Small-scale structure and the Lyman-α forest baryon acoustic oscillation feature

Abstract

The baryon-acoustic oscillation (BAO) feature in the Lyman-α forest is a key probe of the cosmic expansion rate at redshifts z ∼ 2.5, well before dark energy is believed to have become significant. A key advantage of the BAO as a standard ruler is that it is a sharp feature and hence is more robust against broadband systematic effects than other cosmological probes. However, if the Lyman-α forest transmission is sensitive to the initial streaming velocity of the baryons relative to the dark matter, then the BAO peak position can be shifted. Here we investigate this sensitivity using a suite of hydrodynamic simulations of small regions of the intergalactic medium with a range of box sizes and physics assumptions; each simulation starts from initial conditions at the kinematic decoupling era (z ∼ 1059), undergoes a discrete change from neutral gas to ionized gas thermal evolution at reionization (z ∼ 8), and is finally processed into a Lyman-α forest transmitted flux cube. Streaming velocities suppress smallscale structure, leading to less violent relaxation after reionization. The changes in the gas distribution and temperature-density relation at low redshift are more subtle, due to the convergent temperature evolution in the ionized phase. The change in the BAO scale is estimated to be of the order of 0.12% at z = 2.5; some of the major uncertainties and avenues for future improvement are discussed. The predicted streaming velocity shift would be a subdominant but not negligible effect (of order 0.26σ) for the upcoming DESI Lyman-α forest survey, and exceeds the cosmic variance floor.

Inhomogeneous reionization models in cosmological hydrodynamical simulations

“…This is in good agreement with cosmological simulations with 3D radiative transfer that obtain velocities from ∼ 15 h −1 ckpc Myr −1 (∼ 2 × 10 4 km s −1 ) around individual sources at first but accelerate up to ∼ 50 h −1 ckpc Myr −1 (∼ 7 × 10 4 km s −1 ) as larger bubbles form (see e.g. Iliev et al 2006;McQuinn et al 2007;Trac & Cen 2007;Hirata 2018). Recently, Kaurov (2016) presented a detailed analysis of the time-scale of the reionization process using the results of a 3D radiative transfer cosmological simulation.…”

Robust velocity-induced acoustic oscillations at cosmic dawn

“…where v i cb is the value of the DM-baryon relative velocity at kinematic decoupling, z kin [45]. Additionally, we are free to choose a reference frame, which we set at the DM rest frame by setting…”

Impact of inhomogeneous reionization on the Lyman-α forest

“…These simulations generate a map of optical depth with arbitrary normalization (Hirata 2018). The requirement to reproduce the observed mean flux sets the normalization.…”

“…We use our smaller simulations to calculate how the transparency of the Lyman-α forest depends on when reionization occurs. Our small-box simulations ( §3.1) compute Lyman-α transmission by the procedure described in detail in §4.4 of Hirata (2018). Here we briefly describe the strategy used, and the methodology for extracting ψ from our simulations.…”

“…of modeling of these issues led to only minor changes in the simulations by Hirata (2018), so we did not consider them in this paper, but only a few alternative models were tested and more should be explored. In addition, our small-scale simulations do not include fluctuations in the photoionization rate due to the clustering of the ionizing sources (Pontzen 2014;Gontcho A Gontcho et al 2014), which will become especially relevant for the lower redshifts (z < 4).…”