Hierarchical hub-filament structures and gas inflows on galaxy-cloud scales

TL;DR

This study reveals hierarchical hub-filament structures and gas inflows in spiral galaxies, showing gravitational collapse processes across multiple scales from galaxy to dense cores, with slower collapse than free-fall predictions.

Contribution

It demonstrates the presence of multi-scale hierarchical hub-filament structures in galaxy interstellar media, extending previous cloud-scale findings to galaxy scales using CO line data.

Findings

Gas structures exhibit velocity gradients consistent with gravitational collapse.

Hierarchical organization of gas structures from galaxy to dense cores.

Collapse is slower than free-fall gravitational collapse.

Abstract

We investigated the kinematics and dynamics of gas structures on galaxy-cloud scales in two spiral galaxies NGC5236 (M83) and NGC4321 (M100) using CO (2$-$1) line. We utilized the FILFINDER algorithm on integrated intensity maps for the identification of filaments in two galaxies. Clear fluctuations in velocity and density were observed along these filaments, enabling the fitting of velocity gradients around intensity peaks. The variations in velocity gradient across different scales suggest a gradual and consistent increase in velocity gradient from large to small scales, indicative of gravitational collapse, something also revealed by the correlation between velocity dispersion and column density of gas structures. Gas structures at different scales in the galaxy may be organized into hierarchical systems through gravitational coupling. All the features of gas kinematics on galaxy-cloud scale are very similar to that on cloud-clump and clump-core scales studied in previous works. Thus, the interstellar medium from galaxy to dense core scales presents multi-scale/hierarchical hub-filament structures. Like dense core as the hub in clump, clump as the hub in molecular cloud, now we verify that cloud or cloud complex can be the hub in spiral galaxies. Although the scaling relations and the measured velocity gradients support the gravitational collapse of gas structures on galaxy-cloud scales, the collapse is much slower than a pure free-fall gravitational collapse.