HST STIS observations of four QSO pairs

Free access article

Issue		A&A Volume 391, Number 1, August III 2002


Page(s)		1 - 12
Section		Cosmology
DOI		http://dx.doi.org/10.1051/0004-6361:20020733

A&A 391, 1-12 (2002)
DOI: 10.1051/0004-6361:20020733

$\vec{HST}$ STIS observations of four QSO pairs

B. Aracil¹, P. Petitjean^{1, 2}, A. Smette^{3, 4}, J. Surdej^{4, 5}, J. P. Mücket⁶ and S. Cristiani^{7, 8}

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

(Received 26 November 2001 / Accepted 15 May 2002)

Abstract
We present HST STIS observations of four quasar pairs with redshifts $0.84<z_{\rm em}<1.56$ and angular separation 2-3 arcmin corresponding to ~1-1.5 h^-1₅₀ Mpc transverse proper distance at $z\sim0.9$ . We study the distribution of velocity differences between nearest neighbor $\ion{H}{i}$ 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 $\Delta V=500$ 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^-1₅₀ Mpc at $z \sim 1$ . However, out of the 20 detected coincidences within this velocity bin, 12 have $\Delta V >200$ 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 $z_{\rm abs}=1.2412$ toward LBQS 0019-0145A with log N( $\ion{H}{i}$ ) $\sim20.5$ . From the absence of $\ion{Zn}{ii}$ absorption, we derive a metallicity relative to solar $[{\rm Zn/H}]<-1.75$ .

Key words: quasars: absorption lines -- galaxies: ISM -- galaxies: halo

Offprint request: B. Aracil, aracil@iap.fr

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