HST STIS observations of four QSO pairs

We present HST STIS observations of four quasar pairs with redshifts 0.84<z(em)<1.56 and angular separation 2-3 arcmin corresponding to ∼1-1.5 h(50)(-1) Mpc transverse proper distance at zsimilar to0.9. We study the distribution of velocity differences between nearest neighbor HI Lyman-alpha absorption lines detected in the spectra of adjacent QSOs in order to search for the possible correlation caused by the extent or the clustering properties of the structures traced by the absorption lines over such a scale. The significance of the correlation signal is determined by comparison with Monte-Carlo simulations of spectra with randomly distributed absorption lines. We find an excess of lines with a velocity separation smaller than DeltaV=500 km s(-1) significant at the 99.97% level. This clearly shows that the Lyman-alpha forest is correlated on scales larger than 1 h(50)(-1) Mpc at zsimilar to1. However, out of the 20 detected coincidences within this velocity bin, 12 have V>200 km s(-1). This probably reflects the fact that the scale probed by our observations is not related to the real size of individual absorbers but rather to large scale correlation. Statistics are too small to conclude about any difference between pairs separated by either 2 or 3 arcmin. A damped Lyman-alpha system is detected at z(abs)=1.2412 toward LBQS 0019-0145A with log N(HI)similar to20.5. From the absence of ZnII absorption, we derive a metallicity relative to solar [Zn/H]< -1.75.