We present the first statistical survey of the properties of the 12 CO(1-0) and 12 CO(3-2) line emission from the nuclei of a nearly complete subsample of 60 infrared (IR) luminous galaxies selected from SCUBA Local Universe Galaxy Survey (SLUGS). This subsample is flux limited at S 60 μm ≥ 5.24 Jy with far-IR (FIR) luminosities mostly at L FIR > 10 10 L ☉ . We compare the emission line strengths of 12 CO(1-0) and (3-2) transitions at a common resolution of ~15''. The measured 12 CO(3-2) to (1-0) line intensity ratios r 31 vary from 0.22 to 1.72, with a mean value of 0.66 for the sources observed, indicating a large spread of the degree of excitation of CO in the sample. These CO data, together with a wide range of data at different wavelengths obtained from the literature, allow us to study the relationship between the CO excitation conditions and the physical properties of gas/dust and star formation in the central regions of galaxies. Our analysis shows that there is a nonlinear relation between CO and FIR luminosities, such that their ratio L CO / L FIR decreases linearly with increasing L FIR . This behavior was found to be consistent with the Schmidt law relating star formation rate to molecular gas content, with an index N = 1.4 ± 0.3. We also find a possible dependence of the degree of CO gas excitation on the efficiency of star-forming activity. Using the large velocity gradient (LVG) approximation to model the observed data, we investigate the CO-to-H 2 conversion factor X for the SLUGS sample. The results show that the mean value of X for the SLUGS sample is lower by a factor of 10 compared to the conventional value derived for the Galaxy, if we assume the abundance of CO relative to H 2 , Z CO = 10 -4 . For a subset of 12 galaxies with H I maps, we derive a mean total face-on surface density of H 2 + H of about 42 M ☉ pc -2 within about 2 kpc of the nucleus. This value is intermediate between that in galaxies like our own and those with strong star formation.
No takes yet. Share an insight, caveat, or question.
Yao et al. (2003) studied this question.