\begin{table}%t5
\caption{Correlation between the gradients of age and metallicity and the
central velocity dispersion. The brackets show the different indicators used
to calculate the metallicity gradients. The first and the second rows refer to
the values calculated using the ten indicators described in Sect.~\ref{sec-age.metallicity.gradients}.  $t$: $t$-parameter calculated as the
quotient of the slope and its error, computed through Monte~Carlo simulations;
Pnc: probability of no correlation according to a~non-parametric Spearman test.}
\label{table.corr.sigmacentral}
%\centering
\small
\begin{tabular}{@{}lrrr@{}}
\hline\hline
                              & $N$ &  $t$ &  Pnc\\ 
\hline
grad age                      &54  &$-1.736$& 0.168\\
grad [M/H]                    &54  &  0.810 & 0.106\\
grad [M/H] (CN$_2$--H$\beta$) &54  &  1.075 & 0.789\\
grad [M/H] (Ca4227--H$\beta$) &54  &  0.251 & 0.986\\
grad [M/H] (Fe4383--H$\beta$) &54  &  1.303 & 0.244\\
grad [M/H] (C4668--H$\beta$)  &54  &  0.026 & 0.455\\
grad [M/H] (Mgb--H$\beta$)    &54  &  0.578 & 0.934\\   
\hline
\end{tabular}
\end{table}