Christian Theis

The great disk of Milky-Way satellites and cosmological sub-structures

We show that the shape of the observed distribution of Milky Way (MW) satellites is inconsistent with their being drawn from a cosmological sub-structure population with a confidence of 99.5 per cent. Most of the MW satellites therefore cannot be related to dark-matter dominated satellites.

Simulations of the grand design galaxy M51: a case study for analysing tidally induced spiral structure

We present hydrodynamical models of the grand design spiral M51 (NGC 5194), and its interaction with its companion NGC 5195. Despite the simplicity of our models, our simulations capture the present day spiral structure of M51 remarkably well, and even reproduce details such as a kink along one spiral arm, and spiral arm bifurcations. We investigate the oset between the stellar and gaseous spiral arms, and nd at most times (including the present day) there is no oset between the stars and gas to within our error bars. We also compare our simulations with recent observational analysis of M51. We compute the pattern speed versus radius, and like the observations, nd no single global pattern speed. We also show that the spiral arms cannot be tted well by logarithmic spirals. We interpret these ndings as evidence that M51 does not exhibit a quasi-steady density wave, as would be predicted by density wave theory. The internal structure of M51 derives from the complicated and dynamical interaction with its companion, resulting in spiral arms showing considerable structure in the form of short-lived kinks and bifurcations. Rather than trying to model such galaxies in terms of global spiral modes with

DID THE MILKY WAY DWARF SATELLITES ENTER THE HALO AS A GROUP

The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way (MW) halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby universe. In this paper, we address the question whether galaxies in such associations can be the progenitors of the MW satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disklike distribution of the MW and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infall of a progenitor galaxy of the MW satellites, if they are of tidal origin.

One moment in time - modeling star formation in the antennae

We present a new high-resolution N-body/smoothed particle hydrodynamics simulation of an encounter of two gas-rich disk galaxies that closely matches the morphology and kinematics of the interacting Antennae galaxies (NGC 4038/39). The simulation includes radiative cooling, star formation, and feedback from Type II supernovae. The large-scale morphology and kinematics are determined by the internal structure and the orbit of the progenitor disks. The properties of the central region, in particular the starburst in the overlap region, only match the observations for a very short time interval (Δt ≈ 20 Myr) after the second encounter. This indicates that the Antennae galaxies are in a special phase only about 40 Myr after the second encounter and 50 Myr before their final collision. This is the only phase in the simulation when a gas-rich overlap region between the nuclei is forming accompanied by enhanced star formation. The star formation rate as well as the recent star formation history in the central region agree well with observational estimates. For the first time, this new model explains the distributed extra-nuclear star formation in the Antennae galaxies as a consequence of the recent second encounter. The proposed model predicts that the Antennae are in a later merger stage than the Mice (NGC 4676) and would therefore lose their first place in the classical Toomre sequence. (Less)

FULLY COMPRESSIVE TIDES IN GALAXY MERGERS

The disruptive effect of galactic tides is a textbook example of gravitational dynamics. However, depending on the shape of the potential, tides can also become fully compressive. When that is the case, they might trigger or strengthen the formation of galactic substructures (star clusters and tidal dwarf galaxies), instead of destroying them. We perform N-body simulations of interacting galaxies to quantify this effect. We demonstrate that tidal compression occurs repeatedly during a galaxy merger, independently of the specific choice of parameterization. With a model tailored to the Antennae galaxies, we show that the distribution of compressive tides matches the locations and timescales of observed substructures. After extending our study to a broad range of parameters, we conclude that neither the importance of the compressive tides (≈15% of the stellar mass) nor their duration (~107 yr) is strongly affected by changes in the progenitors configurations and orbits. Moreover, we show that individual clumps of matter can enter compressive regions several times in the course of a simulation. We speculate that this may spawn multiple star formation episodes in some star clusters, through, e.g., enhanced gas retention.

Modelling galaxies with a 3d multi-phase ISM

We present a new particle code for modelling the evolution of galaxies. The code is based on a multi-phase description for the interstellar medium (ISM). We include star formation (SF), stellar feedback by massive stars and planetary nebulae, phase transitions, and interactions between gas clouds and ambient diffuse gas, namely condensation, evaporation, drag, and energy dissipation. The last is realised by radiative cooling and inelastic cloud-cloud collisions. We present new schemes for SF and stellar feedback that include a consistent calculation of the star-formation efficiency (SFE) based on ISM properties, as well as a detailed redistribution of the feedback energy into the different ISM phases. As a first test we show a model of the evolution of a present day Milky-Way-type galaxy. Though the model exhibits a quasi-stationary behaviour in global properties like mass fractions or surface densities, the evolution of the ISM is strongly variable locally depending on the local SF and stellar feedback. We start only with two distinct phases, but a three-phase ISM is formed soon and consists of cold molecular clouds, a warm as disk, and a hot gaseous halo. Hot gas is also found in bubbles in the disk accompanied by type II supernovae explosions. The volume-filling factor of the hot gas in the disk is ∼35%. The mass spectrum of the clouds follows a power-law with an index of α ≈ -2. The star-formation rate (SFR) is ∼ 1.6 M ⊙ yr -1 on average, decreasing slowly with time due to gas consumption. In order to maintain a constant SFR, gas replenishment, e.g. by infall, of the order I M ⊙ yr -1 is required. Our model is in fair agreement with Kennicutts (1998, ApJ, 498, 541) SF law including the cut-off at ∼ 10 M ⊙ pc -2 . Models with a constant SFE, i.e. no feedback on the SF, fail to reproduce Kennicutts law. We performed a parameter study varying the particle resolution, feedback energy, cloud radius, SF time scale, and metallicity. In most these cases the evolution of the model galaxy was not significantly different to our reference model. Increasing the feedback energy by a factor of 4-5 lowers the SF rate by ∼0.5 M ⊙ yr -1 , while decreasing the metallicity by a factor of ∼100 increases the mass fraction of the hot gas from about 10% to 30%.

