Kinematic Signature of a Rotating Bar Near a Resonance

There have been several recent suggestions that the Milky Way has rotating bar-like features based on HI and star count data. In this paper, I show that such features cause distinctive stellar kinematic signatures near OLR and ILR. The effects of these resonances may be observable far from the peak density of the pattern and relatively nearby the solar position. The details of the kinematic signatures depend on the evolutionary history of the `bar' and therefore velocity data, both systemic and velocity dispersion, may be used to probe the evolutionary history as well as the present state of the Galaxy. Kinematic models for a variety of simple scenarios are presented. Models with evolving pattern speeds show significantly stronger dispersion signatures than those with static pattern speeds, suggesting that useful observational constraints are possible. The models are applied to the proposed rotating spheroid and bar models; we find: 1) none of these models chosen to represent the proposed large-scale rotating spheroid are consistent with the stellar kinematics; and 2) a Galactic bar with semimajor axis of 3\kpc will cause a large increase in velocity dispersion in the vicinity of OLR (\sim5\kpc) with little change in the net radial motion and such a signature is suggested by K-giant velocity data. Potential future observations and analyses are discussed.Comment: 24 pages, AAS LaTeX macros v3.0, 23 figures (available on request

Effect of the Magellanic Clouds on the Milky Way disk and VICE VERSA

The satellite-disk interaction provides limits on halo properties in two ways: (1) physical arguments motivate the excitation of observable Galactic disk structure in the presence of a massive halo, although precise limits on halo parameters are scenario-dependent; (2) conversely, the Milky Way as a whole has significant dynamical effect on LMC structure and this interaction also leads to halo limits. Together, these scenarios give strong corroboration of our current gravitational mass estimates and suggests a rapidly evolving LMC.Comment: 12 pages, 8 Postscript figures, uses paspconf.sty. To appear in the Third Stromlo Symposium: The Galactic Halo (ASP Conference Series), in press. HTML version available at: http://www-astro.phast.umass.edu/~weinberg/stroml

Adiabatic Invariants in Stellar Dynamics: I. Basic concepts

The adiabatic criterion, widely used in astronomical dynamics, is based on the harmonic oscillator. It asserts that the change in action under a slowly varying perturbation is exponentially small. Recent mathematical results precisely define the conditions for invariance show that this model does not apply in general. In particular, a slowly varying perturbation may cause significant evolution stellar dynamical systems even if its time scale is longer than any internal orbital time scale. This additional `heating' may have serious implications for the evolution of star clusters and dwarf galaxies which are subject to long-term environmental forces. The mathematical developments leading to these results are reviewed, and the conditions for applicability to and further implications for stellar systems are discussed. Companion papers present a computational method for a general time-dependent disturbance and detailed example.Comment: uuencoded compressed PostScript, Preprint 94-

Direct Simulation Monte Carlo for astrophysical flows: II. Ram pressure dynamics

We use the Direct Simulation Monte Carlo (DSMC) method combined with an n-body code to study the dynamics of the interaction between a gas-rich spiral galaxy and intracluster or intragroup medium, often known as the ram pressure scenario. The advantage of this gas kinetic approach over traditional hydrodynamics is explicit treatment of the interface between the hot and cold, dense and rarefied media typical of astrophysical flows and the explicit conservation of energy and momentum and the interface. This approach yields some new physical insight. Owing to the shock and backward wave that forms at the point ICM--ISM contact, ICM gas is compressed, heated and slowed. The shock morphology is Mach-disk-like. In the outer galaxy, the hot turbulent post-shock gas flows around the galaxy disk, while heating and ablating the initially cool disk gas. The outer gas and angular momentum are lost to the flow. In the inner galaxy, the hot gas pressurizes the neutral ISM gas causing a strong two-phase instability. As a result, the momentum of the wind is no longer impulsively communicated to the cold gas as assumed in the Gunn-Gott (1972) formula, but oozes through the porous disk, transferring its linear momentum to the disk en masse. The escaping gas mixture has a net positive angular momentum and forms a slowly rotating sheath. The shear flow caused by the post-shock ICM flowing through the porous multiphase ISM creates a strong Kelvin-Helmholtz instability in the disk that results in Cartwheel-like ring and spoke morphology.Comment: 19 pages, 19 figures, submitted to MNRAS, additional clarifying figures and arguments,revised figures, corrected typos, and incorporated comment

