The Effect of Dissipation on the Shapes of Dark halos

The dissipative infall of gas during the formation of a galaxy modifies the density profile and shape of the dark halo. Gas dissipates energy radiatively and sinks to the center of the dark halo forming the luminous part of a galaxy. The resulting central density enhancement can alter the halo's orbital distribution. We simulate dissipative infall inside of an initially triaxial N-body dark halo by slowly growing a potential in the center of the particle distribution. The dark halo transforms from a prolate-triaxial halo ($T \sim 0.8$) to an oblate-triaxial halo ($T \sim 0.5$) while approximately preserving the flattening ($c/a \sim 0.5$). The main implication is that dark halos are rounder and more oblate than previous predictions of purely collisionless simulations with the new constraint that b/a \gapp 0.7. At the same time, the distribution of intrinsic flattenings of dark halos ($\langle c/a \rangle = 0.5$, $\langle (c/a)^2 \rangle ^{1/2} = 0.15$) is preserved during the period of baryonic infall. The oval distortions of disk galaxies should therefore be slightly less than original predictions from collisionless dark halos. The predicted distribution of shapes for dark halos from cosmological N-body simulations is in better agreement with distribution of ellipticities of elliptical galaxies if we assume all halos and galaxies become oblate-triaxial in response to baryonic dissipation. The observed distribution of kinematic misalignment angles is also consistent with this shape distribution ifComment: uuencoded compressed postscript file 384

The effect of tidal fields on the shapes and kinematics of dark halos

We have carried out a series of N-body simulations to investigate the effect of tidal shear on the structure and kinematics of dark halos. We simulate the collapse of density perturbations using a tree code as described in Dubinski & Carlberg (1991). Density peaks are selected from a random realization of a CDM density field and used as the initial conditions for N-body simulations. We use an experimental approach to examine the effects of tidal shear on collapse. The cosmological tidal field is treated as an external time dependent potential whose strength and orientation can be varied freely. We examine the effects of the tidal field with two experiments. In the first experiment, we simulate a sample of 14 dark halos from the collapse of density peaks in the presence of a 1(sigma) tidal field. In the second experiment, we use the same initial conditions though the tidal field is turned off allowing an experimental control for comparison to highlight the influence of tidal shear on the development of the structure and kinematics of the dark halos

Bars in Cuspy Dark Halos

We examine the bar instability in models with an exponential disk and a cuspy NFW-like dark matter (DM) halo inspired by cosmological simulations. Bar evolution is studied as a function of numerical resolution in a sequence of models spanning 10K to 100M DM particles - including a multi-mass model with an effective resolution of 10G. The goal is to find convergence in dynamical behaviour. We characterize the bar growth, the buckling instability, pattern speed decay through resonant transfer of angular momentum, and possible destruction of the DM halo cusp. Overall, most characteristics converge in behaviour in detail for halos containing more than 10M particles. Notably, the formation of the bar does not destroy the density cusp in this case. These higher resolution simulations clearly illustrate the importance of discrete resonances in transporting angular momentum from the bar to the halo.Comment: 6 pages, 5 figures, IAU Symposium 254 submission. The animations referenced by the paper can be found at http://www.cita.utoronto.ca/~dubinski/IAU25

Dynamical Blueprints for Galaxies

We present an axisymmetric, equilibrium model for late-type galaxies which consists of an exponential disk, a Sersic bulge, and a cuspy dark halo. The model is specified by a phase space distribution function which, in turn, depends on the integrals of motion. Bayesian statistics and the Markov Chain Monte Carlo method are used to tailor the model to satisfy observational data and theoretical constraints. By way of example, we construct a chain of 10^5 models for the Milky Way designed to fit a wide range of photometric and kinematic observations. From this chain, we calculate the probability distribution function of important Galactic parameters such as the Sersic index of the bulge, the disk scale length, and the disk, bulge, and halo masses. We also calculate the probability distribution function of the local dark matter velocity dispersion and density, two quantities of paramount significance for terrestrial dark matter detection experiments. Though the Milky Way models in our chain all satisfy the prescribed observational constraints, they vary considerably in key structural parameters and therefore respond differently to non-axisymmetric perturbations. We simulate the evolution of twenty-five models which have different Toomre Q and Goldreich-Tremaine X parameters. Virtually all of these models form a bar, though some, more quickly than others. The bar pattern speeds are ~ 40 - 50 km/s/kpc at the time when they form and then decrease, presumably due to coupling of the bar with the halo. Since the Galactic bar has a pattern speed ~50 km/s/kpc we conclude that it must have formed recently.Comment: 54 pages, 20 figure