% %Begin Author names Valentina D'Odorico$^1$
Patrick Petitjean$^1$
Stefano Cristiani$^2$ %End Author names
% We present preliminary results of an investigation of the clustering properties of high matter density peaks between redshift $\sim 2$ and $\sim 3$, as traced by Lyman limit and Damped \Lya\ systems in spectra of close QSO pairs and groups. %
vdodorico.tex
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\newcommand{\Lya}{Ly$\alpha$}
\newcommand{\Lyb}{Ly$\beta$}
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\newcommand{\kms}{km s$^{-1}$}
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%\newcommand{\gsim}{\raisebox{-5pt}{$\;\stackrel{\textstyle >}{\sim}\;$}}

\begin{document}
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\heading{%
%Begin Heading
%
Coincidences of High Density Peaks in UVES Spectra of QSO Pairs  
%
%End Heading
} 
\par\medskip\noindent
%
\author{%
%Begin Author names
Valentina D'Odorico$^1$, Patrick Petitjean$^1$, Stefano
Cristiani$^2$ 
%End Author names
}
\address{%
Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 
       F--75014 Paris, France}
\address{
European Southern Observatory, Karl-Schwarzschild-Strasse
2, D--85748 Garching, Germany 
}


\begin{abstract}
%
We present preliminary results of an investigation of
the clustering properties of high matter density peaks
between redshift $\sim 2$ and $\sim 3$, as
traced by Lyman limit and Damped \Lya\ systems in
spectra of close QSO pairs and groups. 

%
\end{abstract}
%
%
\section{Introduction}
%
Paired lines of sight (LOS) toward high redshift quasars,
with angular separations up to a few arcminutes, are a
useful tool to investigate the clustering properties of
absorption lines. 
We have obtained with UVES high resolution spectra
($R\simeq 37000$) of two QSO pairs with separations 1 and
5 arcmin and a QSO triplet with reciprocal separations 1,
8 and 8 arcmin, spanning the redshift range $1.6 \lsim z
\lsim 3.2$ (see \cite{vale01} for further details). 
We assume that high matter density peaks
are traced by optically thick absorbers
(i.e. with column density $N({\rm HI}) \gsim 2\times
10^{17}$ \cm).
The present spectra are scanned to detect the presence of 
high density peaks. We find 5 systems with $N({\rm HI})
\gsim 10^{19}$ \cm\ and 7 with $2\times 10^{17}\lsim\, 
N({\rm HI}) \lsim 10^{19}$ \cm.  
As a second step, we look systematically for
coincident absorptions at the same redshift as the
identified high column density systems. 

%
\section{Results}
%
%Three damped \Lya\ systems ($N(\rm HI) > 2\ 10^{20}$ \cm) are
%present in our spectra while 1 would be expected from the
%corresponding number density as a function of redshift
%\cite{slw00}. 
Out of 5 detected absorption systems with $N({\rm HI})
\gsim 10^{19}$ \cm, 3 of them have a corresponding metal
system in the companion LOS at a velocity difference of
less than 200 \kms. One of them is at less then 1000
\kms\ from the emission redshift of the paired QSO
(also marking a high density peak) and the last
one has a corresponding, weak \Lya\ absorption line but
no metal absorption within $\sim 9000$ \kms. 
%any correspondence in the LOS at a
%transverse spatial separation of $\sim 650\ h_{100}^{-1}$
%kpc.   
From the number density of C~IV systems with rest
equivalent width $w_0 > 0.15$ \AA, as a function of
redshift \cite{steidel90}, we compute the chance
probability (in the hypothesis of null clustering) to
detect a C~IV absorption line within 200 \kms\ between $z
=2$ and 3, ${\cal P}(z) \simeq 0.001$.  
The transverse spatial separation over which these
coincidences happen varies between $\sim 4$ and 7
$h_{100}^{-1}$  comoving Mpc, which suggests that we are
detecting the clustering signal of galactic objects, as
verified in the past 
%for the system at $z_{\rm a} \sim 2.38$
%in the LOS to Q2138-4427 and Q2139-4434
\cite{fh93,francis96,fwd97}.  
These separations could be indeed reasonable correlation 
lengths for normal or dwarf galaxies at this redshift
since at $z \sim 3$ the Lyman break galaxies are found to
show correlation lengths $\sim 2\ h_{100}^{-1}$  Mpc 
\cite{giav98,cris01}.
 
