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

TL;DR

This study measures the clustering of Lyα absorbers across redshifts 1.7 to 3.5, revealing how their spatial correlations evolve and are influenced by physical properties of the intergalactic medium, with implications for cosmic evolution.

Contribution

It provides the first detailed measurements of two- and three-point correlation functions of Lyα absorbers over this redshift range and compares them with simulations to understand underlying physical processes.

Findings

Positive correlation detected up to 8 h^{-1} cMpc in all redshift bins.

Correlation strength increases with HI column density.

Simulations match observations at z~2 but underpredict clustering at higher redshifts.

Abstract

We present redshift space two-point ($\xi$), three-point ($\zeta$) and reduced three-point (Q) correlations of Ly$\alpha$ absorbers (Voigt profile components having HI column density, $N_{\rm HI}>10^{13.5}$cm$^{-2}$) over three redshift bins spanning $1.7< z<3.5$ using high-resolution spectra of 292 quasars. We detect positive $\xi$ up to 8 $h^{-1}$ cMpc in all three redshift bins. The strongest detection of $\zeta =1.81\pm 0.59$ (with Q$=0.68\pm 0.23$), is in $z=1.7-2.3$ bin at $1-2h^{-1}$ cMpc. The measured $\xi$ and $\zeta$ values show an increasing trend with $N_{\rm HI}$, while Q remains relatively independent of $N_{\rm HI}$. We find $\xi$ and $\zeta$ to evolve strongly with redshift. Using simulations, we find that $\xi$ and $\zeta$ 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 Ly$\alpha$ absorbers at small scales, i.e $<0.5h^{-1}$ cMpc, the HI photo-ionization rate ($\Gamma_{\rm HI}$) has a strong influence at all scales. The strong redshift evolution of $\xi$ and $\zeta$ (for a fixed $N_{\rm HI}$-cutoff) is driven by the redshift evolution of the relationship between $N_{\rm HI}$ and baryon overdensity. Our simulation using best-fitted $\Gamma_{\rm HI}(z)$ measurements produces consistent clustering signals with observations at $z\sim 2$ but under-predicts the clustering at higher redshifts. One possible remedy is to have higher values of $\Gamma_{\rm HI}$ at higher redshifts. Alternatively the discrepancy could be related to non-equilibrium and inhomogeneous conditions prevailing during HeII reionization not captured by our simulations.