12CO, 13CO and C18O observations along the major axes of nearby bright
  infrared galaxies
  We present simultaneous observations of CO,13CO and C18O J=1-0 emission in 11
nearby (cz<1000 km/s) bright infrared galaxies. Both 12CO and 13CO are detected
in the centers of all galaxies, except for 13CO in NGC 3031. We have also
detected C18O, CS J=2-1, and HCO+ J=1-0 emission in the nuclear regions of M82
and M51. These are the first systematical extragalactic detections of 12CO and
its isotopes from the PMO 14m telescope. We have conducted half-beam spacing
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 13CO in NGC 3627,
NGC 3628, and M51 can be well fitted by an exponential profile. The 12CO/13CO
intensity ratio,R,decreases monotonically with 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, 13CO/HCO+ and 13CO/CS
(either ours or literature data) show little variations with galactocentric
radius, in sharp contrast with the greatly varied R. This supports the notion
that the observed gradient in R could be the results of the variations of the
physical conditions across the disks. The H_2 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.
