STIS observations of four QSO pairs^*

¹ Institut d'Astrophysique de Paris – CNRS, 98bis boulevard Arago, 75014 Paris, France
² LERMA, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
³ Chercheur qualifié et Collaborateur Scientifique, Fonds National de la Recherche Scientifique, Belgium
⁴ Institut d'Astrophysique et de Géophysique, Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
⁵ Directeur scientifique, Fonds National de la Recherche Scientifique, Belgium
⁶ Astrophysikalisches Institut Potsdam, An der Sternwarte, Potsdam, Germany
⁷ Space Telescope European Coordinating Facility, Karl-Schwarzschild Str. 2, 85748 Garching, Germany
⁸ Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Corresponding author: B. Aracil, aracil@iap.fr

Received: 26 November 2001
Accepted: 15 May 2002

Abstract

We present HST  STIS observations of four quasar pairs with redshifts and angular separation 2–3 arcmin corresponding to ~1–1.5 Mpc transverse proper distance at . We study the distribution of velocity differences between nearest neighbor   Lyman-α 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  km s^-1 significant at the 99.97% level. This clearly shows that the Lyman-α forest is correlated on scales larger than  Mpc at . However, out of the 20 detected coincidences within this velocity bin, 12 have  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-α system is detected at toward LBQS 0019-0145A with log N() . From the absence of  absorption, we derive a metallicity relative to solar .