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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.)