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 H I map of the Magellanic System by Bruns et al. (2005, A&A, 432, 45). The applied search algorithm is a genetic algorithm combined with a code based on the fast, but approximative restricted Nbody method. By this, we were able to analyze more than 106 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 H I, 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 H I observations than models with other halo configurations.

ROTATION OF THE MILKY WAY AND THE FORMATION OF THE MAGELLANIC STREAM

We studied the impact of the revisited values for the local standard of rest (LSR) circular velocity of the Milky Way on the formation of the Magellanic Stream. The LSR circular velocity was varied within its observational uncertainties as a free parameter of the interaction between the Large (LMC) and the Small Magellanic Clouds (SMC) and the Galaxy. We have shown that the large-scale morphology and kinematics of the Magellanic Stream may be reproduced as tidal features, assuming the recent values of the proper motions of the Magellanic Clouds. Automated exploration of the entire parameter space for the interaction was performed to identify all parameter combinations that allow for modeling the Magellanic Stream. Satisfactory models exist for the dynamical mass of the Milky Way within a wide range of 0.6 × 1012-3.0 × 1012 M ☉ and over the entire 1σ errors of the proper motions of the Clouds. However, the successful models share a common interaction scenario. The Magellanic Clouds are satellites of the Milky Way, and in all cases two close LMC-SMC encounters occurred within the last 4 Gyr at t < –2.5 Gyr and t ≈ –150 Myr, triggering the formation of the Stream and of the Magellanic Bridge, respectively. The latter encounter is encoded in the observed proper motions and inevitable in any model of the interaction. We conclude that the tidal origin of the Magellanic Stream implies the previously introduced LMC/SMC orbital history, unless the parameters of the interaction are revised substantially.

Induced planet formation in stellar clusters: a parameter study of star–disc encounters

We present a parameter study of the possibility of tidally triggered disk instability. Using a restricted N-body model which allows for a survey of an extended parameter space, we show that a passing dwarf star with a mass between 0.1 and 1 M ⊙ can probably induce gravitational instabilities in the pre-planetary solar disk for prograde passages with minimum separations below 80–170 AU for isothermal or adiabatic disks. Inclined and retrograde encounters lead to similar results but require slightly closer passages. Such encounter distances are quite likely in young moderately massive star clusters (Scally & Clarke 2001; Bonnell et al. 2001). The induced gravitational instabilities may lead to enhanced planetesimal formation in the outer regions of the protoplanetary disk, and could therefore be relevant for the existence of Uranus and Neptune, whose formation timescale of about 100 Myr (Wuchterl et al. 2000) is inconsistent with the disk lifetimes of about a few Myr according to observational data by Haisch et al. (2001). The relatively small gas/solid ratio in Uranus and Neptune can be matched if the perturbing fly-by occurred after early gas depletion of the solar system, i.e. when the solar system was older than about 5 Myr. We also confirm earlier results by Heller (1993) that the observed 7 degree tilt of the solar equatorial plane relative to the ecliptic plane could be the consequence of such a close encounter.

Spatial Motion of The Magellanic Clouds: Tidal Models Ruled Out?

Recently, Kallivayalil et al. derived new values of the proper motion for the Large and Small Magellanic Clouds (LMC and SMC, respectively). The spatial velocities of both Clouds are unexpectedly higher than their previous values resulting from agreement between the available theoretical models of the Magellanic System and the observations of neutral hydrogen (H I) associated with the LMC and the SMC. Such proper motion estimates are likely to be at odds with the scenarios for creation of the large-scale structures in the Magellanic System suggested so far. We investigated this hypothesis for the pure tidal models, as they were the first ones devised to explain the evolution of the Magellanic System, and the tidal stripping is intrinsically involved in every model assuming the gravitational interaction. The parameter space for the Milky Way (MW)-LMC-SMC interaction was analyzed by a robust search algorithm (genetic algorithm) combined with a fast, restricted N-body model of the interaction. Our method extended the known variety of evolutionary scenarios satisfying the observed kinematics and morphology of the Magellanic large-scale structures. Nevertheless, assuming the tidal interaction, no satisfactory reproduction of the H I data available for the Magellanic Clouds was achieved with the new proper motions. We conclude that for the proper motion data by Kallivayalil et al., within their 1σ errors, the dynamical evolution of the Magellanic System with the currently accepted total mass of the MW cannot be explained in the framework of pure tidal models. The optimal value for the western component of the LMC proper motion was found to be μW lmc –1.3 mas yr–1 in case of tidal models. It corresponds to the reduction of the Kallivayalil et al. value for μW lmc by ≈ 40% in its magnitude.