CO observations of a sample towards nearby galaxies
  We have simultaneously observed $^{12}$CO, $^{13}$CO, and C$^{18}$O (J=1$-$0)
rotational transitions in the centers of a sample of 58 nearby spiral galaxies
using the 13.7-m millimeter-wave telescope of the Purple Mountain Observatory.
Forty-two galaxies were detected in $^{13}$CO emission, but there was a null
detection for C$^{18}$O emission with a sigma upper limit of 2 mK. Central beam
ratios, \R, of $^{12}$CO and $^{13}$CO range mostly from 5 to 13, with an
average value of 8.1$\pm$4.2, which is slightly lower than previous estimates
for normal galaxies. Clear correlations are found between $^{12}$CO and
$^{13}$CO luminosities. An average $X$-factor of $1.44\pm0.84\times10^{20}\
\mathrm{cm}^{-2} (\mathrm{K} \mathrm{km} \mathrm{s}^{-1})^{-1}$ is slightly
lower than that in the Milky Way.
