Impact of Self-shielding Minihalos on the Lyα Forest at High Redshift

Dense gas in minihalos with masses of 10⁶‑10⁸ M _⊙ can shield themselves from reionization for ∼100 Myr after being exposed to the UV background. These self-shielded systems, often unresolved in cosmological simulations, can introduce strong absorption in quasar spectra. This paper is the first systematic study on the impact of these systems on the Lyα forest. We first derive the H I column density profile of photoevaporating minihalos by conducting 1D radiation–hydrodynamics simulations. We utilize these results to estimate the Lyα opacity from minihalos in a large-scale simulation that cannot resolve self-shielding. When the ionization rate of the background radiation is 0.03 × 10^‑12 s^‑1, as expected near the end of reionization at z ∼ 5.5, we find that the incidence rate of damped Lyα absorbers increases by a factor of ∼2‑4 compared to at z = 4.5. The Lyα flux is, on average, suppressed by ∼3% of its mean due to minihalos. The absorption features enhance the 1D power spectrum up to ∼5% at k ∼ 0.1 h Mpc^‑1 (or 10^‑3 km^‑1 s), which is comparable to the enhancement caused by inhomogeneous reionization. The flux is particularly suppressed in the vicinity of large halos along the line-of-sight direction at separations of up to 10 h ^‑1 Mpc at r _⊥ ≲ 2 h ^‑1 Mpc. However, these effects become much smaller for higher ionizing rates (≳0.3 × 10^‑12 s^‑1) expected in the post-reionization Universe. Our findings highlight the need to consider minihalo absorption when interpreting the Lyα forest at z ≳ 5.5. Moreover, the sensitivity of these quantities to the ionizing background intensity can be exploited to constrain the intensity itself.