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 yet 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 absolutely massive, and “die with passion” as Bill Wurtz once put it in a famous YouTube video. A new paper, available in pre-print form on arXiv by Tae Bong Jeon from the Cosmic Frontier Center at the University of Texas at Austin, looks at just 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 time periods that would have been expected given the 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 from hundreds of millions of years later than predicted.

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

Fraser talks about the universe's biggest stars

In the early universe, primordial gas clouds could only collapse under their own gravity if they could cool down. Typically, once they got dense enough they would simply heat up and that heat would cause them to expand back out. 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 actually 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 timeframes where JWST is seeing them. One potential solution to this puzzle is to lower the amount of molecular hydrogen to be used as a coolant near those gas clouds - luckily there is a mechanism to do that, known as Lyman-Werner (LW) radiation.

LW radiation is a type of soft ultraviolet photon that dissociates (i.e. breaks apart) molecular hydrogen on contact, forming atomic hydrogen, which is not nearly as effective as cooling gas clouds. Therefore, if a gas cloud is hit by LW radiation, it delays star formation until it gets to the “atomic cooling” stage, where the amount of material is so massive that the atomic hydrogen that forms after the molecular hydrogen is broken up can finally prompt a catastrophic collapse into a star.

Fraser talks about when we think the first stars formed.

To prove 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 a distinct structure developed where the outer layers of the halo remain extremely hot from the prolonged exposure to external UV radiation, whereas the inner core of the halo becomes more dense, 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 at the time frames of these potential “late” Pop III stars. Metals from other supernovae do as well. However, they do so 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 the Pop III starbursts and the 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 more unlikely 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 utilize gravitational lenses, researchers could find up to nine of these late-blooming Pop III starbursts. If there is a paper in the next few years describing just that, it's likely this paper will at least be one of its reference points, if not the inspiration for the work itself.

Learn More:

T.B. Jeong et al - How Massive Can a Population III Starburst Be? Simulating the First Galaxies with High Lyman-Werner Background

UT - Astronomers are Hunting Down the Elusive Population III Stars

UT - If We Can't Detect the First Stars, Maybe We Can See Their First Galaxies

UT - Astronomers Think They've Found Examples of the First Stars in the Universe