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Dwarf Galaxies: Building Blocks of the Universe. “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 .

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slide1

Dwarf Galaxies: Building Blocks of the Universe

“Definition”

Importance

Evolution and winds

Gas mass and distribution

Magnetic fields

Kinematics and Dark Matter

3-D structure

Winds: case studies

Future studies

IMPRS, April 8

themes of an expiring graduate school ...

slide2

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

but rather ....

M31

NGC6822

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

Kilborn et al. (1999)

slide3

What is a dwarf galaxy?

MB = -17.92

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 = -17.59

MB = -16.36

Bingelli diagramme  linked to galaxy formation

  • shape of potential
  • total mass
slide4

Properties:

POSS HST

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 (?)

GR 8 Im

ESO 410- G005 dSph

The zoo:

Irr’s (Im, IBm, Sm, SBm)

dE’s, dSph’s

LSBDGs

BCDGs, HII galaxies

clumpy irregulars

tidal dwarfs

I Zw 18 BCDG

Importance:

Mkn 297 Cl. Irr.

understanding

  • distant galaxies
  • galaxy evolution
  • ICM evolution
  • nature of Dark Matter
  • structure formation
slide5

Dwarf galaxies are building blocks

CDM: Bottom-up structure formation

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

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)

“missing satellite” problem

Cluster halo 5·1014 M 

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

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 ~ 105M should exist ...
  • soft merging (à la Sagittarius dwarf)

2 Mpc

Galaxy halo 2·1012M 

Moore et al. (1999)

300 kpc

slide6

Mihos & Hernquist (1995)

small perturber ...

large effect!

slide7

Dwarf galaxy evolution

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

subsequent SF gas-rich dI’s

evolution into

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

Larson (1974) : gas depletion through first starburst

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

many models meanwhile ...

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

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

LSB dI’s dSph’s

HSB dI’s dE’s

slide8

Wind models(a selection ....)

Mac Low & Ferrara (1999)

t = 100 Myr

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

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

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 (?)

slide9

How much mass, how much gas?

Bomans et al. (1997)

IZw 18 HI

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

Gentile (in prep.)

total (dynamical) mass:

dwarfs gas-rich (except dE’s, dSph’s)

van Zee et al. (1998)

yet Mtot difficult to assess at low-mass end:

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

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

Hunter et al. (1998)

Hunter (priv. comm.)

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

M31

N6822

cubes

slide10

Molecular (“hidden”?) gas

Kohle (1999)

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 weaker

NGC 4449 (center):

MHI ~ 1.5 ·108 M

MH2 ~ 4.4 ·108 M

Böttner et al. (2001)

methods 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 ....
slide11

virialized clouds: measure

- radius R

- line width v

- CO intensity ICO

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)
slide12

a normal galaxy...

M51

a dwarf galaxy ...

LMC!

slide13

... puzzling cases:

Fritz (2000)

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

slide14

XCO dependence

  • 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

Klein (1999)

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

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

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

bottom line: detailed case studies indispensable!

slide15

Two contrasting examples:

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) from radiative transfer models; requires many transitions, including isotopomers  true gas distribution - strong spatial variation of XCO - blind use of XCO leads to false results ....

slide17

Magnetic fields

Dumke et al. (1995)

Dumke et al. (1995)

B-fields play an important role in SF process

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

NGC4631

B-fields in dwarf galaxies exhibit less coherent structure

NGC4565

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

Klein et al. (1991)

Klein et al. (1996)

Chyy et al. (2000)

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

slide18

Kinematics and Dark Matter

Ho I

early recognition that dwarfs have high M/L Sargent (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)

Ott et al. (2001)

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)

slide19

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)

Blais-Ouellette et al. (2001)

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

Swaters (1999)

need high-quality rotation curves (H + HI)

in particular: undisturbed dwarf galaxies

slide20

3-D structure of dwarf galaxies

IC 2574

Brinks & Walter (1998)

irregular morphologies  inclination often unknown

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

Compare z0 with sizes of largest holes

less gravity  larger z0  larger holes

Galaxy scale height

[pc]

M 31 100

M 33 120

IC 2574 350

Ho I 400

Ho II 625

Brinks & Walter (1998)

slide21

Different masses, different winds ....

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

modern numerical simulations (e.g. Mac Low & Ferrara 1999; Ferrara & Tolstoy 2000): for mechanical luminosity L = 1038erg s-1 blow-out occurs in 109Mgalaxy  only ~30% metals retained

Devine & Bally (1999)

Galaxy D Mtot starburst

[Mpc] [109M ]

M 82 3.6 10 ongoing

NGC 1569 2.2 0.4 post

Ho I 3.6 0.24† past

† visible (stellar) mass

slide22

M 82

Wills et al. (1999)

Kronberg et al. (1981):

LFIR = 1.6 · 1044erg s-1

LX = 2.0 · 1044 erg s-1

SN ~ 0.1 yr-1

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

M82 408 MHz Wills et al. (1997)

slide23

Weiß et al. (2001)

Weiß et al. (1999)

slide24

NGC 1569

Ott (2002)

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

LFIR = 8 · 1041erg s-1

LX = 3 · 1038erg 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

- 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)

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

- strong CO(32) line ICO(3-2)/ICO(21-1) ~ 2 (!)  copious warm gas

- evidence for blown-out/piled-up gas

- radial magnetic fields!

Martin (1999)

slide25

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.)

CO(3  2) Mühle (in prep.)

Mühle (in prep.)

Mühle (in prep.)

Taylor et al. ( 1999)

Hunter et al. (1993)

slide26

Ho I

LSB dwarf galaxy

Mtot ~ 2.4 · 109M (stars + gas)

Ott et al. (2001):

HI arranged in huge shell

Ø  1.7 kpc

MHI  108 M

Einp  1053 erg

kin 80  60Myr (kin. + CMD)

- BCDG phase in the past?

- recollapse?

Minor axis

Major axis

slide27

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

slide30

LB ~ 0.5  LMW

LB ~ 0.06  LMW

LB ~ 0.005  LMW