Large-scale correlations in the Lyα forest at z=3-4

We present a study of the spatial coherence of the intergalactic medium toward two pairs of high-redshift quasars with moderate angular separations observed with Keck ESI, Q1422+2309A/Q1424+2255 (z(em) approximate to 3.63, Deltatheta 39") and Q1439-0034A/B (z(em) approximate to 4.25, Deltatheta = 33"). The cross-correlation of transmitted flux in the Lyalpha forest shows a 5-7 sigma peak at zero velocity lag for both pairs. This strongly suggests that at least some of the absorbing structures span the 230 300 h(70)(-1) 70 proper kpc transverse separation between sight lines. We also statistically examine the similarity between paired spectra as a function of transmitted flux, a measure that may be useful for comparison with numerical simulations. In investigating the dependence of the correlation functions on spectral characteristics, we find that photon noise has little effect for a signal-to-noise ratio of greater than or similar to10 per resolution element. However, the agreement between the autocorrelation along the line sight and the cross-correlation between sight lines, a potential test of cosmological geometry, depends significantly on instrumental resolution. Finally, we present an inventory of metal lines. These include a pair of strong C IV systems at z approximate to 3.4, appearing only toward Q1439B, and an Mg II + Fe II system present toward Q1439A/B at z approximate to 1.68.