ar X iv : a st ro - p h / 04 05 20 5 v 1 1 1 M ay 2 00 4 Large - Scale Correlations in the Ly α Forest at z = 3 − 4 1

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 (zem ≈ 3.63, ∆θ = 39) and Q1439-0034A/B (zem ≈ 4.25, ∆θ = 33). The crosscorrelation of transmitted flux in the Lyα forest shows a 5 − 7σ 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 proper kpc transverse separation between sightlines. We also statistically examine the similarity between paired spectra as a function of transmitted flux, a measure which 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 impact for S/N & 10 per resolution element. However, the agreement between the autocorrelation along the line sight and the crosscorrelation between sightlines, a potential test of cosmological geometry, depends significantly on instrumental resolution. Finally, we present an inventory of metal lines. These include a a pair of strong C iv systems at z ≈ 3.4 appearing only toward Q1439B, and a Mg ii + Fe ii system present toward Q1439 A and B at z ≈ 1.68. Subject headings: cosmology: observations — intergalactic medium — large scale structure of the universe — quasars: absorption lines — quasars: individual (Q1422+2309, Q1424+2255, Q1439-0034A, Q1439-0034B) The observations were made at the W.M. Keck Observatory which is operated as a scientific partnership between the California Institute of Technology and the University of California; it was made possible by the generous support of the W.M. Keck Foundation. Palomar Observatory, California Institute of Technology, Pasadena, CA 91125, USA; [email protected], [email protected] Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA; [email protected]