Evolution of molecular clouds on galaxy-cloud scale revealed by gravitational network analysis: high-mass clouds may deplete nearby gas via accretion or merging

Abstract

Observations show that molecular gas in spiral galaxies is organized into a network of interconnected systems through the gravitational coupling of multiscale hub-filament structures. Building on this picture, we model molecular gas in the galaxy NGC 628 as a gravitational network, where molecular clouds are represented as nodes. Through analysing this network, we can characterize both the gravitational interactions and the physical properties of the clouds using geometry-based network metrics. A strong correlation is observed between the geometric and physical properties of the nodes (clouds). High-mass clouds tend to exhibit less clustering and greater average separations, suggesting that they generally have fewer neighbours. During their formation and evolution, high-mass clouds may deplete nearby gas via accretion or merging, leading to more isolated characteristics within the network. This aligns with observations showing a decrease in the virial ratio of molecular clouds as their mass increases. For clouds at different evolutionary stages, less evolved clouds with lower mass are typically found in tighter gravitational subnetworks, with closer proximity to neighbouring clouds. As a result, they are more prone to accretion or merging during evolution.