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

We present a search for spatial and redshift correlations in a 2 Angstrom, resolution spectroscopic survey of the Ly alpha forest at 2.15 < z < 3.37 toward 10 QSOs concentrated within a 1 degrees diameter field. We find a signal at 2.7 sigma significance for correlations of the Ly alpha absorption-line wavelengths between different lines of sight over the whole redshift range. The significance rises to 3.2 sigma if we restrict the redshift range to 2.60 < z < 3.37, and to 4.0 sigma if we further restrict the sample to lines with rest equivalent widths 0.1 less than or equal to W-0 < 0.9 Angstrom. We conclude that a significant fraction of the Ly alpha forest arises in structures whose correlation length extends at least over 30' (similar to 26 h(-1) comoving Mpc at z = 2.6 for H-0 = 100 h km s(-1) Mpc(-1), Omega = 1.0, Lambda = 0). We have also calculated the three-dimensional two-point correlation function for Ly alpha 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 Ly alpha absorbers in simulated data. We find that the Ly alpha forest redshift distribution provides a more sensitive test for such clustering than the three-dimensional two-point correlation function.