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Dwarf galaxies and the Magnetisation of the IGM
Uli Klein
?
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ICM is magnetized (throughout?)
Galaxy clusters exhibit:
• radio halos
• Faraday rotation
• peripheral radio structures
Clarke et al. (2001)
Coma
Giovannini et al. (1993)
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relativistic electrons have short lifetimes
yr1GHz
1097.821
22
218
21
z
BB
Bt
cmb
B = 1 G, = 1.4 GHz t1/2 = 108 yr @ z = 0
primary electrons require continuous injection, e.g. via
• merger shocks
• galactic wakes
μG)1(25.3 2zBcmb
secondary electrons: hadronic collisions of relativistic protons with thermal gas
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two alternatives:
• primordial magnetic fields; requires extremely efficient amplification
• galactic evolution with injection by early starbursts (Kronberg et al. 1999; Bertone et al. 2006)
in both cases: particle pools required
how did the relativistic plasma get there?
AGN
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FR I/FR II radio galaxies:
P1.4GHz(FRI/II) 1024.7 W Hz-1
starburst dwarf galaxies:
P1.4GHz(dwg) 1020.5 W Hz-1
P1.4GHz(FRI/II) 15000 · P1.4GHz(dwg)
ΛCDM helps …
lifetime of radio galaxy (Bird et al. 2008):
life 1.5·107 yr
duty cycle:
duty 8·108 yr
B09251420
Fornax A
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Dwarf galaxies:
deficiency of synchrotron radiation (at GHz frequencies)
weak synchrotron emission at low-mass end
lack of CR containment
Skillman & Klein (1988)
Klein, Weiland, Brinks (1991)
II Zw 70
synαsyn
.ffν νSνSS
,010
,0
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Kepley et al. (2010)
template: NGC 1569
×103 rad m-2
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- break in synchrotron spectrum: cease of SF burst several Myr ago
- radio halo
- radial magnetic field
- B 40 μG (central region)
Israel & de Bruyn (1988)
Lisenfeld et al. (2004)
synchrotron and IC aging (fast in BCDGs!)
low-frequency halos will be detectable with LOFAR!
10 90, 110 240 MHz
log
log
S
time
-
b
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LOF
AR
low
ban
d
LOF
AR
hig
h ba
nd
e.g. spectrum of total radio continuum from the Coma Cluster halo
b = 0.5 1.0 GHz
Thierbach et al. (2003)
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numerous dwarf galaxies should be surrounded by low-frequency halos of synchrotron radiation
should be detectable with LOFAR; for B = 3 μG
= 50 MHz t1/2 = 3.6 · 108 yr (centre of low band)
= 175 MHz t1/2 = 1.9 · 108 yr (centre of high band)
= 20 MHz t1/2 = 5.6 · 108 yr
should also find lots of ‘idle’ dwarf galaxies!
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NB: powerful central radio galaxies cannot do the job: they are pressure-confined!
Per A
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Spiral galaxies: preponderance of synchroton radiation
spiral galaxies:
• continuous SF and SN rate production of CRs
• magnetic fields store CRs containment, (re)acceleration
Sff/Stot < 10% at 1 GHz
(Gioia et al. 1982; Klein 1990)
synαsyn
.ffν νSνSS 10
optically thin case:
Condon (1992)
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1 kpc
Klein et al. (1996)
Mtot = 5·1010 M
D = 3.7 Mpc
nearby template: NGC 4449• (partly) radial B-field structure
• synchrotron halo
Chyźy et al. (2000)
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A2256
Clarke & Enßlin (2006)
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• injection: starburst and radio galaxies
• dispersion: cluster weather
NGC1265
NGC1275 IC1133