Spiral Structure Properties, Dynamics, and Evolution in MW-mass Galaxy Simulations

TL;DR

This study catalogs dominant $m=2$ spiral episodes in ten Milky Way–mass FIRE-2 galaxies, revealing that spiral arms are transient yet persist on Gyr timescales and recur at relatively fixed pattern speeds $Ω_p$ that generally decrease with radius. By applying a Windowed Discrete Fourier Transform to radially binned stellar mass, the authors quantify spiral amplitudes $a_{2, ext{max}}$, radial extents $ΔR$, and durations, linking spiral properties to environment, bars, and disk kinematics. Key findings include $ΔR$ up to ~10 kpc, $a_{2, ext{max}}$ up to ~0.19, and a median lifetime $ ext{Δtime} approx 1.56$ Gyr, with the strongest spirals often tied to tidal interactions; kinematically colder disks host longer-lived spirals. Importantly, spirals appear in stars of all ages and in star-forming gas, suggesting density-wave–like arms rather than purely material features, and the resulting catalog enables systematic comparisons with current and forthcoming observations to advance our understanding of galaxy evolution.

Abstract

The structure of spiral galaxies is essential to understanding the dynamics and evolution of disk galaxies; however, the precise nature of spiral arms remains uncertain. Two challenges in understanding the mechanisms driving spirals are how galactic environment impacts spiral morphology and how they evolve over time. We present a catalog characterizing the properties, dynamics, and evolution of $m=2$ spiral structure in 10 Milky Way-mass galaxies from the FIRE-2 cosmological zoom-in simulations. Consistent with previous literature, we find that FIRE-2 spirals are transient, recurring features simultaneously present in the disk at varying pattern speeds ($Ω_p$) that broadly decrease with radius. These spirals persist on Gyr timescales (mean duration 1.90 Gyr), but fluctuate in amplitude on timescales of hundreds of Myr. Tidal interactions and bar episodes impact the resulting $m=2$ spiral structure; strong satellite interactions generally produce shorter-lived, stronger spirals with larger radial extent, and bars can increase $Ω_p$. Galactic environment influences spiral structure; kinematically colder disks can support longer-lived, stronger spirals. The properties of identified spirals in FIRE-2 vary widely in radial extent (0.3-10.8 kpc), duration (1.00-6.00 Gyr), and amplitudes ($a_{2,\text{max}}$=0.018-0.192). We find the presence of spirals in all age populations, suggesting these are density wave-driven features. This work represents the first time that spiral structure has been cataloged in this manner in cosmological simulations; the catalog can be leveraged with current and forthcoming observational surveys, enabling systematic comparisons to further our understanding of galaxy evolution.

Figures (15)

• Figure 1: Top: Measure of disk stability (Stellar Toomre $Q_*$, Eq. \ref{['eq:1']}) for each simulation at $z=0$, as a function of radius. The stability criterion ($Q>1$) is indicated by the horizontal dotted line. Bottom: Stellar surface density for each simulation at present day, as a function of radius. The FIRE-2 galaxies we analyze are stable ($Q>1$) beyond the central few kpc, which allows for the presence of recurring spirals in our simulations.
• Figure 2: Density contours reconstructed from the $m=1-5$ Fourier coefficients of stars $<1$ Gyr (contours) over-plotted on the stellar mass distribution of stars $<1$ Gyr (2-D histogram) for Latte simulations m12f (left panel) and m12m (right panel). Note the physical scale is different in each galaxy. The solid and dashed lines show the 25, 15, and 5 percent over and under-densities, respectively. The galaxy m12f is shown $\sim200$ Myr after a pericentric passage with a satellite with a 8:1 mass ratio and displays spiral structure that winds more tightly around the galaxy with less azimuthal spread as compared with m12m, shown after having evolved in relative isolation. The contours of both galaxies display clear spiral structure, although their spirals look vastly different from each other and may be generated through different physical mechanisms (see § \ref{['ssec:star_forming_gas']} for further discussion).
• Figure 3: Global Fourier amplitudes for the m=1-4 multiplicities shown across time for galaxies m12f (top panel) and m12m (bottom panel). The thick gray vertical dashed line indicates when the galaxy transitions from bursty to steady star formation rate. The grayed out region shows the time period that we do not analyze the galaxy. We indicate when the galaxy undergoes merger events (dash-dot line), pericentric passages (dotted line), and bar episodes (hatched rectangle). The $m=1$ (disk lopsidedness) and m=2 (spiral arms) amplitudes are generally dominant across time. The $m=2$ multiplicity can be strongly impacted by bar episodes, as can be seen in m12f at late times. All m-multiplicities are influenced by strong satellite interactions, as can be clearly seen in m12f.
• Figure 4: Power spectra for m12f (left column) and m12m (right column) at two different time intervals. The left side of each plot shows the spectrogram of pattern speeds as a function of radius (Eq. \ref{['eq:6']}), colored by power, and the right side shows the radially integrated power (Eq. \ref{['eq:total_power']}). More power indicates stronger $m=2$ spiral structure present over the time baseline. The darkened rectangles cut out the bar and region beyond the disk and are not included in the integrated spectra. The Inner Linblad, Outer Linblad, and corotation resonances are plotted as the two solid white lines and dashed white line, respectively. The purple horizontal lines on the right panels show peaks in the power spectrum and identify the three most dominant pattern speeds. The structure of $m=2$ spiral amplitudes differs between m12f and m12m; m12f experiences strong tidal interactions whereas m12m evolves secularly in the time baselines shown.
• Figure 5: Frequency evolution of the three most dominant $m=2$ amplitudes for m12f (left) and m12m (right). The top row shows the radius enclosing 90% of the stellar mass, $R_{90}$, as a function of time. Points are color-coded by the radius of peak spiral power relative to $R_{90}$, and the size is proportional to their amplitudes during the relevant time baseline. The time baseline (1.5 Gyr) and frequency resolution ($\Delta\Omega_p\sim 2.1$ km/s/kpc) are shown in the bottom left. The thick gray vertical dashed line indicates when the galaxy transitions from bursty to steady star formation rate. The grayed out region shows the time period that we do not analyze the galaxy. We indicate when the galaxy undergoes merger events (dash-dot lines), pericentric passages (dotted line), and bar episodes (hatched rectangle). The evolution of $m=2$ amplitudes is more segmented in an environment with significant external influence in m12f, compared to a galaxy with primarily secular evolution as m12m. Whereas m12m has a single evolving spiral amplitude that is primarily dominant across time, m12f hosts multiple instances of dominant spiral amplitudes scattered at different pattern speeds across time. This is discussed further in §\ref{['sec:results']} and can be seen more clearly in Fig. \ref{['fig:all_sims_identified_spirals']}.