Abstract We present simultaneous observations of 12CO, 13CO and C18O J=1−0 emission in 11 nearby (cz <1000 km s−1 ) bright infrared galaxies. Both 12CO and 13 CO are detected in the centers of all the galaxies, except for 13CO in NGC 3031. We have also detected C18 O, CSJ=2−1 and HCO+J=1−0 emission in the nuclear regions of M82 and M51. These are the first systematical extragalactic detections of 12 CO and its isotopes from the PMO 14 m telescope. We have conducted half-beam-spaced mapping of M82 over an area of 4' × 2.5' and major axis mapping of NGC 3627, NGC 3628, NGC 4631 and M51. The radial distributions of 12CO and 13 CO in NGC 3627, NGC 3628 and M51 can be well fitted by an exponential profile. The 12CO/13CO intensity ratio, R, decreases monotonically with the galactocentric radius in all mapped sources. The average R in the center and disk of the galaxies are 9.9±3.0 and 5.6±1.9, respectively, much lower than the peculiar R(~24) found in the center of M82. The intensity ratios of 13CO/C18O, 13 CO/HCO+ and 13CO/CS (either our or literature data) show little variation with galactocentric radius, in sharp contrast with the greatly varied R. This supports the notion that the observed gradient in R could be the result of the variations of the physical conditions across the disks. The H2 column density derived from C18O shows that the Galactic standard conversion factor (X-factor) overestimates the amount of the molecular gas in M82 by a factor of ~2.5. These observations suggest that the X-factor in active star-forming regions (i.e., nuclear regions) should be lower than that in normal star-forming disks and the gradient in R can be used to trace the variations of the X-factor.

Keywords galaxies: ISM — ISM: clouds — ISM: molecules — radio lines: ISM

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