Noise-driven evolution in stellar systems: A universal halo profile

Using the theory describing the evolution of a galaxy halo due to stochastic fluctuations developed in the companion paper, we show that a halo quickly evolves toward the same self-similar profile, independent of its initial profile and concentration. The self-similar part of profile takes the form of a double power law with inner and outer exponents taking the values near -1.5 and -3 respectively. The precise value of the inner exponent depends on the magnitude and duration of the noisy epoch and most likely on form of the inner profile to start. The outer exponent is the result of evolution dominated by the external l=1 multipole resulting from the inner halo's response to noise. Three different noise processes are studied: (1) a bombardment by blobs of mass small compared to the halo mass (`shrapnel'); (2) orbital evolution of substructure by dynamical friction (`satellites'); and (3) noise caused by the orbit of blobs in the halo (`black holes'). The power spectra in the shrapnel and satellite cases is continuous and results in the double power law form, independent of initial conditions. The power spectrum for black holes is discrete and has a different form with a much slower rate of evolution. A generic prediction of this study is that noise from transient processes will drive evolution toward the same double power law with only weak constraints on the noise source and initial conditions.Comment: 9 pages, 6 color figures, uses mn.st

Investigating the long-term evolution of galaxies: Noise,cuspy halos and bars

I review the arguments for the importance of halo structure in driving galaxy evolution and coupling a galaxy to its environment. We begin with a general discussion of the key dynamics and examples of structure dominated by modes. We find that simulations with large numbers of particles (N > 1e6) are required to resolve the dynamics. Finally, I will describe some new results which demonstrates that a disk bar can produce cores in a cuspy CDM dark-matter profile within a gigayear. An inner Lindblad-like resonance couples the rotating bar to halo orbits at all radii through the cusp, rapidly flattening it. This resonance disappears for profiles with cores and is responsible for a qualitative difference in bar-driven halo evolution with and without a cusp. Although the bar gives up the angular momentum in its pattern to make the core, the formation epoch is rich in accretion events to recreate or trigger a classic stellar bar. The evolution of the cuspy inner halo by the first-generation bar paves the way for a long-lived subsequent bar with low torque and a stable pattern speed.Comment: 12 pages, 5 figures, to appear in "Astrophysical Supercomputing Using Particles", eds J. Makino and P. Hut, Proc. IAU Symposium 208, Tokyo, July 10-13, 200

Evolution of galaxies due to self-excitation

These lectures will cover methods for studying the evolution of galaxies since their formation. Because the properties of a galaxy depend on its history, an understanding of galaxy evolution requires that we understand the dynamical interplay between all components. The first part will emphasize n-body simulation methods which minimize sampling noise. These techniques are based on harmonic expansions and scale linearly with the number of bodies, similar to Fourier transform solutions used in cosmological simulations. Although fast, until recently they were only efficiently used for small number of geometries and background profiles. These same techniques may be used to study the modes and response of a galaxy to an arbitrary perturbation. In particular, I will describe the modal spectra of stellar systems and role of damped modes which are generic to stellar systems in interactions and appear to play a significant role in determining the common structures that we see. The general development leads indirectly to guidelines for the number of particles necessary to adequately represent the gravitational field such that the modal spectrum is resolvable. I will then apply these same excitation to understanding the importance of noise to galaxy evolution.Comment: 24 pages, 7 figures, using Sussp.sty (included). Lectures presented at the NATO Advanced Study Institute, "The Restless Universe: Applications of Gravitational N-Body Dynamics to Planetary, Stellar and Galactic Systems," Blair Atholl, July 200