Kein Folientitel

1
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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but rather ....
The first stellar system deemed extragalactic
wasnt ....
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)
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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 lt MB lt -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

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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 (dEs, dSphs)
• gas-rich (all others)
• numerous
• DM dominated (?)

GR 8 Im
ESO 410- G005 dSph
The zoo

• Irrs (Im, IBm, Sm, SBm)
• dEs, dSphs
• 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

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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 51014 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 105 M?
  should exist ...
• soft merging (à la Sagittarius dwarf)

2 Mpc
Galaxy halo 21012 M ?
Moore et al. (1999)
300 kpc
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Mihos Hernquist (1995)
small perturber ...
large effect!
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Dwarf galaxy evolution
In bottom-up scenario primordial DM halos filled
with baryonic matter subsequent SF
gas-rich dIs evolution into gas-poor
dSphs 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 dEs 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 dIs dSphs HSB dIs dEs
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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?
DErcole 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
DErcole 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?
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 dEs, dSphs)
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
N6822
M31
cubes
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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
Böttner et al. (2001)
NGC 4449 (center) MHI 1.5 108 M? MH2
4.4 108 M?

• methods to derive molecular masses
• extinction (Dickman 1978) AV NHI 2NH2
• FIR submm emission (Thronson 1986)
• S? NHI 2NH2
• ?-rays (Bloemen et al. 1986)
• I? NHI 2NH2
• virialized clouds (Solomon et al. 1987) most
  widely resorted to ....

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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)

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a normal galaxy...
M51
a dwarf galaxy ...
LMC!
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... 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

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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!
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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, inclu
  ding isotopomers ? true gas distribution -
  strong spatial variation of XCO - blind use of
  XCO leads to false results ....

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Star formation history in dwarf
galaxies
GR 8
Sextans A
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Dumke et al. (1995)
Dumke et al. (1995)
Magnetic fields

• 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)
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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 - dSphs 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)
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• 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
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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)
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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 1038 erg s-1 blow-out
  occurs in 109 M? galaxy ? only 30 metals
  retained

Devine Bally (1999)
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
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M 82
Wills et al. (1999)
Kronberg et al. (1981) LFIR 1.6 1044 erg
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 41.958 (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)
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Weiß et al. (2001)
Weiß et al. (1999)
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NGC 1569
Ott (2002)
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
- 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)
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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)
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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?
Minor axis
Major axis
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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 SFRs - 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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LB 0.5 ? LMW
LB 0.06 ? LMW
LB 0.005 ? LMW
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Ott et al. (in prep.)
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