CO Molecular Gas in Infrared Luminous Galaxies

We present the first statistical survey of the properties of the CO(1-0) and 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 S60μm ≥ 5.24 Jy with far-IR (FIR) luminosities mostly at LFIR > 10 10 L⊙. We compare the emission line strengths of CO(1-0) and (3-2) transitions at a common resolution of 15. The measured CO(3-2) to (1-0) line intensity ratios r31 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 non-linear relation between CO and FIR luminosities, such that their ratio LCO/LFIR decreases linearly with increasing LFIR. 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