arXiv subjects

Christian Theis

Publications and source records attributed to Christian Theis.

27 records · Page 2Linked to original sources

arXiv2001

Exchange Processes in a Multi-Phase ISM

We present a new particle based code with a multi-phase description of the ISM implemented in order to follow the chemo-dynamical evolution of galaxies. The multi-phase ISM consists of clouds (sticky particles) and diffuse gas (SPH): Exchange of matter, energy and momentum is achieved by drag (due to ram pressure) and condensation or evaporation. Based on time scales we show that in Milky-Way-like galaxies the drag force is for molecular clouds only important, if their relative velocities exceed 100 km/s. For the mass exchange we find that clouds evaporate only if the temperature of the ambient gas is higher than one million Kelvin. At lower temperatures condensation takes place at time scales of the order of 1-10 Gyr.

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arXiv2001

Global spiral modes in multi-component disks

We performed two-dimensional non-linear hydrodynamical simulations of two-component gravitating disks aimed at studying stability properties of these systems. In agreement with previous analytical and numerical simulations, we find that the cold gas component strongly affects the growth rates of the unstable global spiral modes in the disk. Already a five percent admixture of cold gas increases approximately two-fold the growth rate of the most unstable global mode while a twenty percent gas admixture enhances the modal growth rate by a factor of four. The local stability properties of a two-component disk coupled by self-gravity are governed by a stability criterion similar to Toomre's Q-parameter derived for one-component systems. Using numerical simulations, we analyse the applicability of a two-component local stability criterion for the analysis of the stability properties of global modes. The comparison of non-linear simulations with the analytical stability criterion shows that the two-component disks can be globally unstable while being stable to the local perturbations. The minimum value of the local stability criterion provides, however, a rough estimate of the global stability properties of two-component systems. We find that two-component systems with a content of cold gas of ten percent or less are globally stable, if the minimum value of the stability parameter exceeds about 2.5.

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arXiv2001

Formation of Twin Clusters in a Galactic Tidal Field

The formation of globular clusters is still an unsolved problem. Though most scenarios assume a massive molecular cloud as the progenitor, it is unclear, how the cloud is transformed into a star cluster. Here a scheme of supernova (SN) induced cluster formation is investigated. In this scenario the expanding SN shell accumulates the mass of the cloud. This is accompanied by fragmentation resulting in star formation in the shell. If this stellar shell expands sufficiently slow, its self-gravity leads to a recollapsing shell, by this forming one or several stellar clusters. In this paper N-body simulations of collapsing shells moving on circular orbits in a galactic potential are presented. It is shown that typical shells (10^5 Msun, 30 pc) evolve to twin clusters in the galactocentric distance range between 3 and 11 kpc. Their masses show a strong radial trend: on orbits inside 5 kpc both clusters have almost equal mass. Outside 5 kpc the more massive twin cluster contains about 55% of the shell's mass, whereas the mass of the smaller decreases linearily to 15% at 11 kpc. Outside 11 kpc the collapsing shells end up in a single cluster. Inside 3 kpc the shells are tidally disrupted and only fragments substantially less massive than the initial shell survive.

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arXiv2001

Multi-method-modeling of interacting galaxies. I. A unique scenario for NGC 4449?

(abridged) We combined several N-body methods in order to investigate the interaction scenario between NGC 4449 and DDO 125, a close companion in projected space. In a first step fast restricted N-body models are used to confine a region in parameter space reproducing the main observational features. In a second step a genetic algorithm is applied for a uniqueness test of our preferred parameter set. We show that our genetic algorithm reliably recovers orbital parameters, provided that the data are sufficiently accurate, i.e. all the key features are included. In the third step the results of the restricted N-body models are compared with self-consistent N-body simulations. In the case of NGC 4449, the applicability of the simple restricted N-body calculations is demonstrated. Additionally, it is shown that the HI gas can be modeled here by a purely stellar dynamical approach. In a series of simulations, we demonstrate that the observed features of the extended HI disc can be explained by a gravitational interaction between NGC 4449 and DDO 125. According to these calculations the closest approach between both galaxies happened $\sim 4-6 \cdot 10^8$ yr ago at a minimum distance of $\sim 25$ kpc on a parabolic or slightly elliptic orbit. In the case of an encounter scenario, the dynamical mass of DDO 125 should not be smaller than 10% of NGC 4449's mass. Before the encounter, the observed HI gas was arranged in a disc with a radius of 35-40 kpc around the center of NGC 4449. It had the same orientation as the central ellipsoidal HI structure. The origin of this disc is still unclear, but it might have been caused by a previous interaction.

