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We present simultaneous observations of CO,13CO and C18O J=1-0 emission in 11nearby (cz<1000 km/s) bright infrared galaxies. Both 12CO and 13CO are detectedin the centers of all galaxies, except for 13CO in NGC 3031. We have alsodetected C18O, CS J=2-1, and HCO+ J=1-0 emission in the nuclear regions of M82and M51. These are the first systematical extragalactic detections of 12CO andits isotopes from the PMO 14m telescope. We have conducted half-beam spacingmapping of M82 over an area of 4'*2.5' and major axis mapping of NGC 3627, NGC3628, NGC 4631, and M51. The radial distributions of 12CO and 13CO in NGC 3627,NGC 3628, and M51 can be well fitted by an exponential profile. The 12CO/13COintensity ratio,R,decreases monotonically with galactocentric radius in allmapped sources. The average R in the center and disk of the galaxies are9.9+/-3.0 and 5.6+/-1.9 respectively, much lower than the peculiar R(~24) foundin the center of M82. The intensity ratios of 13CO/C18O, 13CO/HCO+ and 13CO/CS(either ours or literature data) show little variations with galactocentricradius, in sharp contrast with the greatly varied R. This supports the notionthat the observed gradient in R could be the results of the variations of thephysical conditions across the disks. The H_2 column density derived from C18Oshows that the Galactic standard conversion factor (X-factor) overestimates theamount of the molecular gas in M82 by a factor of ~2.5. These observationssuggest that the X-factor in active star-forming regions (i.e., nuclearregions) should be lower than that in normal star-forming disks, and thegradient in R can be used to trace the variations of the X-factor.

12CO, 13CO and C18O observations along the major axes of nearby bright infrared galaxies