May 17, 2010 MFPO 2010, Krakow 1

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