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arXiv2000

How to Form (Twin) Globular Clusters?

Though it is generally assumed that massive molecular clouds are the progenitors of globular clusters, their detailed formation mechanism is still unclear. Standard scenarios based on the collapse of a smooth matter distribution suffer from strong requirements with respect to cluster formation time scale, binding energy and star formation efficiency. An alternative model assuming cluster formation due to the recollapse of a supernova-induced, fragmented shell can relax these difficulties. In this paper the final collapse stages of the different scenarios are compared by N-body simulations for shells and spheres. It is shown that fragmentation is much more pronounced for shells. Taking a galactic tidal field into account shells preferably form twin (or multiple) systems, whereas spheres end up as single clusters. The twins are characterized by identical metallicities, and stellar mass functions; some of them show counter-rotating cores. Their orbital evolution can result in both, a final merger or well separated twins sharing a common galactic orbit.

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arXiv2000

Studies of Binary Star Cluster Candidates in the Bar of the LMC. I: SL 353 & SL 349

We present a detailed study of the cluster pair SL 353 & SL 349. This candidate binary cluster is located at the northwestern rim of the LMC bar. Based on photometric data we find that both clusters are coeval with an age of 550 +/- 100 Myr. We use the Ca II triplet in the spectra of individual red giants to derive radial velocities. Both components of the binary cluster candidate show very similar mean velocities (approx. 274 +/- 10 km/sec for SL 349 and approx. 279 +/- 4 km/sec for SL 353) while the field stars show lower velocities (approx. 240 +/- 19 km/sec). These findings suggest a common origin of the two clusters from the same GMC. In this sense the cluster pair may constitute a true binary cluster. We furthermore investigate the stellar densities in and around the star clusters and compare them with isopleths created from artificial, interacting as well as non-interacting star clusters. Gravitational interaction leads to a distortion which can also be found in the observed pair.

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arXiv1999

Modeling Interacting Galaxies Using a Parallel Genetic Algorithm

Modeling of interacting galaxies suffers from an extended parameter space prohibiting traditional grid based search strategies. As an alternative approach a combination of a Genetic Algorithm (GA) with fast restricted N-body simulations can be applied. A typical fit takes about 3-6 CPU-hours on a PentiumII processor. Here we present a parallel implementation of our GA which reduces the CPU-requirement of a parameter determination to a few minutes on 100 nodes of a CRAY T3E.

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arXiv1999

Modeling Encounters of Galaxies: The Case of NGC4449

(abridged) Several N-body methods were combined in order to develop a method for the determination of the parameters of interacting galaxies. This method has been applied to the HI distribution of NGC4449. In a first step the fast restricted N-body models were used to confine a region in parameter space reproducing the main observational features. In a second step a genetic algorithm is introduced which allows for an automated search in parameter space as well as for a uniqueness test of special parameter sets. Using artificial data I show that the genetic algorithm reliably recovers orbital parameters. In the third step the results of the restricted N-body models are compared with detailed self-consistent N-body simulations. In the case of NGC4449 the applicability of the simple restricted N-body calculations is demonstrated. NGC4449 is an active star-forming dwarf galaxy of Magellanic type. From radio observations van Woerden et al. (1975) found an extended HI-halo around NGC4449 which is at least a factor of 10 larger than the optical diameter. More recently, Bajaja et al. (1994) and Hunter et al. (1998) discerned details in the HI-halo: a disc-like feature around the center of NGC4449 and a lopsided arm structure. In a series of simulations it is demonstrated that the main features can be obtained by a gravitational interaction between NGC4449 and DDO125, another nearby dwarf galaxy. According to these calculations the closest approach between both galaxies happened 350 Myr ago at a minimum distance of 25 kpc on a nearly parabolic orbit. In case of the encounter scenario, the dynamical mass of DDO125 should not be smaller than 10% of NGC4449's mass. The origin of this disc is still unclear, but it might be caused by a previous interaction.

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arXiv1999

Global Spiral Modes in Star-Forming Gravitating Disks

Using 2D nonlinear simulations, we study the generation and nonlinear evolution of spiral structure in a star-forming multi-component gravitating disk. We confirm in agreement with previous studies the destabilizing role of a cold gaseous component and extend this conclusion for multi-component star-forming disks exchanging mass and momentum between its components. We show that the spiral structure growing on a non-stationary multi-phase background reaches its saturation in a similar manner like the one-component disks. The spiral structure survives even if most of the gas is transformed into stellar remnants of larger velocity dispersion.

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