CO observations towards a sample of nearby galaxies

We have simultaneously observed (CO)-C-12, (CO)-C-13 and (CO)-O-18 (J = 1 - 0) rotational transitions in the centers of a sample of 58 nearby spiral galaxies using the 13.7-m millimeter-wave telescope administered by Purple Mountain Observatory. Forty-two galaxies were detected in (CO)-C-13 emission, but there was a null detection for (CO)-O-18 emission with a sigma upper limit of 2 mK. The central beam ratios, R, of (CO)-C-12 and (CO)-C-13 range mostly from 5 to 13, with an average value of 8.1 +/- 4.2, which is slightly lower than previous estimates for normal galaxies. Clear correlations are found between (CO)-C-12 and (CO)-C-13 luminosities. An average X factor of 1.44 +/- 0.84 x 10(20) cm(-2) (K km s(-1))(-1) is slightly lower than that in the Milky Way.