Dense gas in minihalos (MHs) with masses of 106−108M⊙ can shield
themselves from reionization for about 100 megayears after being exposed to
UV radiation. 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 HI column density profile of photoevaporating MHs
by conducting 1-dimensional radiation-hydrodynamics simulations. We utilize
these results to estimate the Lyα opacity from minihalos in a
large-scale simulation that cannot resolve the self-shielding process. When the
ionization rate of the background radiation is 0.03×10−12s−1, as expected near the end of reionization at z∼5.5, we find that
the incidence rate of damped Lyα absorbers, dN/dX, increases by nearly
a factor of 2 - 4 compared to at z=4.5. The Lyα flux is, on average,
suppressed by 10% of its mean due to absorption by MHs. The extended
absorption features contribute to a ∼20% enhancement in the 1D power
spectrum at k∼0.1hMpc−1 (10−3km−1s),
which is comparable to the enhancement caused by inhomogeneous reionization.
Notably, the flux is particularly suppressed in the vicinity of large halos
along the line-of-sight direction at separations of up to 10h−1Mpc
at r⊥≲2h−1Mpc. However, these effects become much
smaller for higher ionizing rates (0.3 and 1×10−12s−1)
expected in the post-reionization universe z≲5.5. Our findings
highlight the need to consider the absorption by MHs when interpreting the
Lyα forest at z≳5.5. Moreover, the sensitivity of this effect to
the ionizing background intensity can be exploited to constrain the intensity
itself.Comment: 15 pages; 11 figures; Submitted to the Ap