IMPRS, April 8 1

• “Definition”

• Importance

• Evolution and winds

• Gas mass and distribution

• Magnetic fields

• Kinematics and Dark Matter

• 3-D structure

• Winds: case studies

• Future studies

• “Definition”

• Importance

• Evolution and winds

• Gas mass and distribution

• Magnetic fields

• Kinematics and Dark Matter

• 3-D structure

• Winds: case studies

• Future studies

themes of an expiring graduate school ... themes of an expiring graduate school ...

Dwarf Galaxies: Building Blocks of the Universe

2

but rather ....The first stellar system deemed extragalactic wasn‘t ....

L ~ 1 L* L ~ 0.0025 L

*

• Hubble (1925): Cepheids NGC6822 at D = 214 kpc (today: 670 kpc)assumed Gaussian LF....

• Zwicky (1942): LF increases with decreasing luminosity

dwarf galaxies = most numerous stellar systems

M31 NGC6822

Kilborn et al. (1999)

3

Bingelli diagramme linked to galaxy formation

• shape of potential • total mass

What is a dwarf galaxy?

Tamman (1993): “... working definition all galaxies fainter than MB = -16.0 (H0 = 50 km s-1 Mpc-1) and more extended than globular clusters ...”

Gallagher (1998): “... there is consensus that this occurs somewhere around (0.03 ···· 0.1) LB* , ...”LB* = (1.2 ± 0.1) · h-2 · 1010 L -16.9 < MB < -18.2

Binggeli (1994): location in the M - plane formation process!

“Dwarf galaxies lack the E-component!”

MB = -17.92

MB = -17.59 MB = -16.36

dVM

zrGzr

),(4),(

4

• low mass : 106 ··· 1010 M • slow rotators : 10 ··· 100 km s-1

• low luminosity : 106 ··· 1010 L

• low surface brightness (faint end)• high surface brightness (BCDGs)• low metallicity : 1/3 ··· 1/50 Z

• gas-poor (dE’s, dSph’s)• gas-rich (all others)• numerous• DM dominated (?)

POSS HST

GR 8 Im

ESO 410- G005 dSph

I Zw 18 BCDG

Mkn 297 Cl. Irr.

• Irr’s (Im, IBm, Sm, SBm)• dE’s, dSph’s• LSBDGs• BCDGs, HII galaxies • clumpy irregulars• tidal dwarfs

Properties:

The zoo:

understanding

• distant galaxies• galaxy evolution• ICM evolution• nature of Dark Matter• structure formation

Importance:

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Moore et al. (1999)

2 Mpc

300 kpc

Cluster halo 5·1014 M

Galaxy halo 2·1012 M

Dwarf galaxies are building blocks

CDM: Bottom-up structure formation

CDM models predict scale-invariant structures (e.g. Moore et al. 1999, Klypin et al. 1999)galaxy merging important process

power-law mass function dwarf galaxies are most numerous (~10% of mass in substructures)

e.g. HDF: large number of amorphous blue galaxies (B ~ 24) with 1/2 = 0.3” significantly smaller than L* galaxy

“missing satellite” problem

mechanisms to hide low-mass systems:

• remove baryons by SN-driven winds (Dekel & Silk 1986; McLow & Ferrara 1999)

• photo-evaporation from, or prevention of gas collapse into, low-mass systems during reionization at high redshift (Efstathiou 1992; Navarro & Steinmetz 1997) Benson et al. (2001): ‘dark satellites’ with MHI ~ 105 M should exist ...

• soft merging (à la Sagittarius dwarf)

• Stoehr et al. (2002): CDM simulationsobserved kinematics exactly those predcited for stellar populations with the observed spatial structure, orbiting within the most massive satellite substructures

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small perturber ...

large effect!

Mihos & Hernquist (1995)

7

Andersen & Burkert (2000): models including SF, heating, dissipation

- model dwarf galaxies evolving towards equilibrium of ISM balance between input and loss of energy

- dynamical equilibrium: a suitable scenario to produce all types of dwarfs?

- gas consumption time scales are long:

evolution of dE’s must have been different (winds, tidal/ram pressure stripping)

- role of DM halos: self-regulated evolution; exponential profiles

In bottom-up scenario: primordial DM halos filled with baryonic matter

subsequent SF gas-rich dI’s

evolution into

gas-poor dSph’sfirst SF burst(s) decisive?

Dwarf galaxy evolution

yr104 10SFR

M gasdepl

Larson (1974) : gas depletion through first starburst

Vader (1986), Dekel & Silk (1986) : application to dwarf galaxies

many models meanwhile ...

