The diverse nature of spiral arms in the AURIGA SUPERSTARS cosmological hydrodynamic simulations

Grand, RJJ ORCID: 0000-0001-9667-1340, Fragkoudi, F ORCID: 0000-0002-0897-3013, Pakmor, R ORCID: 0000-0003-3308-2420, Gómez, FA ORCID: 0000-0003-4232-8584, van de Voort, FF ORCID: 0000-0002-6301-638X, Bieri, R ORCID: 0000-0002-4554-4488 and Townson, S (2026) The diverse nature of spiral arms in the AURIGA SUPERSTARS cosmological hydrodynamic simulations. Monthly Notices of the Royal Astronomical Society, 548 (4). ISSN 0035-8711

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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 idealized 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 simulations of Milky Way–mass spiral disc galaxies. These simulations combine galaxy formation in a cosmological environment with the high dynamical fidelity afforded by an (Formula presented)  (Formula presented) star particle resolution, giving (Formula presented) million star particles in the disc. We show that several different spiral arm theories are realized 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 time-scales. 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.

Item Type:	Article
Uncontrolled Keywords:	methods: numerical; galaxies: formation; galaxies: kinematics and dynamics; galaxies: spiral; galaxies: structure; 5101 Astronomical Sciences; 51 Physical Sciences; Bioengineering; 5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	21 April 2026
Date of first compliant Open Access:	12 June 2026
Date Deposited:	12 Jun 2026 15:13
Last Modified:	12 Jun 2026 15:13
DOI or ID number:	10.1093/mnras/stag774
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28839

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