As for the 7 Lyman limit systems (LLS) with $N({\rm HI})
\lsim 10^{19}$ \cm, 4 form two coincident pairs 
%at $z_{\rm a} \sim 2.03$ and 2.12 
along two LOS separated by $\sim 1$ arcmin; the
similarity of the coincident absorptions suggests that
the two LOS could be piercing a coherent filament-like
structure. 
The remaining three systems do show corresponding \Lya\
absorption lines but no metal absorption within 3000
\kms, at transverse spatial separations of $\sim 3.8\
h_{100}^{-1}$ Mpc, and in the triplet $\sim 6.7\
h_{100}^{-1}$ Mpc and $\sim 840\ h_{100}^{-1}$ kpc.   
 

%
%\begin{center}
%\begin{tabular}{c r l}
%\multicolumn{3}{l}{{\bf Table 1.} Here is an example for Tables } \\
%\hline
%\\
%\multicolumn{1}{c}{$\lambda_{\rm helio}$}&\multicolumn{1}{c}{$w_{\rm obs}$}&
%\multicolumn{1}{c}{Identification}\\
%\multicolumn{1}{c}{(\AA)}&\multicolumn{1}{c}{(\AA)}&
%\multicolumn{1}{c}{ }\\
%\hline
%\\
%1215.7 & 12.0  & Ly$\alpha$ \\
%3333.3 & 7.1   & H$_2$      \\
%3332.8 & 0.7   & D$_2$      \\
%9797.1 & 0.1   & Pas s\^ur  \\
%\\
%\hline
%\end{tabular}
%\end{center}
%
%
\begin{figure}
\centerline{\vbox{
\psfig{figure=vdodoricoF1.eps,height=6.cm}}}
\caption[]{Summary of the observed coincidences as a
function of redshift. 
The dotted lines mark the observed redshift ranges. The  
angular separations of the quasars are reported between  
the solid vertical lines. The
symbols are: open square for metal systems, solid square
for LLS with $N({\rm HI})<10^{19}$ \cm\ and star for
system with $N({\rm HI})>10^{19}$ \cm. 
The big open circles mark the emission
redshift of the quasars}
\end{figure}
%

\acknowledgements{V.D. is supported by a Marie Curie
individual fellowship  from the European Commission under 
the programme ``Improving Human Research Potential and
the Socio-Economic Knowledge Base'' (Contract
no. HPMF-CT-1999-00029). We thank
C. Ledoux for the UVES spectrum of Q2138-4427.}

\begin{iapbib}{99}{
\bibitem{vale01} D'Odorico V., Petitjean P., Cristiani
S., 2001, in preparation
\bibitem{fh93} Francis P.~J., Hewett P.~C., 1993, AJ, 105
1633 
\bibitem{fwd97} Francis P.~J., Woodgate B.~E., Danks
A.~C., 1997, ApJ, 482 L25
\bibitem{francis96} Francis P.~J., Woodgate B.~E., Warren
S.~J., et al., 1996, ApJ, 457 490
\bibitem{giav98} Giavalisco M., Steidel C.~C., Adelberger
K.~L., et al., 1998, ApJ, 503 543
\bibitem{cris01} Porciani C., Giavalisco M., 2001, ApJ
accepted, astro-ph/0107447
\bibitem{steidel90} Steidel C.~C., 1990, ApJS, 72 1
%\bibitem{slw00} Storrie-Lombardi L.~J., Wolfe A.~M.,
%2000, ApJ, 543 552
%\bibitem{MMM} Marx G., Marx H., \& Marx C., 1938, eds Chaplin C.,
%              in {\it Our lives}. MGM Editions, Paris, p. 129
}
\end{iapbib}
%
\vfill
\end{document}





astro-ph0109373.extracted_bib
\bibitem{vale01} D'Odorico V., Petitjean P., Cristiani
S., 2001, in preparation

\bibitem{fh93} Francis P.~J., Hewett P.~C., 1993, AJ, 105
1633 

\bibitem{fwd97} Francis P.~J., Woodgate B.~E., Danks
A.~C., 1997, ApJ, 482 L25

\bibitem{francis96} Francis P.~J., Woodgate B.~E., Warren
S.~J., et al., 1996, ApJ, 457 490

\bibitem{giav98} Giavalisco M., Steidel C.~C., Adelberger
K.~L., et al., 1998, ApJ, 503 543

\bibitem{cris01} Porciani C., Giavalisco M., 2001, ApJ
accepted, astro-ph/0107447

\bibitem{steidel90} Steidel C.~C., 1990, ApJS, 72 1
%
\bibitem{slw00} Storrie-Lombardi L.~J., Wolfe A.~M.,
%2000, ApJ, 543 552
%
\bibitem{MMM} Marx G., Marx H., \& Marx C., 1938, eds Chaplin C.,
%              in {\it Our lives}. MGM Editions, Paris, p. 129
}