Mayor et al. (2001): tidal stripping in DM galaxy halo (“harassment”)

LSB dI’s dSph’s

HSB dI’s dE’s

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Wind models (a selection ....)

Mc Low & Ferrara (1999):

- dwarfs with masses 106 M M 106 M,

- mechanical luminosities L ~ 1037 ··· 1039 erg s-1

(over 50 Myr)

- significant ejection of ISM only for galaxies with M 106 M

- efficient metal depletion for galaxies with M 109 M

Mac Low & Ferrara (1999)

D’Ercole & Brighenti (1999):

- starburst in typical gas-rich dwarfs NGC 1569

- mechanical luminosities L = 3.8 ·1039 ··· 3.8 ·1040 erg s-1

- efficient metal ejection into IGM

- ‘recovery’ for next starburst after 0.5 ··· 1 Gyr

t = 100 Myr

D’Ercole & Brighenti (1999)

Recchi et al. (2001):

- SNe Ia included

- SN Ia ejecta lost more efficiently (explosions occur in hot and rarefied medium) I Zw 18 seems to fit well

- important for late evolution of starburst ( 500 Myr)

- metal-enriched winds produced more efficiently

models require: - distribution of mass

- distribution and state of ISM

- properties of magnetic field (?)

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How much mass, how much gas?

neutral atomic hydrogen easy to recover (21 cm line):

dwarfs easily tidally disturbed e.g. NGC 4449

- Mtot ~ 2 ·1010 M (?)

- MHI ~ 2 ·109 M

- heavily disturbed by 109 M companion (DDO 125)

- irregular velocity field in centre

M

skm

d

Jy

S

Mpc

DM HI 1

2

51033.2

total (dynamical) mass:

M

skm

RM

kpcR

tot

2

15 )(

1031.2

dwarfs gas-rich (except dE’s, dSph’s) van Zee et al. (1998)

IZw 18 HI

Hunter et al. (1998)

Bomans et al. (1997)

Hunter (priv. comm.)

Gentile (in prep.)

yet Mtot difficult to assess at low-mass end:

- ill-defined inclinations (3-D structure?)

- disturbed velocity fields v ~ vrot at low-mass end

N6822M31 cubes

10

Molecular (“hidden”?) gas

23.5

114 .

1

106.213

cmmol

e

skmd

KT

NexT

b

CO

H2 most abundant molecule, but lacks dipole moment

CO is the tracer [CO/H2] ~ 10-4 (excitation by collisions with H2)

rotational transitions at 115, 230, .... GHz (mm waves)

HI : pervasive Ts ~ 100 K nH ~ 1 ···100 cm-3

H2 : pervasive Tk ~ 10 ··· 30 K nH2 1000 cm-3

GMCs Tk ~ 20 K nH2 ~ 10 2 cm-3

dark clouds Tk ~ 10 K nH2 ~ 10 3 ···10 4 cm-3

cores Tk 40 K nH2 10 4 cm-3

H2 formed on dust grains (catalysts) at nH2 50 cm-3

requires column densities NH2 10 20 cm-2 to shield against dissociation by 11 eV photons

mostly optically thick 12C16O measured

13CO, C18O optically thin, but much weakermethods to derive molecular masses:

• extinction (Dickman 1978): AV ~ NHI + 2·NH2

• FIR & submm emission (Thronson 1986)

S ~ NHI + 2·NH2

• -rays (Bloemen et al. 1986)

I ~ NHI + 2·NH2

• virialized clouds (Solomon et al. 1987)most widely resorted to ....

Kohle (1999)

Böttner et al. (2001)

)(

2

dd TB

DSM

NGC 4449 (center):

MHI ~ 1.5 ·108 M

MH2 ~ 4.4 ·108 M

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virialized clouds: measure

- radius R

- line width v

- CO intensity ICO

11

skmK

skm

d

K

TI b

CO

2121

pcskmKpc

dydx

skm

d

K

TL b

CO

Milky Way: XCO = 2.3 ·1020 mol. cm-2 (K km s-1) -1

implications:

• ICO measures (‘counts’) the number of individual clouds within the telescope beam, weighted by their temperatures

• Mvir (the total cloud mass) equals the sum of the atomic and molecular gas mass

ICO is a good measure for the H2 column density

(or LCO is a good measure for the H2 mass)

Caveat: depends on

• metallicity (C & O abundance)

• radiation fields (dissociation)

• excitation conditions (line intensity)

• density (shielding)

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a normal galaxy...

a dwarf galaxy ...

