\begin{table}%t3
%\centering
\par
\caption{\label{table:slength}Exponential scale lengths of dust and gas emissions from M~31.  }
\begin{tabular}{ l l l l} 
\hline\hline
\noalign{\smallskip}
 &  $l_{\rm N}$~(kpc) &  $l_{\rm S}$~(kpc) & $l$~(kpc)\\
\hline
IR 160~$\mu$m  & 1.86~$\pm$~0.06 & 3.87~$\pm$~0.18 & 2.29~$\pm$~0.11 \\
IR 70~$\mu$m  & 1.38~$\pm$~0.08 & 3.07~$\pm$~0.17  & 1.66~$\pm$~0.09\\
IR 24~$\mu$m  & 1.38~$\pm$~0.07 & 3.44~$\pm$~0.21  & 1.57~$\pm$~0.06 \\
H$_2$           & 1.14~$\pm$~0.07 & 2.40~$\pm$~0.22 &  1.27~$\pm$~0.11 \\
HI           & 4.54~$\pm$~0.24 & 15.90~$\pm$~1.70 & 5.90~$\pm$~0.20 \\
HI+2H$_2$    & 3.80~$\pm$~0.12 & 9.00~$\pm$~0.69  & 4.73~$\pm$~0.21 \\
H$\alpha$    & 1.08~$\pm$~0.03 & 2.92~$\pm$~0.10  & 1.39~$\pm$~0.04 \\
\hline
$\tau_{\rm H\alpha}$ & 2.48~$\pm$~0.07 & 5.06~$\pm$~0.22 & 3.17~$\pm$~0.15\\
%
\hline
\end{tabular}
\tablefoot {The scale lengths were calculated from $R$~=~10.9~kpc to $R$~=~14.9~kpc for the whole galaxy~($l$) and the northern half  ($l_{\rm N}$) and from $R$~=~8.9~kpc to $R$~=~14.9~kpc for the southern half  ($l_{\rm S}$). The scale length of $\tau_{\rm H\alpha}$ is also shown for comparison.}
\end{table}