Systematic uncertainties in models of the cosmic dawn (2012.06588v2)

Abstract: Models of the reionization and reheating of the intergalactic medium (IGM) at redshifts $z \gtrsim 6$ continue to grow more sophisticated in anticipation of near-future 21-cm, cosmic microwave background, and galaxy survey measurements. However, there are many potential sources of systematic uncertainty in models that could bias and/or degrade upcoming constraints if left unaccounted for. In this work, we examine three commonly-ignored sources of uncertainty in models for the mean reionization and thermal histories of the IGM: the underlying cosmology, halo mass function (HMF), and choice of stellar population synthesis (SPS) model. We find that cosmological uncertainties affect the Thomson scattering optical depth at the few percent level and the amplitude of the global 21-cm signal at the $\sim$5-10 mK level. The differences brought about by choice of HMF and SPS models are more dramatic, comparable to the $1 \sigma$ error-bar on $\tau_e$ and a $\sim 20$ mK effect on the global 21-cm signal amplitude. Finally, we jointly fit galaxy luminosity functions and global 21-cm signals for all HMF/SPS combinations and find that (i) doing so requires additional free parameters to compensate for modeling systematics and (ii) the spread in constraints on parameters of interest for different HMF and SPS choices, assuming $5$ mK noise in the global signal, is comparable to those obtained when adopting the "true" HMF and SPS with $\gtrsim 20$ mK errors. Our work highlights the need for dedicated efforts to reduce modeling uncertainties in order to enable precision inference with future datasets.

• The paper shows that uncertainties in cosmological parameters lead to several percent variation in Thomson scattering optical depth and a 5–10 mK change in the global 21-cm signal.
• Different halo mass function and stellar population synthesis models introduce systematics comparable to 1σ errors on optical depth and cause approximately 20 mK variations in the 21-cm signal.
• Joint fitting of galaxy luminosity functions with 21-cm data necessitates additional free parameters to compensate for systematic uncertainties in cosmic dawn modeling.

Systematic Uncertainties in Models of the Cosmic Dawn

The paper examines the systematic uncertainties present in cosmological models of the cosmic dawn, which is characterized by the reionization and thermalization of the intergalactic medium (IGM) at redshifts $z \gtrsim 6$. These models are crucial for interpreting data from upcoming 21-cm, cosmic microwave background (CMB), and galaxy surveys. The authors identify three primary sources of uncertainty in the mean reionization and thermal histories of the IGM: the underlying cosmology, the halo mass function (HMF), and the choice of a stellar population synthesis (SPS) model.

Key Findings

• Cosmological Uncertainties: The impact of uncertainties in cosmological parameters affects the Thomson scattering optical depth ($\tau_e$) at several percent and causes variations in global 21-cm signal amplitude on the order of 5–10 mK. Although these uncertainties alone are significant, they are less consequential than those induced by other model assumptions.
• Halo Mass Function and Stellar Population Synthesis: The disparity among different HMF and SPS choices exerts a larger influence on cosmic dawn models. The influence of different HMF models, specifically those by Press-Schechter (1974), Sheth-Tormen (2001), and Tinker et al. (2010), demonstrates substantial variance, with systematics comparable to the $1\sigma$ errors on $\tau_e$ and creating a $\sim 20$ mK effect on the global 21-cm signal amplitude.
• Joint Fitting and Additional Parameters: A joint analysis fitting galaxy luminosity functions and global 21-cm signals across diverse HMF/SPS combinations reveals that significant modeling systematic compensation is necessary, which often requires introducing additional free parameters. The extent of parameter constraints due to HMF and SPS variances rivals those acquired using the correct HMF and SPS but with larger noise assumptions (20 mK).

Implications and Future Directions

This work underscores the necessity of precise modeling to improve parameter estimation accuracy in forthcoming datasets. Observational noise levels expected in future 21-cm measurements suggest that these systematic uncertainties could be a controlling limit in extracting cosmological information, thereby necessitating refined model treatments.

From a theoretical standpoint, understanding and mitigating these systematic effects is critical for plotting the growth of the early universe and for the precise classification of the nature and properties of early galaxies. This could involve employing more sophisticated numerical simulations, parameterizations that account for all potential uncertainties, and cross-validation with semi-analytic methods.

In practice, the necessity for rigorous model selection and the differentiation of plausible competing models will become crucial as new observatory data becomes available, notably from instruments like the James Webb Space Telescope (JWST) and large-scale radio arrays capturing the 21-cm signal. Such efforts are not only necessary for understanding reionization and early cosmic structures but also for exploring speculative phenomena such as non-standard dark matter interactions.

While the current study focuses on uncertainties tied to HMF and SPS models, future work will likely expand to incorporate systematic modeling uncertainties inherent in improved radiative transfer simulations and other hydrodynamic processes not detailed here. Integrating differing types of observational constraints and thus strengthening these models, will be essential for unraveling the enigmatic period of cosmic dawn.