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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
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.
4
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)
6
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
7
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 (?)
8
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)
2.31105
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)
9
- 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
14
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
10
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)
11
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
13
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 ....
15
Star formation history in dwarf galaxies
GR 8
Sextans A
16
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)
Chyy et al. (2000)
magnetization of IGM by primeval galaxies? (Kronberg et al. 1999)
17
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)
18
• 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
19
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
21
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(32) 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
27
28
29
LB ~ 0.5 LMW
LB ~ 0.06 LMW
LB ~ 0.005 LMW
30
Ott et al. (in prep.)
31
32