Thursday, August 1, 2013

1307.8441 (Alice C. Quillen et al.)

A Vertical Resonance Heating Model for X- or Peanut-Shaped Galactic Bulges    [PDF]

Alice C. Quillen, Ivan Minchev, Sanjib Sharma, Paola Di Matteo
We explore a second order Hamiltonian vertical resonance model for X-shaped galactic bulges. We examine N-body simulations and find that due to the bar slowing down and disk thickening during bar buckling, the resonance and associated peanut-shape moves outward. The peanut-shape is consistent with the location of the resonance, independent of whether the bar buckled or not. We estimate the resonance width from the potential m=4 Fourier component and find that the resonance is narrow, affecting orbits in angular momentum over a range dL/L ~ 0.05. As the resonance moves outward, stars originally in the mid plane are forced out of the mid plane and into orbits just within the resonance separatrix. The height of these orbits, estimated from the Hamiltonian model, is consistent with the peanut-shape height. The X-shape is comprised of stars in these orbits as they are the only ones that would support the peanut-shape in the drifting system. The resonance condition in the Milky Way bulge relates the mid-plane mass density to the rotation curve and bar pattern speed. At an estimated vertical resonance galactocentric radius of ~ 1.3 kpc, we confirm a mid-plane density of ~5x10^8 Msol/kpc^3, consistent with recently estimated mass distributions. Thus the rotation curve, resonance location, X-shape tips, mid plane mass density, and vertical resonance location are all self-consistent in the Milky Way galaxy bulge.
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