Abstract We have simultaneously observed 12CO, 13CO and C18O (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 13CO emission, but there was a null detection for C18O emission with a σ upper limit of 2 mK. The central beam ratios, R, of 12 CO and 13CO 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 12CO and 13CO luminosities. An average X factor of 1.44 ± 0.84 × 1020 cm−2 (K km s −1)−1 is slightly lower than that in the Milky Way.
Keywords galaxies: ISM — molecules: galaxies — millimeter lines: ISM — star formation: ISM
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