The Effect of Dark Matter Halo Shape on Bar Buckling and Boxy/Peanut Bulges (2110.08165v1)

Abstract: It is well established that bars evolve significantly after they form in galaxy discs, often changing shape both in and out of the disc plane. In some cases they may bend or buckle out of the disc plane resulting in the formation of boxy/peanut/x-shape bulges. In this paper we show that the dark matter halo shape affects bar formation and buckling. We have performed N-body simulations of bar buckling in non-spherical dark matter halos and traced bar evolution for 8 Gyr. We find that bar formation is delayed in oblate halos, resulting in delayed buckling whereas bars form earlier in prolate halos leading to earlier buckling. However, the duration of first buckling remains almost comparable. All the models show two buckling events but the most extreme prolate halo exhibits three distinct buckling features. Bars in prolate halos also show buckling signatures for the longest duration compared to spherical and oblate halos. Since ongoing buckling events are rarely observed, our study suggests that most barred galaxies may have more oblate or spherical halos rather than prolate halos. Our measurement of BPX structures also shows that prolate halos promote bar thickening and disc heating more than oblate and spherical halos.

• The paper demonstrates that dark matter halo shape significantly affects the timing and intensity of bar buckling in disk galaxies through N-body simulations.
• It finds that prolate halos trigger earlier and multiple buckling episodes, while oblate halos delay bar formation and suppress repeated buckling.
• The results imply that halo ellipticity may explain why active buckling events are rarely observed in present-day galaxies.

The Effect of Dark Matter Halo Shape on Bar Buckling and Boxy/Peanut Bulges

The paper by Kumar et al. investigates the influence of dark matter halo shapes on bar buckling and the formation of boxy/peanut (BPX) bulges in disk galaxies. Utilizing N-body simulations, the authors focus on how the triaxiality of dark matter halos—ranging from oblate to prolate—affects the temporal evolution and structural characteristics of galactic bars over an 8-billion-year period.

Simulation Details and Model Setup

The authors employ N-body simulations to model isolated galactic disks embedded within dark matter halos of varying shapes. These halos are described using the Hernquist profile, initialized using the GALIC code, with subsequent evolution carried out with Gadget-2. Five distinct halo shapes, characterized by varying axis ratios—oblate (q < 1), spherical (q=1), and prolate (q > 1)—are modeled to examine their impact on bar dynamics.

Each galaxy evolves with a stellar disk absent of a classical bulge, a decision aimed at isolating the effects of halo shape on bar buckling. The simulations assume a static mass profile for the dark matter halo, maintaining a consistent halo mass for comparative analysis across different model setups.

Bar Dynamics and Buckling Instability

The results reveal that the formation and evolution of bars are sensitive to the shape of the encompassing dark matter halo. Oblate halos extend the time necessary for bar formation, while prolate halos facilitate earlier bar initiation. Notably, bars in prolate halos exhibit greater vulnerability to weakening during buckling events, though subsequent re-strengthening post-buckling tends to homogenize the bar's structural properties across different halo shapes.

The buckling process, which temporarily reduces the bar's amplitude by bending it out of the galactic plane, shows similarities across various halo shapes in the duration of the first buckling episode. However, prolate halos not only trigger earlier buckling but also experience additional and prolonged buckling phases. In the most extreme prolate case (q=1.30), a third buckling event is prominently observed, whereas oblate halos consistently suppress such recurrence.

Implications for Observed Galaxies

The paper implies a correlation between galactic bar dynamics and the ellipticity of dark matter halos, speculating that the scarcity of observable ongoing buckling may suggest a prevalence of oblate or spherical halo shapes in the universe. This analysis circumvents the detection challenge posed by buckling's fleeting observational signature, typically complicating empirical verification.

BPX Bulge Formation

The simulations further illustrate how halo shape influences the thickening of bars and subsequent BPX bulge formation. While the inner regions exhibit similar thickening, prolate dark matter halos promote more significant thickening towards the outer edges of the bar. This results in a more pronounced BPX structure, supporting theories that elliptical halo configurations are capable of inducing pronounced bulge evolution via sustained buckling processes.

Comprehensive Discussion

Throughout the work, Kumar et al. incorporate past simulation insights and studies detailing various contributors to bar evolution, including the presence of gas, flyby interactions, and external perturbations. The nuanced understanding provided by the paper establishes foundational knowledge for further exploration into the obscured role of dark matter halo shapes in galactic morphological transformations.

Continued examination incorporating empirical data on halo ellipticity distribution, especially through developing cosmic simulations, will enhance the observational correlation of these theoretical models. Such insights hold substantial potential for refining models of secular galactic evolution and improving our understanding of the intricate ballet performed by bars within the cosmic theater.