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

TL;DR

This study investigates spiral arms in the Auriga Superstars cosmological magnetohydrodynamic simulations of Milky Way-mass discs at roughly 800 Msun stellar resolution, using high-cadence outputs and Fourier spectrograms to measure radial pattern-speed profiles. The analysis reveals a broad spectrum of arm morphologies and pattern-speed behaviors, spanning kinematic density waves, dynamic/co-rotating spirals, manifold spirals tied to bars, and overlapping mode structures. Strong tidal encounters yield grand-design spirals following the ILR, while strong bars produce manifold-like spirals, and in the absence of such perturbations spirals are transient and evolve on sub-Gigayear timescales with multiple concurrent modes. The results underscore that spiral arms in cosmological discs are diverse and time-dependent, shaped by external perturbations and internal structure, offering a framework to interpret observed spiral diversity and guiding future comparisons with observations and targeted simulations over parametric space.

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.

Figures (8)

• Figure 1: Schematic diagrams of spiral pattern speed radial profiles associated with various prominent spiral arm theories, including: kinematic density wave theory (top-left panel); classic density wave theory (bottom-left panel); multiple mode theory (top-middle panel); manifold theory (bottom-middle panel); and dynamic/co-rotating spiral arms (top-right panel). Each profile is depicted with a shaded band and has a qualitatively unique form.
• Figure 2: Face-on projected stellar light images for three auriga Superstars simulations each illustrating a qualitatively different type of spiral structure (see text for details): halo 25 (left panel; $z=0$); halo 18 (middle panel; $z\sim 0.25$); and halo 6 (right panel; $z=0$). The images are synthesized from a projection of the K-, B- and U-band luminosity of stars, which are shown by the red, green and blue colour channels, in logarithmic intervals, respectively. Younger (older) star particles appear bluer (redder).
• Figure 3: Top row: Spectrograms of the $m=2$ Fourier mode for a galaxy with a strong tidal interaction (halo 25; left), a strongly-barred galaxy (halo 18; middle), and a weakly-barred galaxy (halo 6; right) at late times (as indicated in each panel). The greyscale histogram shows the column-normalised power; the power in each frequency and radial bin is divided by the maximum power found in the corresponding radius. Contours show the power distribution with a common (fixed) normalisation. The average amplitude for the spiral structure, $A_{\rm sp}$, for the corresponding time window is given in each panel. The angular rotation curve measured at the middle of the time window is plotted as solid curves; the inner and outer Lindblad resonance loci are plotted as dashed curves. Bottom row: For each auriga Superstars halo, the face-on stellar azimuthal over-density projections at times near the middle of the corresponding spectrogram time window shown above. The colour bar indicates the logarithmic over-density. The galaxy in halo 25 (left column) experiences a strong tidal interaction with a massive flyby. This generates a powerful, large-scale grand-design $m=2$ spiral structure whose pattern speed is very low and closely follows the ILR, which aligns with the rotation profile of kinematic density waves. The galaxy in halo 18 (middle column) shows a clear and strong bar over-density in the central regions, with 2-armed spirals emanating from their ends. This dominant $m=2$ mode shows a single pattern speed of $\sim 55$$\rm km \, s^{-1} \, kpc$ that stretches from the centre of the galaxy to (even slightly beyond) the OLR; the spirals have the same pattern speed as the bar, as expected for manifold theory. The weakly-barred disc galaxy in halo 6 (right) is relatively undisturbed at late times, and exhibits a coherent but complex spiral structure whose pattern speed profile appears to show a mixture of kinematic density waves concentrated mainly in the inner/middle disc and a dynamic spiral in the outer disc. These three clear examples demonstrate the wide-range of spiral structure types found in our simulation suite.
• Figure 4: Top panel: evolution of the amplitude of spiral modes within the radial range $7<R/{\rm kpc}<15$ for halo 6. Shaded regions indicate the time windows of the spectrograms shown in the lower panels, where the left, middle, and right columns correspond to the $m=2$, $m=3$, and $m=4$ Fourier modes, respectively. Different rows correspond to different time windows, as indicated at the top-right corner of each panel. The greyscale histogram shows the column-normalised power; the power in each frequency and radial bin is divided by the maximum power found in the corresponding radius. Contours show the power distribution with a common (fixed) normalisation. The angular rotation curve measured at the middle of the time window is plotted as solid curves; the inner and outer Lindblad resonance loci are plotted as dashed curves. Each panel indicates the time-averaged amplitude, $A_{\rm sp}$, for the corresponding time window and mode, $m$. This figure demonstrates the wide variety of behaviours that spiral arms evolve through for the same galaxy: i) in the 2nd row (window D), the dominant $m=2$ mode shows a kinematic density wave with power following the ILR at almost all radii; ii) in the 3rd and 5th rows (windows C and A), the $m=2$ spectrogram profile indicates an inner kinematic density wave and dynamic spirals at intermediate/outer radii; iii) in the 4th row (window B), $m=2$ mode shows a large-scale density wave with a constant pattern speed of $\sim 14$$\rm km \, s^{-1} \, kpc^{-1}$; iv) the $m=3$ and $m=4$ modes sometimes show evidence of sub-dominant multiple modes.
• Figure 5: Sequence of stellar surface over-density maps for halo 6 for a series of lookback times in time window C of Fig. \ref{['fig:spectrogramh6']}. The left panel shows the locus of a spiral arm (marked by grey triangle symbols) defined by the peak stellar over-density in azimuth for a series of radial annuli in the outer disc ($10<R<15$ kpc), at $t=3.7$ Gyr. The total lifetime of this arm is of order $\sim 100$ Myr (similar to a dynamical time at these radii) and appears to disrupt at $t=3.6$ (right panel). In the middle and right panels, we show the instantaneous location of the positions shown in the left panel advanced in time according to: the circular velocity curve corresponding to dynamic spirals (black crosses); the inner Lindblad resonance curve corresponding to kinematic density waves (green plus symbols); and a radially independent angular pattern speed equal to $15$$\rm km \, s^{-1}$ corresponding to density waves (magenta triangles). The dynamic/co-rotating spirals appear to better describe the rotation profile of this spiral arm during this period of time, which is not easily captured in the relatively long baseline of the spectrogram analysis.