The flux auto- and cross-correlation of the Ly alpha forest. I. Spectroscopy of QSO pairs with arcminute separations and similar redshifts

The Ly alpha forest has opened a new redshift regime for cosmological investigation. At z > 2 it provides a unique probe of cosmic geometry and an independent constraint on dark energy that is not subject to standard candle or ruler assumptions. In Paper I of this series on using the Ly alpha forest observed in pairs of QSOs for a new application of the Alcock-Paczynski test, we present and discuss the results of a campaign to obtain moderate-resolution spectroscopy (FWHM similar or equal to 2.5 angstrom) of the Ly alpha forest in pairs of QSOs with small redshift differences (Delta z < 0:25, z > 2:2) and arcminute separations (theta < 5'). This data set, composed of seven individual QSOs, 35 pairs, and one triplet, is also well suited for future investigations of the coherence of Ly alpha absorbers on similar to 1 Mpc transverse scales and the transverse proximity effect. We note seven revisions for previously published QSO identifications and/or redshifts.