Self-Perpetuating Spiral Arms in Disk Galaxies

The causes of spiral structure in galaxies remain uncertain. Leaving aside the grand bisymmetric spirals with their own well-known complications, here we consider the possibility that multi-armed spiral features originate from density inhomogeneities orbiting within disks. Using high-resolution N-body simulations, we follow the motions of stars under the influence of gravity, and show that mass concentrations with properties similar to those of giant molecular clouds can induce the development of spiral arms through a process termed swing amplification. However, unlike in earlier work, we demonstrate that the eventual response of the disk can be highly non-linear, significantly modifying the formation and longevity of the resulting patterns. Contrary to expectations, ragged spiral structures can thus survive at least in a statistical sense long after the original perturbing influence has been removed.Comment: 16 pages, 11 figures, APJ accepted. Interpretation and conclusions unchanged. Animations can be found at http://www.cfa.harvard.edu/~edonghia/Site/Spiral_Arms.htm

The Halo Density Profiles with Non-Standard N-body Simulations

We propose a new numerical procedure to simulate a single dark halo of any size and mass in a hierarchical framework coupling the extended Press-Schechter formalism (EPSF) to N-body simulations. The procedure consists of assigning cosmological initial conditions to the particles of a single halo with a EPSF technique and following only the dynamical evolution using a serial N-body code. The computational box is fixed with a side of $0.5 h^{-1}$ Mpc. This allows to simulate galaxy cluster halos using appropriate scaling relations, to ensure savings in computing time and code speed. The code can describe the properties of halos composed of collisionless or collisional dark matter. For collisionless Cold Dark Matter (CDM) particles the NFW profile is reproduced for galactic halos as well as galaxy cluster halos. Using this numerical technique we study some characteristics of halos assumed to be isolated or placed in a cosmological context in presence of weak self-interacting dark matter: the soft core formation and the core collapse. The self-interacting dark matter cross section per unit mass is assumed to be inversely proportional to the particle collision velocity: $\sigma/m_{x} \propto 1/v$.Comment: Accepted for publication in MNRAS (2 figures added

Quasi-Resonant Theory of Tidal Interactions

When a spinning system experiences a transient gravitational encounter with an external perturber, a quasi-resonance occurs if the spin frequency of the victim matches the peak orbital frequency of the perturber. Such encounters are responsible for the formation of long tails and bridges of stars during galaxy collisions. For high-speed encounters, the resulting velocity perturbations can be described within the impulse approximation. The traditional impulse approximation, however, does not distinguish between prograde and retrograde encounters, and therefore completely misses the resonant response. Here, using perturbation theory, we compute the effects of quasi-resonant phenomena on stars orbiting within a disk. Explicit expressions are derived for the velocity and energy change to the stars induced by tidal forces from an external gravitational perturber passing either on a straight line or parabolic orbit. Comparisons with numerical restricted three-body calculations illustrate the applicability of our analysis.Comment: 22 pages, 13 figures, ApJ submitted, numerical routines for evaluation of special functions and analytical results are provided upon reques

How galaxies lose their angular momentum

The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disk galaxies that are too compact with respect to the observations. High-resolution N-body/SPH simulations in a cosmological context are presented including cold gas and dark matter. A halo with quiet merging activity since z~3.8 and with a high spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disk. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction plays a minor role,in contrast to previous claims. In fact, after this event a new extended disk begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disk galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered star burst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected as a result of these processes, late-type galaxies could form with a dominant extended disk component, resulting from late infall, a small bulge-to-disk ratio and a low baryon fraction, in agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution figures to the author

The Magellanic Group and the Seven Dwarfs

The Magellanic Clouds were the largest members of a group of dwarf galaxies that entered the Milky Way (MW) halo at late times. This group, dominated by the LMC, contained ~4% of the mass of the Milky Way prior to its accretion and tidal disruption, but ~70% of the known dwarfs orbiting the MW. Our theory addresses many outstanding problems in galaxy formation associated with dwarf galaxies. First, it can explain the planar orbital configuration populated by some dSphs in the MW. Second, it provides a mechanism for lighting up a subset of dwarf galaxies to reproduce the cumulative circular velocity distribution of the satellites in the MW. Finally, our model predicts that most dwarfs will be found in association with other dwarfs. The recent discovery of Leo V (Belokurov et al. 2008), a dwarf spheroidal companion of Leo IV, and the nearby dwarf associations supports our hypothesis.Comment: Contributed talk to IAU Symposium 256: "The Magellanic System: Stars, Gas, and Galaxies