Disentangling Drivers of Disk Warps in Tilted and Tumbling TNG50 Halos

TL;DR

This study investigates what drives disk warps in tilted and tumbling TNG50 halos by analyzing 40 isolated disk galaxies over ~4 Gyr, measuring warp angles $\psi$ with azimuthal harmonic decomposition. It tests correlations between warps and halo tilt, halo figure rotation (pattern speed $\Omega_p$), axis misalignment, and gas angular momentum misalignment using a cross-correlation framework across lags up to $\sim2.3$ Gyr, including normalization by each galaxy’s circular period. The authors find no strong population-level lag or single driver that consistently explains warps, though maximum cross-correlation statistics reveal that halo tilt, figure rotation, and gas misalignment can contribute in specific systems; case studies show that higher warps tend to coincide with stronger figure rotation and/or tilt and gas misalignment. The results imply that while halo figure rotation influences disk structure, disk warps in cosmological environments emerge from a combination of drivers, complicating the use of warps as a clean probe of halo tumbling without additional dynamical tracers.

Abstract

Dark matter (DM) halos in $Λ$ Cold DM cosmological simulations are triaxial. Most exhibit figure rotation. We study 40 isolated halos with stellar disks from the TNG50 simulation suite across $\sim 4$~Gyr to understand whether and how a triaxial halo's tumbling and orientation relative to the disk can drive warps. We measure a warp angle $ψ$ and find even our isolated disks are all at least slightly warped, with each galaxy's maximum $ψ> 1.8^{\circ}$. We perform a modified cross-correlation analysis between $ψ$ and the figure rotation pattern speed, as well as the misalignment between the disk spin axis and (a) the figure rotation axis, (b) the halo minor axis, and (c) the gas angular momentum axis. We use snapshots spanning a lookback time $t_{lb} ~4$ Gyr with 25 linearly-spaced lags from $ 0 - 2.33$ Gyr. We do not find evidence for a consistent lag between the onset of a warp and any of the aforementioned factors on the population level. However, we find significant correlations between individual time-series at various lags. These maximum correlation coefficients were significantly offset from random chance at the population level, suggesting that several of these factors do correlate with disk warping in specific situations. By examining four case studies whose maximum correlation coefficients were significantly higher than random chance, we establish clear qualitative relationships between these factors and warps. While a non-warped galaxy typically shows minimal halo tilt and figure rotation, warped galaxies can have strong/weak tilts and/or strong/weak figure rotation. Keywords: Disk galaxies(391), Galaxy dynamics(591), Hydrodynamical simulations(767), Galaxy DM halos(1880)

Figures (10)

• Figure 1: Fractional mass of disky stars $F_{*,\mathrm{disky}}$ from our BFE-based measurement vs. the estimate of G15 for each of the 550 galaxies that overlap between the catalogs of G15 and AV23. The slope of the correlation is fairly close to unity. G15 considered all galaxies with $F_{*,disky} > 0.5$ to be disks (vertical red line), and we impose the same restriction (horizontal red line). The galaxies in the rectangular region at the top right of the plot are considered disks by both methods.
• Figure 2: Left: Different Fourier modes for Halo 1292 at Snapshot 91 (seen in Figure \ref{['fig:warp fitting']}) as a function of radius with the color signifying $z(R,m)$ (Eq. \ref{['eq:warp']}). The only significant mode is $m = 1$. Right: The maximum Fourier amplitude over all radii as a function of lookback time, again showing that the only significant mode is $m=1$.
• Figure 3: Top: Warp function (Eq. \ref{['eq:warp']}) reflected and plotted over 2-D histogram of our most warped disk. The vertical dotted lines and the vertical dashed lines mark $2r_{0.5}$ and $r_{0.5}$ on the left and right of the galaxy's center, respectively. Bottom: Linear fit of the above warp function in the $r_{0.5}$ to $2r_{0.5}$ region, with the angle of the line annotated.
• Figure 4: KDE of $\psi$ measurements for all 40 galaxies across 25 snapshots (top) and maximum $\psi$ of each of the 40 galaxies (middle). The vertical lines show the 25th, 50th and 75th percentile intervals. The bottom panel shows that the fractional distribution of $\psi$ measurements for our simulated sample (magenta curve) is remarkably similar to the distribution for $\sim$1000 observed edge-on galaxies with 'S-type' warps from refId0.
• Figure 5: Left: Averaged Pearson correlation coefficient $\bar{r}$ (from eq. \ref{['eq:rbar']}) between warp angle $\psi$ and potential drivers as a function of time lag in Gyr (see text for details). Right: The same as the left plot with units in the circular periods of each galaxy calculated at the stellar half mass radius at Snapshot 99. None of the $\bar{r}$ values imply even moderate correlation, implying no uniform lag time over the sample. The p-values averaged over all calculations listed in the legend are also all too high to signify meaningful correlation.