Transcript Kein Folientitel - uni

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 ...
1
The first stellar system deemed extragalactic wasn‘t ....
M31
but rather ....
NGC6822
L~ 1 L
*
• Hubble (1925): Cepheids  NGC6822 at D = 214 kpc (today: 670 kpc)
assumed Gaussian LF....
L ~ 0.0025  L
*
Kilborn et al. (1999)
• Zwicky (1942): LF increases with decreasing luminosity
 dwarf galaxies = most numerous stellar systems
2
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
(r , z )  4  G   (r , z )
• total mass
M     dV
3
Properties:
• 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 (?)
The zoo:
• Irr’s (Im, IBm, Sm, SBm)
• dE’s, dSph’s
• LSBDGs
• BCDGs, HII galaxies
• clumpy irregulars
• tidal dwarfs
POSS
HST
GR 8 Im
ESO 410- G005 dSph
I Zw 18 BCDG
Importance:
understanding
• distant galaxies
• galaxy evolution
• ICM evolution
• nature of Dark Matter
• structure formation
Mkn 297 Cl. Irr.
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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
• 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)
Cluster halo 5·1014 M 
2 Mpc
Galaxy halo 2·1012 M 
Moore et al. (1999)
300 kpc
5
Mihos & Hernquist (1995)
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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)
 depl 
M gas
SFR
 4 1010 yr
- 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
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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
D’Ercole & Brighenti (1999):
- starburst in typical gas-rich dwarfs  NGC 1569
- mechanical luminosities L = 3.8 ·1039 ··· 3.8 ·1040 erg s-1
D’Ercole & Brighenti (1999)
- efficient metal ejection into IGM
- ‘recovery’ for next starburst after 0.5 ··· 1 Gyr
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):
 D   S     d 
   
  
M HI  2.33 10 5  
M
1 
 Mpc   Jy   km  s 
Gentile (in prep.)
2
total (dynamical) mass:
M tot
 R    ( R) 
 
 2.31105  
 km s 1 
kpc


 
2
M
dwarfs gas-rich (except dE’s, dSph’s)
yet Mtot difficult to assess at low-mass end:
van Zee et al. (1998)
Hunter et al. (1998)
- ill-defined inclinations (3-D structure?)
- disturbed velocity fields
v ~ vrot at low-mass end
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)
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- irregular velocity field in centre
M31
N6822
cubes
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)
nH ~ 1 ···100 cm-3
HI : pervasive
Ts ~ 100 K
H2 : pervasive
Tk ~ 10 ··· 30 K nH2  1000 cm-3
GMCs
Tk ~ 20 K
dark clouds Tk ~ 10 K
cores
Tk  40 K
nH2 ~ 10 2 cm-3
nH2 ~ 10 3 ···10 4 cm-3
nH2  10 4 cm-3
H2 formed on dust grains (catalysts) at nH2  50 cm-3
requires column densities NH2  10
against dissociation by  11 eV photons
20
cm-2
to shield
NGC 4449 (center):
Böttner et al. (2001)
MHI ~ 1.5 ·108 M
MH2 ~ 4.4 ·108 M
mostly optically thick 12C16O measured
13CO,
C18O optically thin, but much weaker
methods to derive molecular masses:
• extinction (Dickman 1978): AV ~ NHI + 2·NH2
• FIR & submm emission (Thronson 1986)
N 13 CO
 Tb   d 
  K    km  s 1 
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 2.6 10 
5.3
1 e
S ~ NHI + 2·NH2
• -rays (Bloemen et al. 1986)
I ~ NHI + 2·NH2
• virialized clouds (Solomon et al. 1987)
most widely resorted to ....
S  D 2
Md 
B (Td )  

m ol. cm 2
Tex
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virialized clouds: measure
- radius R
- line width v
- CO intensity ICO
 T   d 
I CO    b   
K km s 1
1 
 K   km  s 
Milky Way: XCO = 2.3 ·1020 mol. cm-2 (K km s-1) -1
 T   d  dx  dy 

 K km s 1 pc2
LCO    b   
1 
2 
 K   km  s  pc 
implications:
• ICO measures (‘counts’) the number of individual
clouds within the telescope beam, weighted by their
temperatures
Caveat: depends on
• Mvir (the total cloud mass) equals the sum of the
atomic and molecular gas mass
• radiation fields (dissociation)
 ICO is a good measure for the H2 column density
(or LCO is a good measure for the H2 mass)
• metallicity (C & O abundance)
• excitation conditions (line intensity)
• density (shielding)
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a normal galaxy...
M51
a dwarf galaxy ...
LMC!
12
... 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
• centre : evolved starburst
• SE
excitation process?
ISM affected
: 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)
Klein (1999)
process: H2 + CR  H2+ + e-(~35 eV) + CR
primary e- heats gas by (ionizing or non-ionizing) energy transfer
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!
14
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 ....
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Star formation history in dwarf galaxies
GR 8
Sextans A
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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  field
• B-fields in dwarf galaxies exhibit less
coherent structure

NGC4565 B  field
• 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
- 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)
Ott et al. (2001)
Mateo (1998)
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• CDM models: e.g. ‘NFW’ (Navarro et al. 1996):
 NFW (r ) 

 r

r
 s
0
 
  1  r
 
r
s
 




2
• 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?
 B (r ) 
 0  r02
r0  r   r02  r 2 
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)
 ( z, R)   (0, R)  sech 2 ( z z0 )
z0 ( R)   gas
1
2  G  tot (0, R)
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)
20
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
Galaxy
M 82
D
Mtot
[Mpc]
[109 M ]
starburst
3.6
10
ongoing
NGC 1569 2.2
0.4
post
Ho I
0.24†
past
3.6
Devine & Bally (1999)
† 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 41.9+58 (most powerful SNR)
M82 408 MHz
Wills et al. (1997)
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
22
Weiß et al. (2001)
Weiß et al. (1999)
23
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)

esc
 2   ( R, z )
- 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)
24
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)
25
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?
26
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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LB ~ 0.5  LMW
LB ~ 0.06  LMW
LB ~ 0.005  LMW
30
Ott et al. (in prep.)
31
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