Quantitative analysis of the molecular gas morphology in nearby disk galaxies

We present a quantitative and statistical analysis of the molecular gas morphology in 73 nearby galaxies using high spatial resolution CO (J=2-1) data obtained from the Atacama Large Millimeter/submillimeter Array (ALMA) by the PHANGS large program. We applied three model-independent parameters: Concentration (C), Asymmetry (A), and Clumpiness (S) which are commonly used to parameterize the optical and infrared morphology of galaxies. We find a significant correlation between A and S, with a Spearman's rank correlation coefficient of 0.62 with a p-value of 4 × 10⁻⁹. This suggests a higher abundance of molecular clumps (i.e. giant molecular cloud associations) in galaxies that display stronger distortion or biased large-scale molecular gas distribution. In addition, the analysis of the C parameter suggests high central molecular concentration in most barred spiral galaxies investigated in this study. Furthermore, we found a positive correlation between the length of the bar structure (Rbar/R₂₅) and the C parameter, with a Spearman's rank correlation coefficient of 0.63 with a p-value of 3.8 × 10⁻⁵ suggesting that larger bar structure can facilitate overall molecular gas transport and yield higher central concentration than galaxies with shorter bars. Finally, we offer a possible classification scheme of nearby disk galaxies which is based on the CAS parameters of molecular gas.