Gas Gas Mixing Mixing + Gas + Gas CyclesCycles in in

DwarfDwarf IrregularsIrregulars GalaxiesGalaxies

Gerhard Hensler(University of Kiel)

Joachim Köppen, Jan Pflamm, Andreas Rieschick

Content:I. Characteristics of dIrrsII. Perturbed HI envelopesIII. Effect of Gas Infall on star formation?IV. OutflowsV. Gas mixing of outflow with the gaseous envelopeVI. Chemical preference for Gas Infall

� low masses� gas rich - HI disky

envelopes� low chemical abundances

Examples:

CharacteristicsCharacteristics of of dIrrsdIrrs

X-ray

HI: λ21cm

optical

LMCLMCLMC NGC 1569NGC 1569A

Prototypical Starburst

Dwarf Galaxy

Stil & Isreal (2002)

���� Hαααα���� X ���� HI

Martin et al. (2002))

HI ≈1.3•108 M�

Hα ����Yun et al. 1994

ChemChemicalical AbundanceAbundances:s:dIrrsdIrrs vs. vs. Cosmological objectsCosmological objects

N/O-O relation

-2,50

-2,00

-1,50

-1,00

-0,50

0,00

5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0 9,5 10,0

12+lo g(O/H)

solarQuasarsQ0000-2619Q2348-147HS 1700+6416,z=1.15HS 1700+6416,z=0.86NGC 4449He 2-10NGC 1569UGC 4483I Zw 18Peg DIGNGC 5253NGC 4214SBS 0355-052II Zw 40VII Zw 403II Zw 70UGCA 292LG dIrrsLMCCNELGsGCsPettini et al.(02)

Characteristics of dIrr Galaxies

Characteristics of Characteristics of dIrrdIrr GalaxiesGalaxies

� dIrrs are gas rich

� small masses: 107...1010 M�

� mostlylow star formation (10-3...10-1 M

�yr-1)

patchy star-formation distribution

various epochs of enhanced star-formation

� some with very bright, blue, compact SFcenters:

Starbursts?

� low chemical abundances (10-2...<1 Z�)

� but: alwaysat least oneold stellar populationexists, widely distributed: I(r) ~ exp{-r/r0)

} Are they young?

No!

Tosi 2002

I Zw 18 - a perturbed dIrr

with gas infall?

II ZwZw 1818 -- a a perturbed dIrrperturbed dIrr

withwith gas infall?gas infall?

(Östlin & Kunth 2000)

(van Zee et al. 1997)

Similarities Similarities to to local Dwarf Galaxieslocal Dwarf Galaxies??

Gas in dSph‘s:almost gas free or infall!

Carignan (1995)

HI gas outside Sculptor dSph? Welsh et al. (1998)

Gas infall in NGC 205 enhances SF

with courtesy from Eva Grebel Fe/H has to increase in simple chem. evolution!

Characteristics of dIrr GalaxiesCharacteristics ofCharacteristics of dIrrdIrr GalaxiesGalaxies

� dIrrs are gas rich

� small masses: 107...1010 M�

� mostlylow star formation: (10-3...10-1 M

�yr-1)

patchy star-formation distributionvarious epochs of enhanced star-formation

� some with very bright, blue, compact SFcenters:

Starbursts?but: always old stellar populationexisting

widely distributed: I(r) ~ exp{-r/r0)

� low chemical abundances (10-2...<1 Z�)

but: no closed-box models fit, low eff. yield

abundance peculiarities

� gaseous envelopes: infall?

� low gravitation� SF self-regulation strongly

affected by energetic events(e.g.stellar energy release, external perturbations,etc.)

� trigger mechanism?� infall

� Are they young?No!No!

� outflows of metal-rich gas?

� infall of low-metallicity gas?

� gas-phase mixing

�� dIrrsdIrrs are ideal laboratories of astrophysical processesare ideal laboratories of astrophysical processes

TheThe rolerole of of hugehuge HHII gas gas reservoirs around dIrrsreservoirs around dIrrs? ?

--Gas infallGas infall from from

perturbedperturbed HHII envelopesenvelopes??

