Probing power spectrum enhancement at small scales with SKA

Abstract

The reionization process is driven by ionizing photons from dwarf galaxies in halos with virial temperature $T_{\rm vir} \gtrsim 10^4$ K, while minihalos whose $T_{\rm vir}\lesssim 10^4$ K consume ionizing photons and have negative contributions to reionization. Since ionizing sources and minihalos have different clustering characteristics, not only the reionization history, but also the morphology of the ionization field, is sensitive to the small-scale power spectrum. If the power spectrum at small scales is enhanced compared with the standard six-parameter $Λ$CDM model, then both the sources and sinks of ionizing photons would be boosted and the net impact depends on the competition between them. Therefore, the 21 cm signal that can probe the morphology of the ionization field will be a useful tool for detecting the small-scale power spectrum. Using the power spectrum proposed by Cielo et al. (2025) (C25) as a demonstration, we investigate the influence of small-scale power spectrum enhancement on the ionization field and the 21 cm signal. We find that for the C25 model, even under the constraints of observed UV luminosity functions for high-$z$ galaxies and reionization history, the 21~cm power spectrum and the bubble size distribution could be still significantly different from the regular $Λ$CDM model. The upcoming SKA-low AA* telescope, and a further imaging telescope, have the potential to detect the small-scale power spectrum more deeply.

Figures (8)

• Figure 1: The power spectrum at $z=6$ for the fiducial model (regular $\Lambda$CDM model) and the C25 model with different $k_{\rm trans}$ values. The vertical dashed lines refer to scales of different typical masses: the left line corresponds to the virial mass at virial temperature $T_\mathrm{vir} = 10^4 \, \mathrm{K}$, set as the lower mass limit for reionization sources and upper limit of minihalos; the right line corresponds to the Jeans mass, representing the minimal mass of minihalos hosting gas.
• Figure 2: Halo mass functions for fiducial model and C25 model with different $k_{\rm trans}$ values at the EoR. In each panel, the left vertical line refers to the Jeans mass, which is the minimal mass of minihalos able to retain gas; the right vertical line corresponds to virial mass of $T_\mathrm{vir} = 10^4 \, \mathrm{K}$, set as the lower mass limit for reionization sources and upper limit of minihalos.
• Figure 3: The predicted UV LFs in fiducial model and in C25 models with different values of $k_{\mathrm{trans}}$, compared with observations mclure2013finkelstein2015bowler2020bouwens2021harikane2023bouwen2023donnan2023donnan2024. The vertical dashed line in each panel indicates the central absolute UV magnitude corresponding to the minimum halo mass required for star formation, $\hat{M}_{\rm UV}\sim -7$.
• Figure 4: The 21 cm fields for fiducial model and C25 models with $k_\mathrm{trans}=400\,\mathrm{Mpc}^{-1}$ and $k_\mathrm{trans}=150\,\mathrm{Mpc}^{-1}$. The first row displays the results for mean ionization fraction $\langle X_{\rm HII}\rangle=0.25$, the second row $\langle X_{\rm HII}\rangle=0.50$ and the third row $\langle X_{\rm HII} \rangle=0.75$. The redshift of each panel is given above the top $x$-axis.
• Figure 5: CMB scattering optical depth as a function of $z$ for the fiducial model and C25 models with different $k_{\rm trans}$ values. The shaded filled regions are observational data and 1$\sigma$ and 2$\sigma$ uncertainties of Planck18 planck18.