| We present the simultaneous observations of 12CO,13CO and C18O emission in 11 nearby (cz<1000 km s-1) bright infrared galaxies with Purple Mountain Observatory (PMO) 13.7m telescope at Delingha, China.12CO (hereafter CO) and 13CO are detected in all 11 galaxies except for NGC3031 which was not detected in 13CO. We completed the half-beam spacing mapping observations along the major axes in four galaxies:NGC 3627, NGC3628, NGC4631 and NGC5194. CO emission is detected in all 46 positions in these four galaxies, while 13CO emission is detected in 31 positions. In addition, M82 is mapped in an area of 4'x2.5'with 47 and 15 positions detected in CO and 13CO, respectively. Besides, C18O emission is also detected in 9 positions in M82 and NGC5194. We find that the radial distributions of CO and 13CO in NGC 3627, NGC 3628 and NGC 5194 both can be fitted well by an exponential law (I(R)=I0 exp(-R/R0)) with CO scale lengths between 2 and 4 kpc, and for the galaxies NGC3627 and NGC5194, the derived CO scale lengths agree well with the optical K-band scale lengths(-3kpc). The CO/13CO integrated intensity ratio, R, diminishes monotonically with galactocentric radius in all of the sources where we have mapped the two isotopologues. R ranges from 3.5±0.9 and 24.8±2.5 with mean value of 9.9±3.0 and 5.6±1.9 for the center and disk of the galaxies, respectively.Comparing the infrared luminosity derived from the photometry in MIPS band with CO and 13CO luminosity, the normalized CO luminosity was found to be still correlated with infrared luminosity, however, the correlation between normalized 13CO and infrared luminosity is weak, which suggests that CO emission could be enhaced by the affection of the physical conditions. The discrepancy between the distribution of SFE, which derived from CO, and dust temperature along the galaxy major axis indicate that the varied conversion factor (X-factor) should be used when estimate the mass of molecular gas in different physical environment, nevertheless,13CO can be used to trace the total molecular gas mass more believable and a constant X-factor of 13CO can be used. Moreover, we suggest the varied line ratio of CO to 13CO can trace the variation in X-factor and we found the ratio has a tendency to be increased with higher SFR. The H2 column density derived from C18O emission show that the standard Galactic X-factor overestimates the amount of molecular gas in M82 by a factor of-2.5, which also confirm that a lower X-factor should be adopted in starburst active regions.We also detected HCO+ and CS emission in the nucleus regions of M82 and NGC5194 which have stronger CO emission. Combining with the data of CO isotopologues emission and data from literatures, we find that the ratios of I(13CO)/I(C18O),I(13CO)/I(HCO+) and I(13CO)/I(CS) varied little in different types despite the extremely high R in active star-forming regions. The results support previous work implying that the variations in physical conditions and the two-type molecular cloud structure in starburst region and nuclear superwind may affect R as well as X-factor. |
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