Measurement of redshift-space two- and three-point correlation of Ly$α$ absorbers at $1.7< z <3.5$: Implications on evolution of the physical properties of IGM

Abstract

We present redshift space two-point (𝜉), three-point (𝜁) and reduced three-point (Q) correlation of Ly𝛼 absorbers (i.e Voigt profile components having H column density 𝑁 HI > 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 is seen in 𝑧 = 1.7 − 2.3 redshift bin at 1 − 2ℎ −1 cMpc with an amplitude of 1.81 ± 0.59 ( ∼ 3.1𝜎 level). The corresponding Q is found to be 0.68 ± 0.23. 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 𝑧 over the redshift range studied. Using hydrodynamical simulations, we find that the 𝜉 and especially 𝜁 seen in real space may be strongly amplified by peculiar velocities in redshift space. Simulations also suggest that while feedback, thermal and pressure smoothing effects affect the clustering of Ly𝛼 absorbers at small scales, i.e < 0.5ℎ −1 cMpc, the H photo-ionization rates (Γ HI ) strongly influences the correlation amplitudes at all scales. We find that the strong redshift evolution shown by 𝜉 and 𝜁 is primarily sourced by the redshift evolution of the relationship between 𝑁 HI and baryon overdensity (Δ). Our simulations that uses available best fitted Γ HI (𝑧) measurements produce consistent clustering signals with observations at 𝑧 ∼ 2 but under-predict the clustering at higher redshifts. One possible remedy is to have higher values of Γ HI at higher redshifts compared to the existing measurements. Alternatively the discrepancy could be related to nonequilibrium and inhomogeneous conditions prevailing during He reionization not captured by our simulations.