2022MNRAS.509.1536M

We present redshift-space two-point (ξ), three-point (ζ), and reduced three-point (Q) correlation of Lyα absorbers (Voigt profile components having H I column density, N_{H I} > 10^13.5 cm^–2) over three redshift bins spanning 1.7 < z < 3.5 using high-resolution spectra of 292 quasars. We detect positive ξ up to 8 h^–1 cMpc in all three redshift bins. The strongest detection of ζ = 1.81 ± 0.59 (with Q = 0.68 ± 0.23) is in z = 1.7-2.3 bin at 1-2 h^–1 cMpc. The measured ξ and ζ values show an increasing trend with N_HI, while Q remains relatively independent of N_HI. We find ξ and ζ to evolve strongly with redshift. Using simulations, we find that ξ and ζ seen in real space may be strongly amplified by peculiar velocities in redshift space. Simulations suggest that while feedback, thermal and pressure smoothing effects influence the clustering of Lyα absorbers at small scales, i.e. <0.5 h^–1 cMpc, the H I photoionization rate (Γ_HI) has a strong influence at all scales. The strong redshift evolution of ξ and ζ (for a fixed N_H I_ cut-off) is driven by the redshift evolution of the relationship between N_H I_ and baryon overdensity. Our simulation using best-fitting Γ_H I_(z) measurements produces consistent clustering signals with observations at z ∼ 2 but underpredicts the clustering at higher redshifts. One possible remedy is to have higher values of Γ_H I_ at higher redshifts. Alternatively the discrepancy could be related to non-equilibrium and inhomogeneous conditions prevailing during He II reionization not captured by our simulations.