Kiloparsec-Scale Simulations of Magnetised Molecular Clouds in Disk Galaxies

TL;DR

This study uses kiloparsec-scale simulations to explore the formation and evolution of dense molecular cloud clumps in galactic disks, highlighting the roles of gravity, turbulence, shear, and magnetic fields in star formation processes.

Contribution

It provides the first large-scale simulations of magnetised molecular clouds in galactic disks, examining their structure, evolution, and star formation rates with magnetic support included.

Findings

Clouds remain roughly in virial equilibrium over time.

Star formation rates are overestimated without magnetic support.

Magnetic fields reduce star formation rates in simulations.

Abstract

We present simulations of the evolution of self-gravitating dense gas on kiloparsec-size scales in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These dense clumps are expected to be the precursors to star clusters and this process may be the rate limiting step controling star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. The evolution of these simulated GMCs and clumps is determined by self-gravity balanced by turbulent pressure support and the large scale galactic shear. While the cloud structures and densities significantly change during their evolution, they remain roughly in virial equilibrium for time scales exceeding the free-fall time of GMCs, indicating that energy from the galactic shear continuously cascades down. We implement star formation at a slow, inefficient rate of 2% per local free-fall time, but this yields global star formation rates that are more than ~two orders of magnitude larger than the observed Kennicutt-Schmidt relation due to the over-production of dense clump gas. To explain this discrepancy, we anticipate magnetic fields to provide additional support. Low-resolution simulations indeed show that the magnetic field reduces the star formation rate.