Evidence for large-scale structure in the Ly alpha forest at z > 2.6

We present a search for spatial and redshift correlations in a 2 A resolution spectroscopic survey of the Lyα forest at 2.15 < z < 3.37 toward 10 QSOs concentrated within a 1° diameter field. We find a signal at 2.7 σ significance for correlations of the Lyα absorption-line wavelengths between different lines of sight over the whole redshift range. The significance rises to 3.2 σ if we restrict the redshift range to 2.60 < z < 3.37, and to 4.0 σ if we further restrict the sample to lines with rest equivalent widths 0.1 ≤ W0 < 0.9 A. We conclude that a significant fraction of the Lyα forest arises in structures whose correlation length extends at least over 30' (~26 h-1 comoving Mpc at z = 2.6 for H0 ≡ 100 h km s-1 Mpc-1, Ω = 1.0, Λ = 0). We have also calculated the three-dimensional two-point correlation function for Lyα absorbers; we do not detect any significant signal in the data. However, we note that line blending prevents us from detecting the signal produced by a 100% overdensity of Lyα absorbers in simulated data. We find that the Lyα forest redshift distribution provides a more sensitive test for such clustering than the three-dimensional two-point correlation function.