\begin{table}%T1
\caption{\label{totalgas_m51_properties}Properties of $\Sigma_{\rm gas}$ and $\Sigma_{\rm
H2}$/$\Sigma_{\rm HI}$ in M~51 averaged over the areas shown in
Fig.~\ref{total_gas}.} 
\small%\centering
\par
\begin{tabular}{lrr}   
\hline \hline   
                             & $\Sigma_{\rm gas}$  &
                             $\Sigma_{\rm H2}$/$\Sigma_{\rm HI}$ \\    
                             & [\msun~pc$^{-2}$] &
                              \\    
\par
\noalign{\smallskip} \hline \noalign{\smallskip}   
inner spiral arms & 26.8  & 2.3  \\               
outer spiral arms & 18.8  & 0.21  \\                   
interarm region & 9.4  & 0.71 \\                   
outer disk      & 6.4     &   --   \\
NGC~5195 -- companion & 20.2 & 0.4 \\
\hline             
ratio inner/outer spiral & 1.4 & 11.0 \\
inner arm/interarm       & 2.9 & 3.2 \\
outer arm/interarm       & 2.0 & 0.3  \\
 \hline   
\end{tabular} 
\end{table}