Modelling Dwarf Galaxies with a Multi-Phase ISM Stefan Harfst 1,2 with: Ch. Theis 3,2 and G. Hensler 3,2 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 Introduction Introduction galaxies – systems made of stars and gas galaxies – systems made of stars and gas how to model galaxies how to model galaxies cosmological models cosmological models chemo-dynamical models chemo-dynamical models The Model The Model the multi-phase ISM the multi-phase ISM star formation star formation Models of isolated MW-type Galaxies Models of isolated MW-type Galaxies Dwarf Galaxy Dwarf Galaxy isolated and interacting isolated and interacting

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 disk bulge halo

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

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

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

Star Formation what observation tell Schmidt-Law (Schmidt, 1959) SFR/area ~  n gas with n  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 (Kennicutt, 1998) (Elmegreen & Efremov, 1997)

M str,emb  Process of Star Formation t0t0 t1t1 t2t2 M cl  gas, T gas  ia  t 0 – t b,cl  gas, T gas  ia  t 1 – t b,cl M cl  M low  gas, T gas r(E SNII ) v(E SNII ) · M cl   ( M cl,P gas )

Star Formation Efficiency (Elmegreen & Efremov, 1997)

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

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

Star Formation Rate mean SFR ~1.5 M  yr -1 mean SFR ~1.5 M  yr -1 similar to Milky Way similar to Milky Way but SFR decreases but SFR decreases reason is consumption of gas reason is consumption of gas observation show constant SFR  infall of gas? observation show constant SFR  infall of gas? SF follows a Schmidt-Law SF follows a Schmidt-Law SFR/area ~  n gas mit n  1.7

Star Formation Law

SF Efficiency

A model of a Dwarf Galaxy start with an isolated SMC-like galaxy start with an isolated SMC-like galaxy (Widrow&Dubinski, 2005) total mass ~3*10 9 M  total mass ~3*10 9 M  ratio baryonic/dark matter 1:1 ratio baryonic/dark matter 1:1 components disk and halo and a tiny bulge components disk and halo and a tiny bulge extend of disk/halo ~6/10 kpc extend of disk/halo ~6/10 kpc gas content ~30% gas content ~30% cloud/diffuse gas mass ratio is roughly constant and ~10 cloud/diffuse gas mass ratio is roughly constant and ~10

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

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

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

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