The diverse nature of spiral arms in the Auriga Superstars cosmological hydrodynamic simulations

TL;DR

This study uses high-resolution cosmological simulations to explore the diverse and evolving nature of spiral arms in Milky Way-like galaxies, revealing multiple formation mechanisms and complex dynamics.

Contribution

It demonstrates that various spiral arm theories are realized in simulations, showing the transient and complex nature of spiral structures without a dominant, long-lived wave.

Findings

Different spiral theories are realized in simulations, including density waves, manifold spirals, and dynamic spirals.

Strong tidal interactions produce clear kinematic density waves.

Spiral pattern speeds can evolve on sub-Gigayear timescales, indicating transient structures.

Abstract

The dynamical nature and formation mechanism(s) of galactic spiral arms remain long-standing problems in astrophysics. Most theoretical work is based on analytic calculations or idealised simulations, which has yielded several theories of spiral structure. The radial profile of the spiral arm rotation speed - the pattern speed - is a key observable prediction of these theories. However, observations that infer spiral pattern speeds reveal a mixed picture with no clear consensus. Here, we expand on theoretical efforts by examining the pattern speed profiles in the Auriga Superstars set of high-resolution cosmological magnetohydrodynamic simulatons of Milky Way-mass spiral disc galaxies. These simulations combine galaxy formation in a cosmological environment with the high dynamical fidelity afforded by an $\sim 800$ $\rm M_{\odot}$ star particle resolution, giving $\sim 100$ million star particles in the disc. We show that several different spiral arm theories are realised among our simulations, including large-scale kinematic density waves, manifold spirals, dynamic (co-rotating) spirals, and overlapping modes. In particular, we demonstrate that a strong tidal interaction leads to clear kinematic density waves, and that manifold spirals are present in a strongly-barred galaxy. Interestingly, we find that the same galaxy may show qualitative evolution of their spiral pattern speed profiles, indicating that the nature of spiral arms can evolve on potentially sub-Gigayear timescales. Our results demonstrate that in the absence of a strong external encounter or a strong bar, galactic spiral structure is highly transitional and complex with no clear long-lived underlying wave.