Spiral-arm instability - II: magnetic destabilisation

TL;DR

This paper extends the analysis of spiral-arm instability by incorporating magnetic fields, demonstrating that toroidal magnetic fields can destabilize spiral arms and lead to clump formation, with predictions matching observed giant clump masses.

Contribution

It introduces a linear perturbation theory for magnetized spiral-arm instability, including the effects of toroidal magnetic fields, and validates it with MHD simulations.

Findings

Magnetic fields can destabilize spiral arms via magneto-Jeans instability.

The analysis predicts clump formation with masses around 10^7-10^8 solar masses.

Magnetic destabilization causes fragmentation independent of magnetic field strength at long wavelengths.

Abstract

Fragmentation of spiral arms can drive the formation of giant clumps and induce intense star formation in disc galaxies. Based on the spiral-arm instability analysis of our Paper I, we present linear perturbation theory of dynamical instability of self-gravitating spiral arms of magnetised gas, focusing on the effect of toroidal magnetic fields. Spiral arms can be destabilised by the toroidal fields which cancel Coriolis force, i.e. magneto-Jeans instability. Our analysis can be applied to multi-component systems that consist of gas and stars. To test our analysis, we perform ideal magneto-hydrodynamics simulations of isolated disc galaxies and examine the simulation results. We find that our analysis can characterise dynamical instability leading arms to fragment and form clumps if magnetic fields are nearly toroidal. We propose that dimensionless growth rate of the most unstable perturbation, which is computed from our analysis, can be used to predict fragmentation of spiral arms within an orbital time-scale. Our analysis is applicable as long as magnetic fields are nearly toroidal. Using our analytic model, we estimate a typical mass of clumps forming from spiral-arm fragmentation to be consistent with observed giant clumps $\sim10^{7-8}~{\rm M_\odot}$. Furthermore, we find that, although the magnetic destabilisation can cause low-density spiral arms to fragment, the estimated mass of resultant clumps is almost independent from strength of magnetic fields since marginal instability occurs at long wavelengths which compensate the low densities of magnetically destabilised arms.