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Creators/Authors contains: "Madigan, Ann-Marie"

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  1. ; ; (, Monthly Notices of the Royal Astronomical Society)
    ABSTRACT An overwhelming majority of galactic spiral arms trail with respect to the rotation of the galaxy, though a small sample of leading spiral arms has been observed. The formation of these leading spirals is not well understood. Here we show, using collisionless N-body simulations, that a barred disc galaxy in a retrograde dark matter halo can produce long-lived (∼3 Gyr) leading spiral arms. Due to the strong resonant coupling of the disc to the halo, the bar slows rapidly and spiral perturbations are forced ahead of the bar. We predict that such a system, if observed, will also host a dark matter wake oriented perpendicular to the stellar bar. More generally, we propose that any mechanism that rapidly decelerates the stellar bar will allow leading spiral arms to flourish. 
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  2. ; ; ; ; ; (, The Astronomical Journal)
    Abstract Axisymmetric disks of eccentric orbits in near-Keplerian potentials are unstable and undergo exponential growth in inclination. Recently, Zderic et al. showed that an idealized disk then saturates to a lopsided mode. Here we show, using N -body simulations, that this apsidal clustering also occurs in a primordial Scattered Disk in the outer solar system, which includes the orbit-averaged gravitational influence of the giant planets. We explain the dynamics using Lynden-Bell's mechanism for bar formation in galaxies. We also show surface density and line-of-sight velocity plots at different times during the instability, highlighting the formation of concentric circles and spiral arms in velocity space. 
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