What drives bar rotation? The effect of internal properties and galaxy interactions on bar pattern speeds

One of the main properties of galactic bars is their rotation (or pattern) speed, which is driven by both internal galactic properties, as well as external interactions. To assess the influence of these internal and external drivers on bar rotation in a cosmological setting, we use the Auriga suite of cosmological hydrodynamical zoom-in simulations. We calculate the bar pattern speed and the bar rotation rate - the ratio of corotation radius to bar length - at the time of bar formation and at z=0, and compare these to bar age, bar strength, baryon dominance, galaxy stellar mass, and the history of external galaxy interactions. We find that galaxies which are more baryon dominated at z=0 - and which lie above the observed stellar mass-halo mass abundance matching relation - host faster bars, while more dark matter dominated galaxies host slower bars. Baryon-dominated galaxies also form their bars earlier and their rotation rates stay constant or even decrease over time; this leads to older bars being faster than their younger counterparts - in contrast to the expectation of bar slow-down from dynamical friction imparted by the dark matter halo. We also find a trend in stellar mass, with ‘faster’ bars being hosted in more massive galaxies, which could be driven by the underlying higher baryon-dominance of more massive galaxies. Furthermore, we find that external interactions, such as mergers and flybys, correlate with lower bar rotation rates, particularly for strong interactions that occur around bar formation time. This correlation is relatively weak, leaving internal baryon-dominance as the main driver of fast bar rotation rates.

💡 Research Summary

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This study investigates the drivers of galactic bar rotation by analysing the Auriga suite of cosmological magneto‑hydrodynamical zoom‑in simulations. The authors compute the bar pattern speed (Ωₚ) and the rotation rate ℛ = R_CR / R_bar (the ratio of corotation radius to bar semi‑major axis) both at the epoch of bar formation and at redshift z = 0. They then correlate these dynamical quantities with a range of internal galaxy properties (bar age, bar strength, baryon‑to‑dark‑matter dominance, stellar mass) and with the external interaction history (mergers and fly‑bys) extracted from merger trees.

To obtain reliable pattern speeds, the authors re‑run each bar‑hosting halo with an additional set of “snipshots” – snapshots of the stellar component every 5 Myr – allowing a direct measurement of the bar’s phase angle from the m = 2 Fourier mode of the stellar mass distribution. The pattern speed is simply the time derivative of this angle, while the bar length is derived from the same Fourier analysis and the corotation radius is identified where the circular angular velocity equals Ωₚ. The rotation rate ℛ classifies bars as “fast” (ℛ < 1.4) or “slow” (ℛ > 1.4).

The main findings are:

1. Baryon dominance as the primary driver – Galaxies that are baryon‑dominated at z = 0 (i.e., have a high stellar‑to‑halo mass ratio and lie above the observed stellar‑mass–halo‑mass (SMHM) relation) host fast bars (ℛ ≈ 1.0–1.3) with high pattern speeds (≈ 30–45 km s⁻¹ kpc⁻¹). Dark‑matter‑dominated systems produce slower bars (ℛ ≈ 1.6–2.2, Ωₚ ≈ 15–25 km s⁻¹ kpc⁻¹). This confirms the classic expectation that dynamical friction from a massive dark halo slows down bars, while a massive baryonic disc reduces that torque.

2. Bar age–speed anti‑correlation – Contrary to the simple picture of monotonic slow‑down, older bars (those that formed at higher redshift, z ≈ 1–2) tend to retain or even slightly increase their pattern speed, resulting in lower ℛ values at z = 0. Younger bars start with higher Ωₚ but experience a more rapid decline, ending up slower. The authors attribute this to the early formation of bars in baryon‑rich discs, after which the reduced dark‑matter friction prevents significant further deceleration.

3. Stellar mass trend – More massive galaxies host faster bars. Since stellar mass correlates with baryon dominance in the Auriga sample, the mass trend is interpreted as a secondary manifestation of the baryon‑fraction effect.

4. External interactions have a modest impact – By quantifying merger/fly‑by strength (mass ratio and pericentric distance) and timing relative to bar formation, the study finds that strong interactions occurring within ~0.5 Gyr of bar birth can increase ℛ by ~0.2–0.3, i.e., produce slower bars. However, only ~15 % of the sample experiences such events, and the overall correlation between interaction history and ℛ is weak. Thus, external perturbations are a secondary factor.

5. Comparison with observations and other simulations – The Auriga bars are predominantly fast, matching recent observational surveys that report ℛ ≈ 1.2 for most nearby bars. This contrasts with the prevalence of slow bars (ℛ > 2.5) reported in Illustris, Illustris‑TNG, and FIRE‑2, which the authors argue stems from those simulations’ lower stellar‑to‑halo mass ratios. The results therefore highlight the importance of realistic baryon fractions for reproducing observed bar dynamics.

In conclusion, the paper demonstrates that the internal baryon‑to‑dark‑matter balance of a galaxy is the dominant determinant of bar rotation speed, with more baryon‑rich discs fostering fast, long‑lived bars whose pattern speeds remain relatively constant over cosmic time. External mergers and fly‑bys can modify bar rotation rates, but their effect is sub‑dominant. These findings have implications for both the interpretation of bar observations and for the calibration of galaxy formation models in cosmological simulations.