this study, the researchers focused on ultraviolet photons with enough energy to ionize hydrogen, making them central to understanding how the early universe became transparent.
What Hubble found in nearby starburst galaxies
To investigate how radiation escapes, the international team used Hubble to study nearby starburst galaxies. These galaxies have undergone periods of especially intense star formation and can serve as modern examples of the kinds of galaxies that may have contributed to reionization.
Hubble, a NASA mission launched with contributions from the European Space Agency, enabled the researchers to examine the galaxies’ emitted light in detail.
The team analyzed each galaxy’s spectrum to determine how much light was present at different wavelengths. Spectral patterns can reveal information about the gas surrounding the stars, including how quickly that gas is moving.
Gas absorbs light at specific wavelengths, producing identifiable dips in a galaxy’s spectrum. The positions and shapes of those features provide clues about the speed and motion of galactic winds.
When the researchers compared wind speeds with the amount of ionizing radiation emitted by each galaxy, they found an unexpected relationship: galaxies with the fastest winds allowed only a small fraction of their ionizing radiation to escape.
The result directly contradicts the expectation that stronger winds would remove more gas and make it easier for radiation to reach intergalactic space.
“The energy input from the supernova appears to be perturbing the surrounding environment, increasing rather than decreasing the range of neutral gas and dust that suppresses the emission of ionizing radiation,” said Stephen McCandriss, the study’s senior author and director of the Johns Hopkins University Center for Astrophysical Sciences.
In other words, energy from stellar explosions may be redistributed in a way that causes more gas and dust to block ultraviolet light instead of clearing a path for it to escape.
Young stars may be central to reionization
Although large galactic winds appear to suppress the escape of radiation, smaller winds and other activity around young stars may still create openings in the surrounding gas.
These processes can occur before the first supernova explosion, giving radiation an opportunity to escape while conditions are more favorable.
The findings suggest that astronomers may need to focus more closely on the earliest stages of star formation rather than concentrating primarily on the effects of stellar explosions.
Kerr and colleagues are seeking additional Hubble observing time to study more galaxies and determine whether the same pattern appears elsewhere. Further observations could clarify how important young-star activity is in allowing ionizing radiation to escape.
Implications for models of the early universe
The findings also have implications for computer simulations of the early universe. These simulations seek to recreate how galaxies formed, how gas moved, and how radiation changed intergalactic space as it filled with hydrogen.
“Supernovae play a central role in cosmological hydrodynamic simulations that explain how ionizing radiation escapes from galaxies,” said Renyue Sen, senior author of the study and professor and director of the Center for Cosmology and Computational Astrophysics at Zhejiang University. “Our findings highlight the need to investigate how radiation and winds from young stars form the surrounding gas before the first supernova explodes.”
More accurately representing the early interactions among stars, radiation and gas could help researchers build better models of reionization.
The discovery does not settle what caused the universe’s last major phase transition. It does, however, provide an important clue: the radiation that helped clear the universe’s ancient hydrogen fog may have escaped from galaxies before powerful stellar explosions began reshaping their surroundings.
International collaboration and funding
The study involved nearly 30 scientists from more than a dozen institutions worldwide. Sally Ooi, a professor of astronomy at the University of Michigan, also contributed to the research.
The project received additional financial support from Zhejiang University, the Zhejiang Provincial Top-Level Research Support Program, the National Natural Science Foundation of China, the Spanish Ministry of Science and Innovation Universities, the European Regional Development Fund, and the European Union’s Horizon 2020 Research and Innovation Program.
The research was supported in part by federal funding from NASA and the National Science Foundation. Additional support for the Hubble Space Telescope observations came through grants administered by the Space Telescope Science Institute.
Source: www.sciencedaily.com
