How big can the universe’s first starbursts get?

As our telescopes have improved and we’ve been able to peer farther back in time, we’ve begun finding more fascinating features of the universe. But one thing we haven’t found for sure is Population III (Pop III) stars. These were the earliest stars in the universe, formed completely from pristine hydrogen and helium, with no “metals” (i.e., elements heavier than those two) polluting their processes.

They are also theorized to be extremely massive and to “die with passion,” as Bill Wurtz once put it in a famous YouTube video. A new paper, available on the arXiv preprint server by Tae Bong Jeon of the Cosmic Frontier Center at the University of Texas at Austin, looks at how massive those starbursts could be and whether the James Webb Space Telescope could detect one.

So far, JWST has found hints of them—and at much later periods than would have been expected based on theoretical models from before the telescope was launched. Some galaxies near the end of the Epoch of Reionization have features that appear to come from these pristine starbursts, but hundreds of millions of years later than predicted.

For that to happen, two existential traps must be avoided. First, the hydrogen and helium gas clouds that form these stars can’t collapse too early. Second, they must avoid being “contaminated” by metals from neighboring supernovae. Let’s first look at how to delay a gas cloud’s collapse.

In the early universe, primordial gas clouds could collapse under their own gravity only if they could cool down. Typically, once they got dense enough, they would simply heat up, causing them to expand again. Crucially, the only “coolant” they had was molecular hydrogen (H₂). This critical molecule was a key feature in the formation of the first Pop III stars, as it allowed dark matter to pull enough gas close enough together for fusion to start.

In later phases of the universe, however, molecular hydrogen was more abundant, so, at least in theory, Pop III stars would form more readily and, given their short life spans, burn out well before the periods in which JWST is seeing them. One potential solution to this puzzle is to reduce the amount of molecular hydrogen available as a coolant near those gas clouds. Luckily, there is a mechanism to do that, known as Lyman-Werner (LW) radiation.

LW radiation consists of soft ultraviolet photons that dissociate (i.e., break apart) molecular hydrogen on contact, forming atomic hydrogen, which is not nearly as effective at cooling gas clouds. Therefore, if a gas cloud is hit by LW radiation, star formation is delayed until it reaches the “atomic cooling” stage. At that point, the amount of material is so massive that the atomic hydrogen formed after the molecular hydrogen is broken up can finally prompt a catastrophic collapse into a star.

To demonstrate this mechanism, the authors modeled a dark matter “halo” that was in the process of cooling before collapse and exposed it to different levels of LW radiation. They noted that a distinct structure developed, in which the outer layers of the halo remained extremely hot from prolonged exposure to external UV radiation, while the inner core became denser, turning into a type of shield.

Essentially, the outer layer takes the brunt of the heat, while the inner layer cools down and eventually forms a star.

But LW radiation isn’t the only thing floating around the universe during the periods of these potential “late” Pop III stars. Metals from other supernovae do as well. However, they move much more slowly. The authors point out that LW radiation from nearby supernovae can reach these pristine gas and dust clouds hundreds of millions of years more quickly than the metal particles that can “pollute” them into Pop II stars (the next generation of stars, which has some level of metal content).

Distinguishing between Pop III starbursts and their more mundane Pop II cousins remains a technical challenge, though. The authors calculate that, with some help from gravitational lensing, we can pick up Pop III starbursts with our modern suite of equipment. But using gravitational lenses requires a significant amount of luck, as what you’re looking for has to be directly in line with the galaxy doing the lensing.

That luck is made even less likely by the rarity of the conditions needed to form these starbursts—an intense UV bath and a pristine, unpolluted pocket of gas. Still, the authors calculate that using surveys like GLIMPSE, which is designed specifically to use gravitational lenses, researchers could find up to nine of these late-blooming Pop III starbursts. If a paper in the next few years describes just that, this paper will likely be one of its reference points, if not the inspiration for the work itself.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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