[Paper Review] Properties of barred spiral disks in hydrodynamical cosmological simulations

David Goz, Pierluigi Monaco+2 more|arXiv (Cornell University)|Dec 9, 2014

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This study investigates bar formation in hydrodynamical cosmological simulations of spiral galaxies (GA and AqC), finding that bars arise either via secular evolution in Toomre-unstable disks (Q ≤ 1) or through minor mergers with mass ratios ≥1:30. The GA galaxy forms a stable 8.8 kpc bar through secular processes, while the AqC galaxy shows merger-triggered bar formation at high resolution, with bar lengths of 6.5–11 kpc depending on merger timing and resolution.

ABSTRACT

We present a quantification of the properties of bars in two N-body+SPH cosmological simulations of spiral galaxies, named GA and AqC. The initial conditions were obtained using the zoom-in technique and represent two dark matter (DM) halos of $2-3 imes10^{12}\ { m M}_\odot$, available at two different resolutions. The resulting galaxies are presented in the companion paper of Murante et al. (2014). We find that the GA galaxy has a bar of length $8.8$ kpc, present at the two resolution levels even though with a slightly different strength. Classical bar signatures (e.g. pattern of streaming motions, high $m=2$ Fourier mode with roughly constant phase) are consistently found at both resolutions. Though a close encounter with a merging satellite at $z\sim0.6$ (mass ratio $1:50$) causes a strong, transient spiral pattern and some heating of the disk, we find that bar instability is due to secular process, caused by a low Toomre parameter $Q\lesssim1$ due to accumulation of mass in the disk. The AqC galaxy has a slightly different history: it suffers a similar tidal disturbance due to a merging satellite at $z\sim0.5$ but with a mass ratio of $1:32$, that triggers a bar in the high-resolution simulation, while at low resolution the merging is found to take place at a later time, so that both secular evolution and merging are plausible triggers for bar instability.

Motivation & Objective

• To understand the physical mechanisms triggering bar formation in isolated spiral galaxies within cosmological simulations.
• To disentangle the roles of secular evolution (driven by disk instability) versus external perturbations (e.g., minor mergers) in bar formation.
• To assess the robustness of bar properties across different simulation resolutions and initial conditions.
• To examine how disk formation affects the triaxiality of dark matter halos and its potential feedback on bar evolution.
• To determine whether transient features from tidal interactions can be mistaken for true bars using Fourier analysis.

Proposed method

• Simulated two Milky Way-sized dark matter halos (GA and AqC) using the zoom-in technique with N-body + SPH hydrodynamics.
• Employed high and low resolution runs to test convergence and stability of bar formation across resolution levels.
• Tracked bar properties via Fourier decomposition of surface density (m=2 mode), phase coherence, and streaming motions.
• Monitored the Toomre Q parameter to assess disk stability and identify secular bar triggers.
• Analyzed the impact of minor mergers (mass ratios 1:50 and 1:32) on bar emergence and transient features.
• Used the inertia tensor of dark matter halos to quantify changes in triaxiality due to disk formation and evolution.

Experimental results

Research questions

• RQ1 What physical mechanisms—secular evolution or minor mergers—primarily trigger bar formation in simulated spiral galaxies?
• RQ2 How does the Toomre Q parameter influence the onset of bar instability in disk galaxies?
• RQ3 To what extent do minor mergers with mass ratios of 1:50 or 1:32 produce transient features that mimic true bars?
• RQ4 How does the presence of a baryonic disk affect the triaxiality of the dark matter halo?
• RQ5 Are bar properties (length, strength, phase coherence) robust across different simulation resolutions?

Key findings

• The GA galaxy forms a stable bar of length 8.8 kpc via secular evolution, driven by a Toomre parameter Q ≤ 1 due to disk mass accumulation.
• The AqC galaxy forms a bar of length 6.5 kpc at high resolution (AqC5) due to a minor merger with a 1:32 mass ratio, while a later merger in AqC6 leads to a longer, developing bar of 11 kpc.
• A minor merger with a 1:50 mass ratio in GA induces a transient spiral pattern and high A2 coefficient, but Fourier analysis confirms it is not a true bar due to phase incoherence.
• The bar in GA is robust across resolutions, indicating it is a physical feature rather than a numerical artifact.
• Disk formation transforms the inner dark matter halo from triaxial (in collisionless runs) to roughly oblate, with the major eigenvector aligned to the galaxy's angular momentum.
• Bar formation is sensitive to fine details of disk structure and merger timing, with different outcomes in AqC5 and AqC6 despite similar initial conditions.