Hydrogen Glow Pushed Gas Away From Baby Galaxies, Simulations Suggest

Light released by hydrogen atoms may have blown thin gas and dust out of some of the universe's earliest galaxies, making their dense star clusters easier to see. That is the conclusion of a new study in The Astrophysical Journal, as reported by Universe Today.

The work is theoretical. The two authors analyzed existing computer simulations rather than observing the galaxies directly. Their results bear on a puzzle raised by the James Webb Space Telescope: why bright galaxies from the Cosmic Dawn, only a few hundred million years after the Big Bang, were more numerous than expected.

Who did the research and what did they find?

The paper was written by Shyam H. Menon, a research fellow at the Flatiron Institute's Center for Computational Astrophysics, and Aaron Smith, an assistant professor of physics at the University of Texas at Dallas. According to Universe Today, they found that the radiation could push away low-density gas and dust very quickly.

Menon said this can “clear the ‘fog,’” letting astronomers see early stars more clearly than they otherwise could.

What is Lyman-alpha radiation?

Lyman-alpha is light produced when an electron in a hydrogen atom drops from an excited state to its ground state. Because hydrogen is by far the most abundant element in the universe, Universe Today notes, this light is a key tool for studying the earliest galaxies and the gas in and between them.

It behaves unusually. Atoms normally re-emit absorbed energy at a different wavelength. Hydrogen instead absorbs Lyman-alpha and re-emits it almost instantly, and the light then meets more hydrogen. That repeated cycle makes the light a useful probe, and it also lets the radiation push on hydrogen gas.

How did the team study something so hard to model?

Astronomers have long known the radiation carries a lot of force, Menon said, but they could not gauge its importance in realistic conditions because it is so hard to model. The authors attribute earlier omissions to its complexity and heavy computing demands.

Rather than simulate the radiation from scratch, the pair took snapshots from existing radiation-hydrodynamic simulations of dense, star-cluster-forming clouds with a range of dust levels. They processed those snapshots with a code called COLT, short for Cosmic Lyman-alpha Transfer, which Smith created. They then calculated how much force the radiation would exert. One visualization from the work covers a 2.5-million-year simulation of a gas cloud being cleared as stars form.

Did the radiation clear all the gas?

No. According to the report, the radiation was not strong enough to blow away gas in the densest star-forming regions that Webb has found, because gravity from that dense gas was too strong. Outside those dense filaments, however, it could drive away the more diffuse gas and dust.

The authors summarize this in two conclusions. Lyman-alpha pressure is unlikely to prevent efficient star formation, and it can drive significant winds once star formation has saturated. They write that star formation efficiency is therefore likely to stay high in the dense clusters forming in the early universe.

Why does it matter?

The finding is about how astronomers read the faint light from the first galaxies. Webb has revealed compact, dense star clusters in lensed fields, including some seen about 460 million years after the Big Bang in a strongly lensed galaxy called SPT0615-JD1. If thin gas and dust were pushed out once star formation was well underway, the clusters' light could escape more readily and be easier to detect.

The authors say a large share of the gas that fills the volume could be ejected this way. They link that to the visibility of cluster light, the escape of ionizing photons and the launching of outflows from galaxies. For general readers, the practical meaning is scientific: it offers one possible explanation for what Webb sees in the early universe and a new factor for models of galaxy formation.

How firm is the result?

The study appears in a peer-reviewed journal, but it rests on processing simulation snapshots, not on new telescope observations. It shows what Lyman-alpha pressure could do in the modeled conditions. The report does not describe any direct observational test of the idea.

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Around Prague, based on reporting by Universe Today