LMC!

M51

13

NGC 4214 D = 4.1 Mpc

Walter et al. (2001):

• 3 molecular complexes in distinct evolutionary stages

• NW : no massive SF yet excitation process?

• centre : evolved starburst ISM affected

• SE : SF commenced recently ICO as in NW

canonical threshold column density for SF: NHI ~ 1021 cm-2

comparison with HI above 1021 cm-2 primarily molecular

Haro 2 D = 20 Mpc

Fritz (2000):

• complex velocity field and distribution of (visible!) molecular gas advanced merger?

• CO and HI concentrated

• strong starburst, SFR ~1.5 M yr-1

• de Vaucouleurs stellar profile (r1/4)

CO emission from regions with rather different properties

Fritz (2000)... puzzling cases:

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• certainly depends on spatial scale ....

Milky Way, Local Group, Virgo Cluster, ULIRGs, high-z galaxies

• metallicity (Wilson 1995)

• CR heating (Glasgold & Langer 1973)

heating by

- energetic particles (1 ··· 100 MeV CRs)

- hard X-rays ( 0.25 keV)

process: H2 + CR H2+ + e-(~35 eV) + CR

primary e- heats gas by (ionizing or non-ionizing) energy transfer

XCO dependence

Klein (1999)

bottom line: detailed case studies indispensable!

circumstantial evidence for this process on large (~ 200 ··· 400 pc) scales

but: CR flux at E 100 MeV not known in galaxies ....

heating rate (Cravens & Dalgarno 1978; van Dishoek & Black 1986):

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• WLM D = 0.9 Mpc: - little SF, weak radiation field & CR flux- XCO ~ 30 XGal (Taylor & Klein 2001)- below 12 + log(O/H) = 7.9 no CO detections

of galaxies (Taylor et al. 1998)• M 82 D = 3.6 Mpc:

- intense SF, strong radiation field and CR flux high gas density, large amount of dust

- XCO ~ 0.3 XGal in central region (Weiß 2000) fromradiative transfer models; requires many transitions,including isotopomers true gas distribution- strong spatial variation of XCO

- blind use of XCO leads to false results ....

Two contrasting examples:

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Star formation history in dwarf galaxies

GR 8 Sextans A

17

• B-fields play an important role in SF process

• B-fields provide a large-scale storage for relativistic particles

• low-mass galaxies may have strong winds less containment for CRs (Klein et al. 1991)

NGC4565

NGC4631

fieldB

fieldB

• B-fields in dwarf galaxies exhibit less coherent structure

Dumke et al. (1995) Dumke et al. (1995)

Klein et al. (1991)

Klein et al. (1996)

Chyy et al. (2000)

Magnetic fields

magnetization of IGM by primeval galaxies? (Kronberg et al. 1999)

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Ott et al. (2001)

Ho I

Kinematics and Dark Matter

• early recognition that dwarfs have high M/LSargent (1986): “The estimated M/L are high . . . . 10 ··· 3. This is not

simply a consequence of the objects being rich in HI gas”.

• at low-mass end:- mostly rigid rotation- v v - annular distribution of HI- dSph’s show high M/L (stellar v in Local Group galaxies, e.g. Mateo 1998)

• large number of HI rotation curves: WHISP (de Block 1997; Stil 1999; Swaters 1999)- systematic production of rotation curves of LSBGs and

dwarfs- probably DM dominated, but:

maximum disk solution fits rotation curves well scaling the HI “ “ “ “ “

- problem of beam smearing and velocity resolution(van den Bosch et al. 2000) Mateo (1998)

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2

1

0)(

srr

srr

rNFW

• CDM models: e.g. ‘NFW’ (Navarro et al. 1996):

• problems:- reconcile with TF relation (Navarro & Steinmetz 2000)- number of satellites around MW (Moore et al. 1999) effects of reionization (Benson et al. 2001)

- no spirals (Steinmetz et al. 2000)- rotation curves seem to be at odds with NFW.

beam smearing? (van den Bosch et al. 2000) stellar feedback? (Gnedin & Zhao 2001)

Swaters (1999)

need high-quality rotation curves (H + HI)

in particular: undisturbed dwarf galaxies

• better fit to inner RCs: ‘Burkert’ profile (Burkert 1995) no cusps?

