On stability and spiral patterns in polar disks

To investigate the stability properties of polar disks we performed two-dimensional hydrodynamical simulations for flat polytropic gaseous self-gravitating disks which were perturbed by a central S0-like component. Our disk was constructed to resemble that of the proto-typical galaxy NGC 4650A. This central perturbation induces initially a stationary two-armed tightly-wound leading spiral in the polar disk. For a hot disk (Toomre parameter Q>1.7), the structure does not change over the simulation time of 4.5 Gyr. In case of colder disks, the self-gravity of the spiral becomes dominant, it decouples from the central perturbation and grows, until reaching a saturation stage in which an open trailing spiral is formed, rather similar to that observed in NGC4650A. The timescale for developing non-linear structures is 1-2 Gyr; saturation is reached within 2-3 Gyr. The main parameter controlling the structure formation is the Toomre parameter. The results are surprisingly insensitive to the properties of the central component. If the polar disk is much less massive than that in NGC4650A, it forms a weaker tightly-wound spiral, similar to that seen in dust absorption in the dust disk of NGC2787. Our results are derived for a polytropic equation of state, but appear to be generic as the adiabatic exponent is varied between \gamma = 1 (isothermal) and \gamma = 2 (very stiff).Comment: 14 pages including 23 figures, accepted for publication in Astronomy & Astrophysic

Chemo-dynamical Evolution of the ISM in Galaxies

Chemo-dynamical models have been introduced in the late eighties and are a generally accepted tool for understanding galaxy evolution. They have been successfully applied to one-dimensional problems, e.g. the evolution of non-rotating galaxies, and two-dimensional problems, e.g. the evolution of disk galaxies. Recently, also three-dimensional chemo-dynamical models have become available. In these models the dynamics of different components, i.e. dark matter, stars and a multi-phase interstellar medium, are treated in a self-consistent way and several processes allow for an exchange of matter, energy and momentum between the components or different gas phases. Some results of chemo-dynamical models and their comparison with observations of chemical abundances or star formation histories will be reviewed.Comment: 10 Pages, 5 Figures, to appear in "From Observations to Self-Consistent Modelling of the ISM in Galaxies", 2003, eds M. Avillez et a

Is the dark matter halo of the Milky Way flattened?

We performed an extended analysis of the parameter space for the interaction of the Magellanic System with the Milky Way (MW). The varied parameters cover the phase space parameters, the masses, the structure, and the orientation of both Magellanic Clouds, as well as the flattening of the dark matter halo of the MW. The analysis was done by a specially adopted optimization code searching for the best match between numerical models and the detailed HI map of the Magellanic System by Bruens et al. (2005). The applied search algorithm is a genetic algorithm combined with a code based on the fast, but approximative restricted N-body method. By this, we were able to analyze more than 10^6 models, which makes this study one of the most extended ones for the Magellanic System. Here we focus on the flattening q of the axially symmetric MW dark matter halo potential, that is studied within the range 0.74<=q<=1.20. We show that creation of a trailing tail (Magellanic Stream) and a leading stream (Leading Arm) is quite a common feature of the Magellanic System-MW interaction, and such structures were modeled across the entire range of halo flattening values. However, important differences exist between the models, concerning density distribution and kinematics of HI, and also the dynamical evolution of the Magellanic System. Detailed analysis of the overall agreement between modeled and observed distribution of neutral hydrogen shows that the models assuming an oblate (q<1.0) dark matter halo of the Galaxy allow for better satisfaction of HI observations than models with other halo configurations.Comment: 19 pages, 20 figures, 2 appendices, accepted for publication in A&

Measuring dark matter by modeling interacting galaxies

The dark matter content of galaxies is usually determined from galaxies in dynamical equilibrium, mainly from rotationally supported galactic components. Such determinations restrict measurements to special regions in galaxies, e.g. the galactic plane(s), whereas other regions are not probed at all. Interacting galaxies offer an alternative, because extended tidal tails often probe outer or off-plane regions of galaxies. However, these systems are neither in dynamical equilibrium nor simple, because they are composed of two or more galaxies, by this increasing the associated parameter space.We present our genetic algorithm based modeling tool which allows to investigate the extended parameter space of interacting galaxies. From these studies, we derive the dynamical history of (well observed) galaxies. Among other parameters we constrain the dark matter content of the involved galaxies. We demonstrate the applicability of this strategy with examples ranging from stellar streams around theMilkyWay to extended tidal tails, from proto-typical binary galaxies (like M51 or the Antennae system) to small group of galaxies.Comment: 4 pages, 3 figures, Conf.: Hunting for the dark, Malta 200

Early evolution of Tidal Dwarf Galaxies

Our aim is to study the evolution of tidal dwarf galaxies. The first step is to understand whether a model galaxy without Dark Matter can sustain the feedback of the ongoing star formation. We present tests of the evolution of models in which star formation efficiency, temperature threshold, initial distribution of gas and infall are varied. We conclude that it is feasible to keep a fraction of gas bound for several hundreds of Myr and that the development of galactic winds does not necessarily stop continuous star formation.Comment: 2 pages, 1 figure, to appear in the Proceedings of the CRAL conference "Chemodynamics: from first stars to local galaxies", Lyon, France, 10-14 July 200