Exploring the effect of different cosmologies on the Epoch of Reionization 21-cm signal with POLAR

TL;DR

The paper investigates how varying cosmological parameters, notably $h$ and $\sigma_8$, influence the Epoch of Reionization (EoR) 21-cm signal using the POLAR pipeline that couples Gadget-4 N-body simulations with the L-Galaxies semi-analytic model and the Grizzly 1D radiative transfer code. It introduces two scenarios—unconstrained (fixed galaxy physics across cosmologies) and constrained (UVLF-tuned per cosmology)—to assess degeneracies between cosmology and astrophysics in reproducing UV luminosity functions and the 21-cm power spectrum in LOFAR's redshift window. The results show that UVLFs can be matched across models, but the 21-cm signal can remain broadly similar across cosmologies when astrophysical parameters are tuned, underscoring the need for broad parameter priors and joint constraints from multiple observables (JWST, Euclid, SPHEREx, SKA-Low tomography). The work highlights the importance of treating cosmology and galaxy physics as interdependent in inference pipelines and outlines future directions, including higher-resolution techniques and ML-based emulators, to robustly forecast and extract cosmological information from upcoming 21-cm and galaxy surveys.

Abstract

A detection of the 21-cm signal power spectrum from the Epoch of Reionization is imminent, thanks to consistent advancements from telescopes such as LOFAR, MWA, and HERA, along with the development of SKA. In light of this progress, it is crucial to expand the parameter space of simulations used to infer astrophysical properties from this signal. In this work, we explore the role of cosmological parameters such as the Hubble constant $H_0$ and the matter clustering amplitude $σ_8$, whose values as provided by measurements at different redshifts are in tension. We run $N$-body simulations using GADGET-4, and post-process them with the reionization simulation code POLAR, that uses L-GALAXIES to include galaxy formation and evolution properties and GRIZZLY to execute 1-D radiative transfer of ionizing photons in the intergalactic medium (IGM). We compare our results with the latest JWST observations and explore which astrophysical properties for different cosmologies are necessary to match the observed UV luminosity functions at redshifts $z = 10$ and $9$. Additionally, we explore the impact of these parameters on the observed 21-cm signal power spectrum, focusing on the redshifts within the range of LOFAR 21-cm signal observations ($z \approx 8.5-10$). Despite differences in cosmological and astrophysical parameters, our models cannot be ruled out by the current upper limits. This suggests the need for broader physical parameter spaces for inference modeling to account for all models that agree with observations. However, we also propose stronger constraining power by using a combination of galactic and IGM observables.

Figures (15)

• Figure 1: Halo mass functions (HMFs) for our four models: fiducial (cyan), $h$ high (red), $\sigma_8$ low (purple) and $\sigma_8$ high (green) at $z = 10$ (top panel) and 9 (bottom). The A- and B-series are shown with dashed and dotted lines, respectively. The corresponding fit for each simulation using the Tinker_2008 model is shown with solid lines of the corresponding colours.
• Figure 2: A and B-series averaged UVLFs for the unconstrained (top) and constrained (bottom) cases at $z = 10$ and 9 for the fiducial (cyan), $h$ high (red), $\sigma_8$ low (purple) and $\sigma_8$ high (green). The averaging of the F&P pairs allows to compensate for stochasticity in underfilled bins of the UVLFs, by suppressing the impact of cosmic variance. We also show the dust attenuated (solid) and unattenuated (dashed) UVLFs, and JWST and HST observations Finkelstein_2015Bouwens_2015Bouwens_2021Harikane_2022Bouwens_2023aBouwens_2023bHarikane_2023Leung_2023McLeod_2024Adams_2024.
• Figure 3: Maps of $\delta T_{\rm b}$ of the A-series middle slices of single cell thickness (i.e., $\approx 600~h^{-1}~\rm ckpc$) for the four cosmological models in the constrained case at $z = 12, 10, 8,~\rm and~6$ (from top to bottom). Here the dark areas represent the ionized regions with $\delta T_{\rm b} = 0$. Note that $\delta T_{\rm b}$ cannot have negative values due to the assumption of $T_{\rm S} \gg T_{\rm CMB}$.
• Figure 4: Redshift evolution of the average of the A and B-series volume-averaged neutral hydrogen fraction $\langle x_{\rm HI} \rangle$ for the fiducial (cyan), $h$ high (red), $\sigma_8$ low (purple) and $\sigma_8$ high (green) models for the unconstrained (top) and constrained (bottom) cases. Dotted lines refer to a fit to the curves, which is used for a better estimate of the redshift of reionization when the redshift resolution is too coarse. The vertical grey dashed lines indicate the redshifts observationally relevant for LOFAR ($z = 10.11$, 9.16 and 8.3), and the black solid line is drawn at $\langle x_{\rm HI} \rangle = 0.5$ to guide the eye. Grey circles are a collection of observational constraints Fan_2006bTotani_2006Ota_2008Ouchi_2010Bolton_2011Dijkstra_2011McGreer_2011Mortlock_2011Ono_2012Chornock_2013Jensen_2013Robertson_2013Schroeder_2013Pentericci_2014Schenker_2014McGreer_2015Sobacchi_2015Choudhury_2015Mesinger_2015Greig_2017Davies_2018Mason_2018Hoag_2019Greig_2019Jones_2024.
• Figure 5: A and B-series averaged star formation rate (SFR) as a function of stellar mass for the unconstrained (top) and constrained (bottom) cases, for the fiducial (cyan), $h$ high (red), $\sigma_8$ low (purple) and $\sigma_8$ high (green) models at $z = 10$ and 9. Solid lines refer to the median star formation rate in each mass bin, with 16th and 84th percentiles shown as shaded regions. Grey circles are observations from various JWST programs Treu_2023Fujimoto_2023Looser_2023Bouwens_2023bPapovich_2023Arrabal_2023Arrabal_2023bLong_2023Leethochawalit_2023Atek_2023Robertson_2023Heintz_2023Heintz_2023bJin_2023Helton_2024Jung_2024.