Impact of inhomogeneous reionization on the Lyman-α forest

Abstract

The Lyman-α forest at high redshifts is a powerful probe of reionization. Modeling and observing this imprint comes with significant technical challenges: inhomogeneous reionization must be taken into account while simultaneously being able to resolve the web-like small-scale structure prior to reionization. In this work we quantify the impact of inhomogeneous reionization on the Lyman-α forest at lower redshifts (2 < z < 4), where upcoming surveys such as DESI will enable precision measurements of the flux power spectrum. We use both small box simulations capable of handling the small-scale structure of the Lyman-α forest and semi-numerical large box simulations capable of representing the effects of inhomogeneous reionization. We find that inhomogeneous reionization could produce a measurable effect on the Lyman-α forest power spectrum. The deviation in the 3D power spectrum at z obs = 4 and k = 0.14 Mpc −1 ranges from 19 -36%, with a larger effect for later reionization. The corrections decrease to 2.0 -4.1% by z obs = 2. The impact on the 1D power spectrum is smaller, and ranges from 3.3 -6.5% at z obs = 4 to 0.35 -0.75% at z obs = 2, values which are comparable to the statistical uncertainties in current and upcoming surveys. Furthermore, we study how can this systematic be constrained with the help of the quadrupole of the 21 cm power spectrum.1 More precisely, the optical depth is proportional to the neutral hydrogen density, which in turn depends on the density and temperature of the gas and the photoionization rate. This is then smeared by the Doppler width of the Lyman-α line.

Imprints of temperature fluctuations on the z ∼ 5 Lyman-α forest: a view from radiation-hydrodynamic simulations of reionization

“…On even larger scales still, the clustering of the ionizing sources and the size of the ionized bubbles during reionization leave their imprints on the Lyα flux via temperature fluctuations. A number of works have shown that large-scale correlations in the IGM temperature field can bring excess power in the lowest observable wavenumbers in the power spectrum (Cen et al 2009;D'Aloisio et al 2018;Oñorbe et al 2019;Montero-Camacho et al 2019), but the small-scale effects of temperature fluctuations have still not been studied in detail.…”

Extracting the astrophysics of reionization from the Ly$α$ forest power spectrum: a first forecast

“…Using the same high-resolution simulations, Montero-Camacho et al (2019) reported for the first time the impact of inhomogeneous reionization in the Ly𝛼 forest 3D and 1D power spectrum at lower redshifts 1 . The expected effect of the memory of inhomogeneous reionization in terms of the relative difference is currently estimated to range from six per cent at 𝑧 = 2 up to fifty per cent at 𝑧 = 4 for the 3D Ly𝛼 power spectrum, and it ranges from a per cent at 𝑧 = 2 up to roughly twenty per cent at 𝑧 = 4 for the 1D power spectrum (Montero-Camacho & Mao 2020; hereafter "MM20").…”

“…(1) Small-scale simulations: we utilize the modified Gadget2 (Springel 2005) simulations described and tested in detail in Hirata 1 The 1D results in Montero-Camacho et al (2019) were compromised by an error in their mapping from 3D to 1D that was later corrected in Montero-Camacho & Mao (2020). (2018).…”

“…Nevertheless, these new measurements also come with new challenging systematics. One of such systematics is the thermal relics from inhomogeneous reionization in the Ly𝛼 forest, which includes the impact of the evolution of the small-scale structure (Montero-Camacho et al 2019;Montero-Camacho & Mao 2020).…”

The effect of inhomogeneous reionisation on the Lyman-$α$ forest power spectrum at redshift $z>4$: implications for thermal parameter recovery

“…Cen et al 2009;Keating et al 2018;D'Aloisio et al 2019) or semi-numerical reionisation models (e.g. Lidz & Malloy 2014;Montero-Camacho et al 2019), these neglect the dynamical effect of patchy heating on the small scale structure of the Ly𝛼 forest (i.e. the scales at wavenumbers 𝑘 ∼ 0.1 km −1 s that are most sensitive to the IGM thermal state and the coldness of dark matter at 𝑧 > 4).…”