Star Formation in Disk Galaxies. I. Formation and Evolution of Giant Molecular Clouds via Gravitational Instability and Cloud Collisions

TL;DR

This study uses high-resolution simulations to explore how giant molecular clouds form and evolve in disk galaxies, emphasizing gravitational instability and cloud collisions without including star formation or feedback processes.

Contribution

It provides a detailed simulation-based analysis of GMC formation and evolution driven by gravitational instability and collisions, matching many observed properties without feedback effects.

Findings

GMCs reach ~10^6 solar masses with a mass spectrum similar to Galactic GMCs.

Cloud collision times are about 1/5 of the orbital period, effectively injecting turbulence.

The disk maintains a quasi-steady state with properties consistent with observations.

Abstract

We investigate the formation and evolution of giant molecular clouds (GMCs) in a Milky-Way-like disk galaxy with a flat rotation curve. We perform a series of 3D adaptive mesh refinement (AMR) numerical simulations that follow both the global evolution on scales of ~20kpc and resolve down to scales ~<10pc with a multiphase atomic interstellar medium (ISM). In this first study, we omit star formation and feedback, and focus on the processes of gravitational instability and cloud collisions and interactions. We define clouds as regions with n_H>=100cm^-3 and track the evolution of individual clouds as they orbit through the galaxy from their birth to their eventual destruction via merger or via destructive collision with another cloud. After ~140Myr a large fraction of the gas in the disk has fragmented into clouds with masses ~10^6 Msun and a mass spectrum similar to that of Galactic GMCs. The disk settles into a quasi steady state in which gravitational scattering of clouds keeps the disk near the threshold of global gravitational instability. The cloud collision time is found to be a small fraction, ~1/5, of the orbital time, and this is an efficient mechanism to inject turbulence into the clouds. This helps to keep clouds only moderately gravitationally bound, with virial parameters of order unity. Many other observed GMC properties, such as mass surface density, angular momentum, velocity dispersion, and vertical distribution, can be accounted for in this simple model with no stellar feedback.