\begin{table}%t4
\caption{Linear fits and probability of no correlation between the metallicity
gradient and the gradient of velocity dispersion. $N$: number of galaxies in
the fit; $b+\sigma(b)$: slope and error of the linear fit (the errors have been
calculated by Monte~Carlo simulations, as described in the text); $t$:
$t$-statistic to verify the hypothesis "$b=0$'' (a~$t$~value higher than~1.96
allows the hypothesis to be rejected, with a~significance level lower than~0.05);
Pnc: probability of no correlation in a~non-parametric Spearman test.}\label{table.gradsigma}
%\centering
\small
\begin{tabular}{@{}l rrrr@{}}
\hline\hline
                               & $N$ & \multicolumn{1}{c}{$b+\sigma(b)$} & 
                               $t$  & Pnc\\
\hline
grad age                       & 54  &$  1.727\pm 3.099$ & 0.56 & 0.885\\
grad [M/H]                     & 54  &$  0.175\pm 0.269$ & 0.65 & 0.238\\ 
grad [M/H] (CN$_2$--H$\beta$)  & 54  &$  1.134\pm 0.555$ & 2.04 & 0.002\\
grad [M/H] (Ca4227--H$\beta$)  & 54  &$  0.449\pm 0.686$ & 0.65 & 0.063\\
grad [M/H] (Fe4383--H$\beta$)  & 54  &$ -0.392\pm 0.442$ & 0.89 & 0.900\\
grad [M/H] (C4668--H$\beta$)   & 54  &$  0.451\pm 0.356$ & 1.27 & 0.062\\
grad [M/H] (Mgb--H$\beta$)     & 54  &$  0.876\pm 0.463$ & 2.11 & 0.001\\
\hline
\end{tabular}
\end{table}