Stefan Harfst

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%.

Modelling Galaxies with a 3D Multi-Phase ISM

We present a modified TREE-SPH code to model galaxies in three dimensions. The model includes a multi-phase description of the interstellar medium which combines two numerical techniques. A diffuse warm/hot gas phase is modelled by SPH, whereas a cloudy medium is represented by a sticky particle scheme. Interaction processes (such as star formation and feedback), cooling, and mixing by condensation and evaporation, are taken into account. Here we apply our model to the evolution of a Milky Way type galaxy. After an initial stage, a quasi-equilibrium state is reached. It is characterised by a star formation rate of ~1 Mo yr–1. Condensation and evaporation rates are in balance at 0.1–1 Mo yr–1.

Star formation in a multi-phase ISM

We present a 3d code for the dynamical evolution of a multi-phase interstellar medium (ISM) coupled to stars via star formation (SF) and feedback processes. 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 processes. The cycle of matter is completed by SF and feedback by SNe and PNe. A SF scheme based on a variable SF efficiency as proposed by Elmegreen and Efremov (1997) is presented. For a Milky Way type galaxy we get a SF rate of ∼1 M⊙ yr-1 with an average SF efficiency of ∼5%.

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.

Genes and Galaxies

A major problem in modeling encounters of galaxies is the extended parameter space. Traditional search strategies suffer from very large CPU-requirements or from a strong dependence on initial conditions. An efficient alternative are genetic algorithms (GA) (e.g. Charbonneau: 1995, ApJS 101, 309). In combination with fast restricted N-body-codes (e.g. Toomre and Toomre: 1972, ApJ 178, 623) they allow for an efficient search in parameter space which can be used for both, an automatic search of interaction parameters (provided sufficiently accurate data are available) and/or a uniqueness test of a preferred parameter combination (Wande: 1998, AA Theis: 1999, Rev. Mod. Astron. 12, 309).