Eventful Evolution of Giant Molecular Clouds in Dynamically Evolving Spiral Arms

TL;DR

This study uses high-resolution simulations to show that giant molecular clouds in evolving spiral arms are highly dynamic, short-lived, and influenced by stellar feedback, challenging traditional static models of their evolution.

Contribution

It introduces a simulation-based analysis of GMC evolution in a dynamically evolving spiral galaxy, emphasizing the role of feedback and environmental effects over traditional models.

Findings

GMCs exhibit no systematic evolutionary sequence across spiral arms.

GMC lifetimes are short, only a few tens of millions of years.

Most GMCs without HII regions are collapsing, while those with HII regions tend to expand due to feedback.

Abstract

The formation and evolution of giant molecular clouds (GMCs) in spiral galaxies have been investigated in the traditional framework of the combined quasi-stationary density wave and galactic shock model. However, our understanding of the dynamics of spiral arms is changing from the traditional spiral model to a dynamically evolving spiral model. In this study, we investigate the structure and evolution of GMCs in a dynamically evolving spiral arm using a three-dimensional N-body/hydrodynamic simulation of a barred spiral galaxy at parsec-scale resolution. This simulation incorporated self-gravity, molecular hydrogen formation, radiative cooling, heating due to interstellar far-ultraviolet radiation, and stellar feedback by both HII regions and Type-II supernovae. In contrast to a simple expectation based on the traditional spiral model, the GMCs exhibited no systematic evolutionary sequence across the spiral arm. Our simulation showed that the GMCs behaved as highly dynamic objects with eventful lives involving collisional build-up, collision-induced star formation, and destruction via stellar feedback. The GMC lifetimes were predicted to be short, only a few tens of millions years. We also found that, at least at the resolutions and with the feedback models used in this study, most of the GMCs without HII regions were collapsing, but half of the GMCs with HII regions were expanding owing to the HII-region feedback from stars within them. Our results support the dynamic and feedback-regulated GMC evolution scenario. Although the simulated GMCs were converging rather than virial equilibrium, they followed the observed scaling relationship well. We also analysed the effects of galactic tides and external pressure on GMC evolution and suggested that GMCs cannot be regarded as isolated systems since their evolution in disc galaxies is complicated because of these environmental effects.