\begin{table}%t4
\par
\caption{\label{tab:comp_samples}Summary of the main properties of our and the comparison samples.}
\small%\centerline
{
\begin{tabular}{llrrrr}
\hline \hline
Ref. & Type & $z$ & $N$ & Area & Ly$\alpha$ luminosity \\
 & & & & sq.arcmin & erg~s$^{-1}$ \\
\hline
This sample & Spectroscopic/photometric&  $\sim$3.0 & 59/83 & 133 & $\ga$$0.3\times 10^{42}$\\
& Around absorption systems\\
Serendipitous & Spectroscopic & $2.30\leq z\leq 3.61$ & 6 & $-$ & $-$ \\
& Field\\
\cite{gronwall2007}$^a$ & Photometric only& $\sim$3.1 & 162 & 1008 & $\ga$$1.3\times 10^{42}$\\
 & Field\\
\cite{ouchi2008}$^b$ & Photometric only & $\sim$$3.1$ & 356 & 3538 & $\ga$$10^{42}$\\
& Field\\
\cite{rauch2008} & Long-slit spectroscopy & $ 2.67\leq z\leq 3.75 $ & 27 & $-$ & $(0.015{-}1.73)\times 10^{42}$\\
& Field\\
\hline
\end{tabular}}
\smallskip
$^a$ These authors have also carried out spectroscopic observations for a subsample of 92 candidates (Gawiser et~al. 2007).\\
$^b$   These authors have also carried out spectroscopic observations for a subsample of 41 candidates.  
\end{table}