CasesCases of of peculiarpeculiar HHIIkinematicskinematics::

I Zw 18 - a perturbed dIrr

with gas infall?

II ZwZw 1818 -- a a perturbed dIrrperturbed dIrr

withwith gas infall?gas infall?

(Östlin & Kunth 2000)

(van Zee et al. 1997)

NGC 4449a triggered starburst

NGC 4449a triggered starburst

(Hunter et al. 1995)

NGC 1569Gas Infall

confirmed!?

NGC 1569Gas InfallGas Infall

confirmedconfirmed!?!?

(Stil & Isreal, 2002)

Hαααα

HI

1.1. questionquestion::What triggers the high star-formation rates?Gas Infall?Gas Infall?

Consider the effects Consider the effects of of externalexternal gas infall!gas infall!

hot gas

clouds

stars

remnants

gas energy

cloud energy

Star-formation rate

Analytical InvestigationsAnalytical Investigations of Gas Infallof Gas Infall

}exp{),( KTcCTc cn

nc410

331) AA, (1998, G.H. Theis, Köppen,in astreatment

−=Ψ

solar vicinity;

units in Myrs, M�, pc

Star-formation is inherently self-regulated

Köppen, Theis, G.H. 1995, AA, 296

Köppen, Theis, G.H. 1998, AA, 331

SelfSelf--regulated evolution regulated evolution without without gas infallgas infall

Infall of a Infall of a cloud with cloud with Jeans Jeans mass mass ofof

• 105 M�

(curves from left to right): 400, 100, 50,10, 1 km/s

• 104 M�

(curves from left to right): 100,10, 1 km/s

Starburst

Pflamm (2003) thesis

Pflamm, G.H. (2003) in prep.

• How many metals from SNeII are stored in the hot ISM?• How much metals can be lost from a galaxy by galactic winds?• How efficiently is hot halo gas removed by gas stripping?• Are outflows facilitated or hampered by cluster environments?

2. 2. questionquestion::What consequences What consequences of of high starhigh star--formation ratesformation rates??

SNeSNeIIII ⇒⇒⇒⇒⇒⇒⇒⇒ superbubbles superbubbles ⇒⇒⇒⇒⇒⇒⇒⇒ outflowsoutflows, , galactic windsgalactic winds!!!!

GalacticGalactic wind in M82wind in M82

Yun et al. (1994)

Garnett (2002)

Effective yields of dIrrs smaller than solar!Outflow of SNII gas reduces O and yeff

MacLow & Ferrara (1999)Ferrara & Tolstoy (2000)

Galactic blowGalactic blow--away is almost away is almost impossible impossible !!

pressure of external gasand DM gravitational potentialmostly hamper galactic winds

Proofs:• many objects reveal

Hα loops and arcs:e.g. NGC 1705, I Zw 18

• Cluster DGs more evolved

NGC 1705

• Single SSCformed:

age ≈ 10 Myrs, Mvir ≈ 105 M�

• SSC embedded in HI disk: MHI ≈ 108 M�

• X-ray maxima surrounded byHα loops,representing tips of asuperbubble, expanding vertically to the HI disk, but confined

X-ray contours Hαoverlaid on HI

Hα(Hensler et al. 1998) 2 kpc

Meurer et al. (1998)

10 kpc

But: super star cluster is not formed in the center !!

3. 3. questionquestion::Can outflows explain abundance Can outflows explain abundance peculiaritiespeculiarities??

N/O production: • O is produced in massive stars and

released by supernovae II (hot gas);• N is mainly produced in intermediate-

mass stars (warm gas);

• Massive stars live shorter than IMS;

• (N also produced and released by massive stars as primary and secondary element)

N/O signatures:• HII regions in gSs along second.-N

production track;• outer HII regions resemble dIrrs scatter;

• dIrrs show low N/O (~ -1.6) at low O!• radial abundance homogeneity in dIrrs ⇒

global homogenisation

Pagel, B.N.P. (1985) ESO Workshop“ ... C,N,O Elements”

Henry, R.B.C. & Worthey, G. (1999)

the N/O problemthe N/O problem

solutions:• dIrrs are very young like DLAs: no!• O loss by galactic winds: O/H-N/O�

• Starbursts produce fresh O: O/H-N/O �

• Infall of pristine gas: O/H-N/O �

N/O N/O evolution modelsevolution models

Garnett (1990)

Pilyugin(1992)

Henry, Edmunds, Köppen, (1999)

early evolution: track through DLA regime

at later epochs:models settle at secondary N-line,

But: no no returnreturn to to dIrr regimedIrr regime !!

