We use numerical simulations to follow evolution of barred galaxies in a suite of models with progressively more massive stellar bulges, with bulge-to-total (disc+bulge) mass ratios of B/T ∼ 0–0.25, embedded in dark matter (DM) haloes with spin $\lambda\sim 0\!-\!0.09$. We focus on models with a sequence of initial rotational support for bulges, and analyse their spinup and spindown. We find that (1) the presence of a bulge affects evolution of bars, i.e. the time-scale of bar instability, bar pattern speed, and its decay, and the vertical buckling instability. Bar strength is nearly independent of B/T in haloes with spin $\lambda=0$, and is suppressed by a factor ∼2 for haloes with $\lambda=0.09$; (2) The main effect of the bulge is the destruction of the harmonic core which affects the buckling; (3) The bulge plays a minor role in the exchange of angular momentum between the barred disc and the DM halo, during its spinup and spindown; (4) Buckling process triggers different response above/below the disc mid-plane, which anticorrelates with the bulge mass; (5) In spinning haloes, the buckling process has a prolonged amplitude tail, extending by few Gyr, as verified by measuring distortions in the Laplace plane; (6) Furthermore, as verified by orbital spectral analysis, the bulge gains its spin from the bar mainly via the inner Lindblad resonance, while losing it via a number of resonances lying between the outer and inner Lindblad resonance. The corollary is that we do not expect to find non-rotating bulges in barred galaxies.
The short-lived buckling instability is responsible for the formation of at least some box/peanut (B/P) shaped bulges, which are observed in most massive, z = 0, barred galaxies. Nevertheless, it has also been suggested that B/P bulges form via the slow trapping of stars on to vertically extended resonant orbits. The key difference between these two scenarios is that when the bar buckles, symmetry about the mid-plane is broken for a period of time. We use a suite of simulations (with and without gas) to show that when the buckling is sufficiently strong, a residual mid-plane asymmetry persists for several Gyrs after the end of the buckling phase, and is visible in simulation images. On the other hand, images of B/P bulges formed through resonant trapping and/or weak buckling remain symmetric about the mid-plane. We develop two related diagnostics to identify and quantify mid-plane asymmetry in simulation images of galaxies that are within 3° of edge-on orientation, allowing us to test whether the presence of a B/P-shaped bulge can be explained by a past buckling event. We apply our diagnostics to two nearly edge-on galaxies with B/P bulges from the Spitzer Survey of Stellar Structure in Galaxies, finding no mid-plane asymmetry, implying these galaxies formed their bulges either by resonant trapping or by buckling more than ∼5 Gyr ago. We conclude that the formation of B/P bulges through strong buckling may be a rare event in the past ∼5 Gyr.
more » « less- NSF-PAR ID:
- 10381882
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 518
- Issue:
- 2
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 2300-2319
- Size(s):
- ["p. 2300-2319"]
- Sponsoring Org:
- National Science Foundation
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