\begin{table}%t4
\caption{\label{tab:summary-z}Relative velocities in the 8~o'clock arc.} 
%\centering
 \small
\begin{tabular}{lcc}   
\hline\hline \noalign{\smallskip}     
Spectral features & \multicolumn{1}{c}{$z$\tablefootmark{a}} & \multicolumn{1}{c}{$\upsilon$\tablefootmark{b}} \\
                  & \multicolumn{1}{c}{}                & \multicolumn{1}{c}{(km~s$^{-1}$)} \\   
\hline                     
Stellar photospheric absorption lines & $z_{\rm stars}$  	            = 2.7347 & --24 \\
\ion{H}{ii} emission lines            & $z_{\rm C~\mathsc{iii}]}$    = 2.7350 & 0 \\
Recombination emission line           & $z_{{\rm Si~\mathsc{ii}}^*}$ = 2.7352 & +16 \\
Interstellar absorption lines         & $z_{\rm ISM}$    = 2.7335 & --120 \\
Ly$\alpha$ emission line              & $z_{\rm Ly\alpha}$	    = 2.7420 & +570 \\
\hline                       
\end{tabular}
\tablefoot {\tablefoottext{a}{Vacuum heliocentric;}
\tablefoottext{b}{relative to the systemic redshift $z_{\rm sys} = 2.7350$ that is the mean of $z_{\rm stars}$ and $z_{\rm em}$ (the~mean redshift of emission lines).}}
\end{table}