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Modelling Dwarf Galaxies with a Multi-Phase ISM

Modelling Dwarf Galaxies with a Multi-Phase ISM. Stefan Harfst 1,2 with: Ch. Theis 3,2 and G. Hensler 3,2 1 Rochester Institute of Technology, Rochester 2 Institut für Theoretische Physik und Astrophysik, Universität Kiel 3 Institut für Astronomie, Universität Wien. Content.

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Modelling Dwarf Galaxies with a Multi-Phase ISM

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  1. Modelling Dwarf Galaxies with a Multi-Phase ISM Stefan Harfst1,2 with: Ch. Theis3,2 and G. Hensler3,2 1Rochester Institute of Technology, Rochester 2Institut für Theoretische Physik und Astrophysik, Universität Kiel 3Institut für Astronomie, Universität Wien

  2. Content • Introduction • galaxies – systems made of stars and gas • how to model galaxies • cosmological models • chemo-dynamical models • The Model • the multi-phase ISM • star formation • Models of isolated MW-type Galaxies • Dwarf Galaxy • isolated and interacting

  3. halo disk bulge Structure of Galaxies morphological classification(Hubble, 1936) • elliptical galaxies • disk or spiral galaxies • disk, bulge und halo galaxies consist of • stars • interstellar medium (ISM) • different phases, e.g. cold molecular clouds, warm diffus gas, hot halo gas • dark matter • non-baryonic system bound by gravitation

  4. Evolution of Galaxies • dynamical evolution • stellar dynamics • dissipative gas dynamics • processes • star formation (SF) • feedback of stars • heating and cooling • exchange of matter between different phases of the ISM description of the ISM is an important aspect in modelling galaxies

  5. Modelling Galaxies two different approaches: • cosmological models (e.g. Navarro&White, 1993; Steinmetz&Müller, 1995; Brook et al., 2003) • usually based on particle methods • geometrical flexible, 3d-description • single-phase ISM • chemo-dynamical models (Theis et al., 1992; Samland et al., 1997; Samland&Gerhard, 2003) • detailed description of physical processes • multi-phase ISM • at first only 1d and 2d, restricted by grid next step: new method combining the advantages of both approaches

  6. cooling feedback SF feedback condensation & evaporation, drag (ram pressure) Schematic Model of a Galaxy • different particle types • dark matter • stars with IMF and stellar life times • diffuse gas SPH(e.g. Monaghan, 1992) • radiative cooling • clouds Sticky Particles(Theis & Hensler, 1993) • dissipation by cloud-cloud collisions • coupling of gas phases • star formation DM halo diffuse gas stars clouds gravitation withTREE-method (Barnes&Hut, 1986; Dehnen, 2002)

  7. (Kennicutt, 1998) (Elmegreen & Efremov, 1997) Star Formation • what observation tell • Schmidt-Law (Schmidt, 1959) SFR/area ~ Sngas with n » 1.4-2 • threshold density ~5-10 Mpc-2 • basis for SF in many simulations • constant SF efficiency • here new approach • SF described for molecular clouds • variable SF efficiency allows self-regularisation

  8. halo disk bulge Initial Conditions • pure stellar model(Kuijken&Dubinski, 1995) • similar to Milky Way • three components • stellar disk and bulge • dark matter halo • dynamical stable • clouds • randomly select 20% of disk particles • diffuse gas • spherical homogenous distribution • constant temperature

  9. Results for a MW-type Galaxy • stable evolution of galaxy • stellar disk thickens • velocity dispersion: stars increase, clouds constant • weak transient spiral arms • three-phase interstellar medium • clouds • warm diffuse gas (~0.1cm-3, ~104K) in disk • hot gas (~10-4cm-3, ~106-7K) in halo and in bubbles in the disk

  10. Star Formation Rate • mean SFR ~1.5 Myr-1 • similar to Milky Way • but SFR decreases • reason is consumption of gas • observation show constant SFR infall of gas? • SF follows a Schmidt-Law SFR/area ~ Sngas mit n » 1.7

  11. Star Formation Law

  12. SF Efficiency

  13. A model of a Dwarf Galaxy • start with an isolated SMC-like galaxy (Widrow&Dubinski, 2005) • total mass ~3*109 M • ratio baryonic/dark matter 1:1 • components disk and halo and a tiny bulge • extend of disk/halo ~6/10 kpc • gas content ~30% • cloud/diffuse gas mass ratio is roughly constant and ~10

  14. SFR in isolated Dwarf • low average SFR • no SF after 1Gyr • cloud mass constant

  15. Cloud mass spectrum • in general cloud mass spectra compare well to observations • GMC are described on a per particle basis

  16. SFR in an interacting model • dwarf is orbiting at ~100kpc in an iso-thermal potential (220 km/s) • SF bursts • probably result of model parameters • still confirms that our new SF prescription is sensitive to perturbations

  17. Summary • new model works well for MW-type galaxies • reproduces eg. SF law, cloud mass spectra • predicts SF efficiencies but… • more work needs to be done to adapt to dwarf galaxies • choice of model parameters • initial conditions for interacting models

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