Modelling Dwarf Galaxies Modelling Dwarf Galaxies with a Multi-Phase ISMwith a Multi-Phase ISM

Stefan HarfstStefan Harfst1,21,2 with: Ch. Theiswith: Ch. Theis3,23,2 and and

G. HenslerG. Hensler3,23,2

1Rochester Institute of Technology, Rochester

2Institut für Theoretische Physik und Astrophysik, Universität Kiel

3Institut für Astronomie, Universität Wien

ContentContent

IntroductionIntroduction galaxies – systems made of stars galaxies – systems made of stars and gasand gas

how to model galaxieshow to model galaxies cosmological modelscosmological models chemo-dynamical modelschemo-dynamical models

The ModelThe Model the multi-phase ISMthe multi-phase ISM star formationstar formation

Models of isolated MW-type Models of isolated MW-type GalaxiesGalaxies

Dwarf GalaxyDwarf Galaxy isolated and interactingisolated and interacting

Structure of GalaxiesStructure of Galaxies

morphological classificationmorphological classification (Hubble, (Hubble, 1936)1936)

elliptical galaxieselliptical galaxies disk or spiral galaxiesdisk or spiral galaxies

disk, bulge und halodisk, bulge und halo

galaxies consist ofgalaxies consist of starsstars interstellar medium (ISM)interstellar medium (ISM)

different phases, different phases, e.g. cold molecular clouds, e.g. cold molecular clouds, warm diffus gas, hot halo warm diffus gas, hot halo gasgas

dark matterdark matter non-baryonicnon-baryonic

system bound by gravitationsystem bound by gravitationdiskdisk bulgebulge

halohalo

Evolution of GalaxiesEvolution of Galaxies

dynamical evolutiondynamical evolution stellar dynamicsstellar dynamics dissipative gas dynamicsdissipative gas dynamics

processesprocesses star formation (SF)star formation (SF) feedback of starsfeedback of stars heating and coolingheating and cooling exchange of matter exchange of matter between different phases between different phases of the ISMof the ISM

description of the ISM is description of the ISM is anan

important aspect in important aspect in modellingmodelling

galaxiesgalaxies

Modelling GalaxiesModelling Galaxies

two different approaches:two different approaches:

cosmological models cosmological models (e.g. Navarro&White, 1993; (e.g. Navarro&White, 1993; Steinmetz&Müller,Steinmetz&Müller,

1995; Brook et al., 1995; Brook et al., 2003) 2003) usually based on particle methodsusually based on particle methods geometrical flexible, 3d-descriptiongeometrical flexible, 3d-description single-phase ISMsingle-phase ISM

chemo-dynamical models chemo-dynamical models (Theis et al., 1992; Samland et (Theis et al., 1992; Samland et al., 1997;al., 1997; Samland&Gerhard, Samland&Gerhard, 2003)2003) detailed description of physical processesdetailed description of physical processes multi-phase ISMmulti-phase ISM at first only 1d and 2d, restricted by gridat first only 1d and 2d, restricted by grid

next stepnext step: : new method combining the advantages of both approachesnew method combining the advantages of both approaches

Schematic Model of a GalaxySchematic Model of a Galaxy

stars

diffusegas

clouds

DM halo

gravitationwith TREETREE-method-method

(Barnes&Hut, 1986; Dehnen, 2002)(Barnes&Hut, 1986; Dehnen, 2002)

condensation & evaporation,drag (ram pressure)

SFfeedback

feedback

different particle different particle typestypes dark matterdark matter starsstars with IMF and with IMF and stellar life timesstellar life times

diffuse gasdiffuse gas SPH SPH (e.g. Monaghan, (e.g. Monaghan, 1992)1992)

radiative radiative coolingcooling cloudsclouds Sticky Sticky ParticlesParticles (Theis & Hensler, (Theis & Hensler, 1993)1993)

dissipationdissipation by by cloud-cloud collisionscloud-cloud collisions

coupling of gas coupling of gas phasesphases

star formationstar formation

coolingcooling

Star FormationStar Formation

what observation tellwhat observation tell Schmidt-Law Schmidt-Law (Schmidt,

1959)

