Quantitative analysis of the molecular gas morphology in nearby disk galaxies

Abstract 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 parametrize the optical and infrared morphology of galaxies. We find a clear apparent correlation between A and S, with a Spearman’s rank correlation coefficient of 0.52 with a p-value of $2\times 10^{-6}$. 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 ($R_\mathrm{bar}/R_{25}$) and the C parameter, with a Spearman’s rank correlation coefficient of 0.63 with a p-value of $3.8 \times 10^{-5}$, suggesting that a larger bar structure can facilitate overall molecular gas transport and yield a higher central concentration than galaxies with shorter bars. Finally, we offer a possible classification scheme for nearby disk galaxies based on the CAS parameters of molecular gas.