Dying Stars May Have Slowed, Not Sped Up, the Clearing of the Early Universe’s Fog

A new study suggests that exploding stars may not have helped lift the fog that once filled the young universe. According to Live Science, astronomers who examined nearby galaxies found that the ones with the strongest supernova-driven winds let the least ionizing ultraviolet light escape, the opposite of what many models assume.

The work, published Oct. 15 in the journal Nature Astronomy, points instead to the intense light and winds of newborn stars as the more likely engine behind the event known as the Epoch of Reionization.

How the early universe cleared up

Around 400,000 years after the Big Bang, the universe cooled enough for atoms to form, mostly hydrogen. Those neutral atoms made space opaque to ultraviolet light. Later, once the first stars formed and gathered into galaxies, bursts of radiation ionized the hydrogen, splitting it into protons and electrons and letting light travel freely across space.

The researchers call this the last major phase transition in the universe. Lead author Cody Carr, an astronomer at the University of Michigan, said in a statement that scientists know it happened but do not have complete answers about which objects caused it or how it unfolded.

What the Hubble observations showed

Because the earliest galaxies are too distant to study in this detail, the team used the Hubble Space Telescope to look at local stand-ins: six star-forming, wind-launching galaxies about 3.5 billion light-years away. These compact starburst galaxies are generally smaller than 6,500 light-years across and form about 10 solar masses’ worth of stars per year.

Their spectra, the light-based fingerprints of their gas, revealed the origin, chemical makeup, speed and direction of the winds blowing through them. The team found a two-phase process:

• First 3 million years: winds from newly formed massive stars cut through the galaxies’ neutral gas and dust at nearly 2,000 miles per second (3,000 kilometers per second). That let Lyman continuum light, the ionizing radiation that clears the fog, escape into intergalactic space.
• After about 3 million years: the situation appeared to reverse. As the hottest, most massive stars died young, the material they threw off added to the galactic gas and dust, which created a trap for the ionizing light.

The oldest galaxies in the sample, which therefore had the fastest supernova-generated winds, let the least ionizing radiation out.

Why it matters

For readers, this is a story about how the universe got its present, transparent look. Astrophysicists had previously thought the fog-clearing effect depended on supernovas. Carr told Live Science that the most interesting result is that the strongest emitters of this light do not have fast galactic winds, which challenges the idea that galaxies need fast supernova-driven winds for the light to escape.

He also said the findings disagree with most cosmological simulations, which typically show the escape of this light peaking after supernova-driven winds begin. In other words, according to the study, supernovas did not drive reionization and may even have dampened it, leaving the intense light of newborn stars as the most viable source.

The result is a single study based on six galaxies, so it does not settle the question. It does suggest that parts of the standard picture of cosmic evolution may need revisiting.

What comes next

The team says future research could gather additional galaxy samples and make higher-resolution observations across other wavelengths, which would show whether the pattern seen in these six galaxies holds more broadly.

Around Prague, based on reporting by Live Science