SFR/area ~ SFR/area ~ nngasgas

withwith n n 1.4-21.4-2 threshold density threshold density ~5-10~5-10 M Mpcpc-2-2

basis for SF in many basis for SF in many simulationssimulations constant SF efficiencyconstant SF efficiency

here new approachhere new approach SF described for molecular SF described for molecular

cloudsclouds variable SF efficiency allows variable SF efficiency allows

self-regularisation self-regularisation

(Kennicutt, 1998)

(Elmegreen & Efremov, 1997)

Initial ConditionsInitial Conditions

pure stellar modelpure stellar model (Kuijken&Dubinski, (Kuijken&Dubinski,

1995)1995) similar to Milky Waysimilar to Milky Way three componentsthree components

stellar disk and bulge stellar disk and bulge dark matter halodark matter halo

dynamical stabledynamical stable cloudsclouds

randomly select 20% of randomly select 20% of disk particlesdisk particles

diffuse gasdiffuse gas spherical homogenous spherical homogenous distributiondistribution

constant temperatureconstant temperature

diskdisk bulgebulge

halohalo

Results for a MW-type Results for a MW-type GalaxyGalaxy

stable evolution of galaxystable evolution of galaxy stellar disk thickensstellar disk thickens velocity dispersion: stars increase, velocity dispersion: stars increase, clouds constant clouds constant

weak transient spiral armsweak transient spiral arms three-phase interstellar mediumthree-phase interstellar medium

cloudsclouds warm diffuse gas (warm diffuse gas (~0.1cm~0.1cm-3-3, ~10, ~1044KK) in disk) in disk hot gas (hot gas (~10~10-4-4cmcm-3-3, ~10, ~106-76-7KK) in halo and in ) in halo and in bubbles in the diskbubbles in the disk

Star Formation RateStar Formation Rate

mean SFR mean SFR ~1.5 M~1.5 Myryr-1-1 similar to Milky similar to Milky WayWay

but SFR decreasesbut SFR decreases reason is reason is consumption consumption of gasof gas

observation show observation show constant SFRconstant SFR infall of gas? infall of gas?

SF follows a SF follows a Schmidt-LawSchmidt-Law

SFR/area ~ SFR/area ~ nngasgas

mit n mit n 1.71.7

Star Formation LawStar Formation Law

SF EfficiencySF Efficiency

A model of a Dwarf A model of a Dwarf GalaxyGalaxy

start with an isolated SMC-start with an isolated SMC-like galaxylike galaxy

(Widrow&Dubinski, 2005)(Widrow&Dubinski, 2005)

total mass total mass ~3*10~3*1099 M M ratio baryonic/dark matter 1:1ratio baryonic/dark matter 1:1 components disk and halo components disk and halo and a tiny bulgeand a tiny bulge

extend of disk/halo extend of disk/halo ~6/10~6/10 kpc kpc gas content gas content ~30%~30% cloud/diffuse gas mass ratio is cloud/diffuse gas mass ratio is roughly constant and ~10roughly constant and ~10

SFR in isolated DwarfSFR in isolated Dwarf

low average low average SFRSFR

no SF after no SF after 1Gyr1Gyr cloud mass cloud mass constantconstant

Cloud mass spectrumCloud mass spectrum

in general in general cloud mass cloud mass spectra compare spectra compare well to well to observationsobservations

GMC are GMC are described on a described on a per particle per particle basisbasis

SFR in an interacting SFR in an interacting modelmodel

dwarf is orbiting dwarf is orbiting at ~100kpc in an at ~100kpc in an iso-thermal iso-thermal potential (220 potential (220 km/s)km/s)

SF burstsSF bursts probably result of probably result of model parametersmodel parameters

still confirms still confirms that our new SF that our new SF prescription is prescription is sensitive to sensitive to perturbations perturbations

SummarySummary

new model works well for MW-type new model works well for MW-type galaxiesgalaxies reproduces eg. SF law, cloud mass spectrareproduces eg. SF law, cloud mass spectra predicts SF efficienciespredicts SF efficiencies

but…but… more work needs to be done to adapt more work needs to be done to adapt to dwarf galaxiesto dwarf galaxies choice of model parameterschoice of model parameters initial conditions for interacting modelsinitial conditions for interacting models