2200

200)(

rrrr

rrB

Blais-Ouellette et al. (2001)

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• irregular morphologies inclination often unknown

• HI holes in low-mass galaxies grow larger thicker disks (e.g. Brinks & Walter 1998)

),0(2

1)(0

RGRz

tot

gas

)(sech),0(),( 02 zzRRz

Compare z0 with sizes of largest holes

less gravity larger z0 larger holes

Brinks & Walter (1998)

Brinks & Walter (1998)

Galaxy scale height

[pc]

M 31 100

M 33 120

IC 2574 350

Ho I 400

Ho II 625

IC 2574

3-D structure of dwarf galaxies

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Galactic winds:

• winds play an important role in the evolution of (small) galaxies (Matteucci & Chiosi 1983); may explain- metal deficiency of dwarf galaxies- enrichment of IGM

Different masses, different winds ....

• modern numerical simulations (e.g. Mac Low & Ferrara 1999;Ferrara & Tolstoy 2000):for mechanical luminosity L = 1038 erg s-1 blow-out occurs in109 M galaxy only ~30% metals retained

Galaxy D Mtot starburst

[Mpc] [109 M ]

M 82 3.6 10 ongoing

NGC 1569 2.2 0.4 post

Ho I 3.6 0.24† past

† visible (stellar) mass

Devine & Bally (1999)

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Kronberg et al. (1981):

LFIR = 1.6 · 1044 erg s-1

LX = 2.0 · 1044 erg s-1

SN ~ 0.1 yr-1

M 82

Weiß et al. (1999):

discovery of expanding molecular superbubble, broken out of the disk result of high ambient pressure and dense ISM

centred on 41.9+58 (most powerful SNR)

main contributor to high-brightness X-ray outflow!

vexp 45 km s-1

Ø 130 pc

M 8 ·106 M

Einp 1054 erg

kin 106 yr

SN ~ 0.001 yr-1

10% of Einp hot X-ray gas

10% of Einp expansion of molecular shell

Wills et al. (1999)

M82 408 MHz Wills et al. (1997)

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Weiß et al. (2001)

Weiß et al. (1999)

24

),(2 zResc

- prominent HI hole around star clusters (Israël & van Driel (1990)

- inner gaseous disk completely disrupted (Stil 1999)

- partly vw vesc (H velocities: Martin 1998; X-ray temperature: Della Ceca et al. 1996; Martin 1999)

Heckman et al. (1995), Della Ceca et al. (1996):

LFIR = 8 · 1041 erg s-1

LX = 3 · 1038 erg s-1

SN ~ 0.01 ··· 0.001 yr-1

Israël & de Bruyn (1988), Greggio et al. (1998):

starburst ceased ~5 ··· 10 Myr ago

SFR 0.5 M yr-1

NGC 1569

Martin (1999)

- giant molecular clouds near central HI holeformed by shocks from central burst?

- strong CO(32) lineICO(3-2)/ICO(21-1) ~ 2 (!) copious warm gas

- evidence for blown-out/piled-up gas

- radial magnetic fields!

Ott (2002)

25Hunter et al. (1993)

Taylor et al. ( 1999)

Mühle (in prep.)

Mühle (in prep.)

CO(3 2) Mühle (in prep.)

Disrupted gas in a dwarf galaxy:

• kinematics of HI (Stil 1999): inner part (r 0.6 kpc) completely disrupted by starburst

• just two regions of dense gas left (Taylor et al. 1999)

• warm, diffuse gas out to ~400 pc (Mühle in prep.)

• radial configuration of magnetic field (Mühle in prep.)

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Ho I

LSB dwarf galaxy

Mtot ~ 2.4 · 109 M (stars + gas)

Ott et al. (2001):

HI arranged in huge shell

Ø 1.7 kpc

MHI 108 M

Einp 1053 erg

kin 80 60 Myr (kin. + CMD)

- BCDG phase in the past?

- recollapse?

Major axis

Min

or a

xis

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Outlook

• study of low-mass galaxies important for our understanding of galaxies in the early universe

• detailed case studies indispensable (dwarf galaxies are individuals!)- different environments (field, group, cluster)- different masses and SFR’s- recover full gas content- derive gravitational potentials (DM)- study interplay between SF and ISM (disk - halo)

• numerical simulations must incorporate realistic conditions- gas distribution- mass distribution- attempt to ‘reproduce’ observed galaxies

• interpreting distant galaxies requires scrutiny of nearby ones,in particular at low-mass end

• relevant observations of (more) distant galaxies - SKA- ALMA- NGST- X-ray satellites

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29

30

LB ~ 0.5 LMW

LB ~ 0.06 LMW

LB ~ 0.005 LMW

31

Ott et al. (in prep.)

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