Gas InfallGas Infall can explain can explain

the chemical evolution the chemical evolution ((rejuvenationrejuvenation) )

and and abundance peculiaritiesabundance peculiarities

Gas Infall and its Effect on AbundancesGas Infall and its Effect on Abundances

Model assumptions:� Yieldssame as in Henry,

Edmunds, Köppen (2000): van der Hoek & Groenewegen (1997), Maeder (1992)

� Galaxy models evolvefor 13 Gyrs with different yeff of 0.1 ... 1⇒ different locations in (N/O)-(O/H) diagram

� Infall of clouds with primordial abund. and masses of 106... 108 M

�

� Extension of tracks depends on yeff

� (N/O) scatter reproduc-ible by age differences of start models

Köppen, G.H. (2003) in prep.

Main Main issuesissues::� Gas infall can explain the most significant

observational signatures of gaseous galaxies both 1. Modes of star formation 2. Chemical refreshment

� Gas infall is the main driver of star formation

Further comparisonsFurther comparisons4. 4. questionquestion::

On On what timescales are released what timescales are released metals incorporated into the metals incorporated into the cool cool ISM? ISM?

What can What can chemochemo--dynamical models dynamical models teach usteach us??

low-mass stars0.1-1 Mo

massive stars,10-100 Mo

star formation

remnants

dissipation

evaporationcondensation

SNeII

SNeIa

WNM, WIM M ≈≈≈≈ 105-107 Mo T ≈≈≈≈ 102 -104 K

ChemodynamicalChemodynamical TreatmentTreatmentCNM

M<104 MoT≤≤≤≤100 K

HIM T≥≥≥≥105 K

.M.E

Lyc, stellar winds

O,Si...Fe

Fe

C,N

WD NS BH

cooling

cooling

coolingGerhard Hensler, Univ. Kiel

Lyc

Clouds:formation

collisions

intermediate-mass stars 1-10 Mo

planetary

nebulae

all chemodynamical processes given bytheor. + empirical results from literaturefree parameters: initial cond., IMF

initial conditions:starting from the recombination timemass: Mg = 109 M� Ms=0DM: 1010 M� (Burkert 1995)

rini = 20 kpc

ρρρρ(r), L/M(r)

evolution:� collapse sets in due to dissip. + cooling

� ISM phases approach equlibrium

� different evolutionary phases

ChemoChemo--dynamicaldynamical dIrrdIrr ModelModel

2 kpc

Chemo-dynamical treatment:Theis, Burkert, G.H. (1992) AA, 265, 465Samland, G.H., Theis (1997) ApJ, 476,544

Rieschick, G.H. (2003) AA subm.

Brightness of Stellar ComponentsBrightness of Stellar Componentsmassive stars

low-mass stars5. 5. questionquestion::

What isWhat is thethe effecteffect of of heatheatconductionconduction??

Evaporation vs. Evaporation vs. Condensation Condensation in in the the wind wind phase phase of of chemochemo--dynamical dynamical dir dir modelmodel

Local Gas Mixing vs. Local Gas Mixing vs. LargeLarge--scale Circulationscale Circulation

ρρρρcond - ρρρρevap (Hensler et al. , 1999, ASP Conf. Ser. 187)

parameter ββββ = ---------------ρρρρcond + ρρρρevap

collapse phase (left), wind phase (half left); wind phase:(for red β=1, blue β= -1.) CM (half rigth),OCM (right) distributions

problems:• Abundances determined from HII regions: Abundances of which component?• SNII explosions release metals to the hot ISM. • What is the mixing time to the cool ISM?• No DGs with pristine gas observed. Self-enriched or ICM polluted?

