The SILCC project - V. The impact of magnetic fields on the chemistry and the formation of molecular clouds

TL;DR

This study uses 3D MHD simulations to explore how magnetic fields influence the chemical evolution, structure, and formation of molecular clouds in the interstellar medium, revealing delayed dense gas formation and altered cloud morphology.

Contribution

It provides new insights into the role of magnetic fields in molecular cloud formation, including their impact on gas fragmentation, structure, and accretion processes in the ISM.

Findings

Magnetic fields thicken the galactic disc and delay dense gas formation by ~25 Myr.

Magnetized gas forms fewer, differently shaped clumps with subcritical mass-to-flux ratios.

Cloud collisions occur at high velocities of a few 10 km/s, influencing cloud dynamics.

Abstract

Magnetic fields are ubiquitously observed in the interstellar medium (ISM) of present-day star-forming galaxies with dynamically relevant energy densities. Using three-dimensional magneto-hydrodynamic (MHD) simulations of the supernova (SN) driven ISM in the flux-freezing approximation (ideal MHD) we investigate the impact of the magnetic field on the chemical and dynamical evolution of the gas, fragmentation and the formation of molecular clouds. We follow the chemistry with a network of six species (H$^{+}$, H, H$_2$, C$^+$, CO, free electrons) including local shielding effects. We find that magnetic fields thicken the disc by a factor of a few to a scale height of $\sim100\,\mathrm{pc}$, delay the formation of dense (and molecular) gas by $\sim25\,\mathrm{Myr}$ and result in differently shaped gas structures. The magnetised gas fragments into fewer clumps, which are initially at subcritical mass-to-flux ratios, $M/\Phi\approx0.3(M/\Phi)_\mathrm{crit}$, and accrete gas preferentially parallel to the magnetic field lines until supercritial mass-to-flux ratios of up to order 10 are reached. The accretion rates onto molecular clouds scale with $\dot{M}\propto M^{1.5}$. The median of the inter-cloud velocity dispersion is $\sim2-5\,\mathrm{km\,s}^{-1}$ and lower than the internal velocity dispersion in the clouds ($\sim3-7\,\mathrm{km\,s}^{-1}$). However, individual cloud-cloud collisions occur at speeds of a few $10\,\mathrm{km\,s}^{-1}$.