A Universal Accounting Problem: Tension in Reionization Estimates

Paper Title: Reionization after JWST: a photon budget crisis?

Authors: Julian B. Muñoz, Jordan Mirocha, John Chisholm, Steven R. Furlanetto, Charlotte Mason

First Author’s Institution: Department of Astronomy, University of Texas at Austin, Austin, Texas, USA

Status: published on arXiv [open access]

Heating Up the Universe

Following the Big Bang, the universe was incredibly hot and violent. Particles popped into and out of existence from the massive concentrations of energy permeating space. Subatomic particles like protons, neutrons, and electrons existed during this period, but they had far too much energy to form atoms, meaning that the matter in the universe was ionized. These energetic particles frequently interacted with photons, which is part of how they gained and lost energy, meaning that any photons that were produced traveled a very short distance before interacting. Over time, the universe rapidly expanded due to inflation, causing particles to cool and slow down. This led to the formation of atoms like hydrogen and helium and molecules like molecular hydrogen (H₂) during a process called recombination. When this occurred, the universe was no longer ionized, and it allowed photons to stream freely through the universe. We can still see these photons today as the cosmic microwave background (CMB).

At this point, stars had not yet formed – in fact, the formation of H₂ was a critical step in catalyzing star formation. When the first generation of stars formed, shining hot and bright, they sent large amounts of UV and X-ray radiation out into the universe. This radiation reionized the remaining gas between galaxies (the intergalactic medium, or IGM). Reionization is believed to have been a very “patchy” process as reionization bubbles formed around galaxies and then slowly grew into each other (see this bite’s featured image). Reionization has been studied heavily in numerical simulations, but it is difficult to measure observationally because it happened so long ago. However, recent measurements from the James Webb Space Observatory (JWST) are creating discord with our previous measurements and our insights from numerical models.

Reionization with JWST

There are a few main numerical parameters that quantify reionization. The first is the number of photons produced by a galaxy that can ionize hydrogen, which is described by the galaxy’s UV luminosity (its total UV energy output rate) and its ionizing photon production efficiency ξ_ion. The distribution of galaxy UV luminosities is called the UV luminosity function (UVLF), which has a low-luminosity cutoff below which galaxies can’t produce ionizing radiation. The second is the escape fraction f_esc, which is the fraction of these photons that escape the galaxy and ionize hydrogen in the intergalactic medium. The third is the average number of recombinations each hydrogen atom undergoes, which serves to hinder reionization.

As highlighted in Figure 1, there was a good consensus on the allowed values of the UVLF cutoff and f_esc for a given value of ξ_ion prior to JWST. However, calibrating ξ_ion to recent JWST data leads to the lack of an overlapping region among measurements from Hubble and JWST, from CMB measurements, and from studies of f_esc in low-redshift galaxies.

Resolving Reionization Tension

The authors of today’s paper suggest a handful of possible avenues to resolve the tension among different reionization parameter estimates, shown in Figure 2.

1. One possibility is that ξ_ion is too high. This could be the case if JWST is observationally biased towards galaxies with high ionization efficiencies and is “missing out” on the low-efficiency galaxies. Samples of galaxies that have large UV luminosities or strong UV emission lines, which can make them more detectable, could be correlated with high ionization efficiencies.
2. Another possibility is that f_esc is too high. As with the ionization efficiency, there could be an observational bias in detecting galaxies with high escape fractions. It could also be the case that the mechanism driving the escape fraction is different between low- and high-redshift galaxies, which would lead to incorrect assumptions about how f_esc depends on the cutoff luminosity, ξ_ion, and redshift.
3. A third scenario is that the end of the UVLF is not well modeled, resulting in poor estimates. Different cutoff points and different shapes near the cutoff can affect the contributions of low-luminosity galaxies to reionization.
4. A fourth possibility is having a too-simplistic model for recombinations, which affect how long it takes for the universe to become fully reionized. The model used by today’s authors is relatively simple, so more sophisticated models may reduce or resolve the tension among different reionization measurements.

Solutions 2 and 3 can be calibrated to reproduce the CMB measurements, either by increasing the cutoff luminosity (accelerating reionization) or lowering f_esc (slowing reionization); these points are highlighted in the right panel in Figure 1 (the red diamond and black circle, respectively). However, as noted in Figure 2, these models can be difficult to distinguish, indicating that further measurements are necessary to determine which models are more likely.

Astrobite edited by Skylar Grayson

Featured image credit: Wikipedia

• About the Author

About Brandon Pries

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I've also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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