Gas mixing and cycles: metal self-enrichment

Gas mGas mixingixing and and cyclescycles: : metal metal selfself--enrichmentenrichment

� star formation and resulting SNII explosions: ⇒ evaporationof local CM, mass-loaded flows

� condensationand sweep up of local gas in superbubble shells: ⇒ local self-enrichmentof star-forming regions by 25%

� outflow of hot SN-enriched gas: ⇒ gradual mixing by condensationon slowly infalling(primordial) clouds (few km/s)

� enrichment timescales:

⇒ 2 mixing cycles:

•for instantaneous recycling (locally 25%) = few 10 Myrs;

•for fall back (from > 3 kpc) > 1 Gyr

Chemodynamical Abundance Evolution Chemodynamical Abundance Evolution of a 10of a 1099 MM�������� dIrrsdIrrs

N/O-O relation

-2,50

-2,00

-1,50

-1,00

-0,50

0,00

5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0 9,5 10,0

12+lo g(O/H)

solarQuasarsQ0000-2619Q2348-147HS 1700+6416,z=1.15HS 1700+6416,z=0.86NGC 4449He 2-10NGC 1569UGC 4483I Zw 18Peg DIGNGC 5253NGC 4214SBS 0355-052II Zw 40VII Zw 403II Zw 70UGCA 292LG dIrrsLMCCNELGsGCsPettini et al.(02)

Large-scale streaming and gas-phase mixingLargeLarge--scale streaming scale streaming and gasand gas--phase mixingphase mixing

Hot-gas outflow mixes with infalling HI� Cloud evaporation leads to mass-loaded flows

(with N) and outflow; � Condensation of expanding + cooling hot gas on

infalling clouds leads to cloud enrichment;� ISM is homogenized on scales up to 1 kpc

(NGC 1569: Kobulnicki & Skillman 97, I Zw 18: Izotov 99);

� Slow fall back of metal-enriched clouds;� Remaining part: blowout, blowaway, stripping:

What metal fraction goes to ICM?

Timescales of return� cooling of blow-out hot gas and fall back: ~ Gyrs (TT 1986)� turbulent mixing: ~ 20 Myrs (Recchi et al. 2001) [Poster 259]� cloud evap. enhances bubble cooling; HII gas studies: Recchi, G.H., et al. (in prep.)

Recchi et al., MN 322 (2001)

Moving Cloud Models Moving Cloud Models

Th=5.6 106K, nh=6.6 10-4 cm-3,vrel=0.3 Ma

Results

� heat conduction stabilizesclouds against KH instability

� mass accretion by condensation almost compensates mass loss

� accreted material (metals!) mixed by internal turbulence

without and with heat conduction

at 25, 50, 75 Myrs

Vieser & Hensler (2002a) AA subm.Conclusions for theConclusions for the dIrrdIrr evolutionevolution

Present ISM abundances not observable in HII regions

Galactic winds possible but HI envelopes/ICM pressure

Blown-up material can be stripped

gas-phase mixing due to evap./cond.+large-scale dynamics� metals are only partly expelled ⇒ chemical abundances change � gas cycles from instantaneous (10 Myrs) to .... several 100 Myrs� abundance homogenisation

gas infall triggers star formation and produces starbursts + chemical peculiarities� rejuvenation of BCDGs� environmental effects determine evolution of dIrrs

�Requirements for Observations:gas infall, Z of single stars and gaseous envelopes of dIrrs, IG clouds, metal content of hot gas, ...

Problems in Understanding DG EvolutionProblems in Understanding DG Evolution

because of lower gravitation DG evolution is strongly affected by other forms of energetic events:

stellar energy, gas infall, tidal fields etc. lead to� large-scale streaming motions� long cooling timescales� gas-phase mixing processes� star-gas interactions� metals are lost ⇒ chemical abundances change

DGs are ideal laboratories of astrophysical processes

chemical, dynamical, energetical, materialistic processes are coupled + environmental effects

� chemo-dynamical treatment is required combining� Astrophysics (stellar evol., gravitation, yields, etc.)� Dynamics (2 gas phases, stars)� Plasmaphysics (cooling, heating, etc.)