Title: Early Bright Galaxies from Helium Enhancements in High-Redshift Star Clusters
Authors: Harley Katz, Alexander P. Ji, O. Grace Telford, and Peter Senchyna
First Author’s Institution: Department of Astronomy and Astrophysics, University of Chicago
Access: Preprint on arXiv
Among the many recent mysteries JWST has provided (that astronomers three years ago might have laughed at) is the issue that some galaxies are simply too bright. Researchers are finding an “overabundance” of early-universe galaxies (z>10) shining way brighter than we expect, specifically in the UV wavelength region. And, of course, this is a problem because our models didn’t see this coming! So back to the drawing board we go to try to figure out what kinds of processes in the early universe could create this mysterious UV brightness boost.
Because the high redshift universe is so uncertain, there are actually a lot of potential explanations for this brightness. Researchers have proposed “solutions” ranging from there being less dust in high-redshift galaxies, to there being more massive stars created in these galaxies (referred to as a “top-heavy” initial mass fraction), or to the idea that stars simply form more efficiently in the early universe. It’s generally thought that there’s likely a combination of all these effects happening at the same time (hence the air quotes), but researchers are still interested in quantifying the kinds of contributions to the brightness each of these phenomena could have.
The authors of today’s paper add another piece to the puzzle by considering how a potential enhancement of helium in the stars of these galaxies could contribute to their UV brightness, using some nifty elemental abundance relations seen in local globular clusters. They focus on three z>10 UV-bright galaxies that have also been observed to have high levels of nitrogen, another important property in this investigation.
Helium’s role in boosting brightness
The temperature and luminosity of a star depend on the opacity of the star (i.e. how easy is it for light to get through) and the mean molecular weight of the star (i.e. what metals are in the star). For stars over twice the mass of our Sun, the opacity of the star is mainly determined by how many electrons are scattering off atoms; These atoms are mostly all hydrogen, so it all depends on how much hydrogen is in the star. Because a star is mainly composed of hydrogen and helium, you can then invert the luminosity equation (equation 1) to be defined entirely in terms of the amount of helium in a star (after you make some simplifying assumptions, of course). Figure 1 shows how different levels of helium (also called the mass fraction Y) affect the luminosity.
Figure 1. Luminosity of a star as a function of the level of helium (Y) in the star. The green line (“homology”) shows the model based on equation 1. The colored lines show MESA models (a common computational stellar evolution code) of stars with different metallicities to see how metallicity may change this relationship between luminosity and helium. Figure 1 in original paper.
Equation 1. Luminosity relationship entirely defined in terms of the mass fraction of helium Y, assuming star is in radiative equilibrium and over 2 solar masses with a fully ionized medium. Equation 2 in original paper.
The amount of helium usually assumed to be in stars is Y = 0.24, which is the fractional amount of helium made through Big Bang Nucleosynthesis. In the model based on equation 1, we can see that an increase to Y~0.4 can increase the luminosity by about 80%. And even in the MESA models, the UV luminosity is boosted by around 30% at Y~0.4.
Globular Clusters as a translation tool
Unfortunately, helium is nearly impossible to observe directly in these galaxies. The wavelengths of the necessary helium lines can’t be seen with JWST at redshifts higher than 6.5, so, like always, astrophysicists have to get creative with ways to estimate their helium abundance.
It is well-reported in the literature that these three galaxies have relatively high levels of nitrogen, which has itself been a puzzle for scientists. But in this work, the authors are primarily interested in what these high levels of nitrogen can tell us about the potential levels of helium in these galaxies.
To connect the two, they turn to local observations of globular clusters. Globular clusters have odd chemical abundance patterns, which basically means they don’t have the same ratios of elements as our Sun. One particular oddity is that the amount of nitrogen-enriched stars increases with the mass of the globular cluster, and by coincidence (or not) the amount of helium also increases with that mass. Simply put, globular clusters with higher levels of nitrogen also seem to have higher levels of helium than expected. While this has mostly been seen in the local universe, it has also been seen at high redshift (z~6), making this hypothesis a bit more feasible for these three galaxies.
To calculate the theoretical helium levels in these three high-redshift galaxies, the authors fit a relationship between the nitrogen level and the helium fraction based on findings from another 2024 paper, as shown in Figure 2. They find that the suggested Helium levels range from Y ~ .34 to Y ~ .47 for these three galaxies, which correspond to luminosity boosts of ~18% to ~48% respectively.
Piecing together the puzzle
These results come with the caveat that it’s incredibly difficult to get that much helium in a star. Because of Big Bang Nucleosynthesis, stars on average always have at least a helium mass fraction of Y~.24. Stars then create helium through stellar nucleosynthesis, but the average star creates nowhere near the Y~.40 mass fractions these observations would need. But perhaps these galaxies don’t contain your average star–maybe they’re biased with AGB stars that produce Y ~ .35-.40, or binary stars that can (potentially) produce mass fractions up to Y ~ .63. There are avenues for heightened helium production, but it’s definitely on the rarer side.
These considerations of helium are a good addition to the puzzle, but it will take many more observations to understand what processes are at play in the high redshift universe. It’s another reminder that interesting effects are happening everywhere, and that no one “solution” is ever the end-all-be-all.
Astrobite edited by Magnus L’Argent
Featured image credit: mwphillips75, licensed under CC BY-NC 2.0.
