18 September, 2008 Astrophysics with E-LOFAR, Hamburg 1
Dwarf galaxies and the Magnetisation of the IGM Uli Klein 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 2
Galaxy clusters: Feretti & Giovannini (1998) • ‘radio halos’: secondary electrons or MHD turbulence • ‘radio relics’ cluster mergers: reacceleration • Faraday rotation ICM is magnetised (throughout? ) Coma Röttgering et al. (1994) A 2256 18 September, 2008 Clarke et al. (2001) Astrophysics with E-LOFAR, Hamburg 3
relativistic electrons have short lifetimes B = 1 G, = 1. 4 GHz t 1/2 = 108 yr @ z = 0 primary electrons require continuous injection secondary electrons: hadronic collisions of relativistic protons thermal gas 18 September, 2008 Astrophysics with E-LOFAR, Hamburg with 4
How did the relativistic gas and magnetic field get there? Two alternatives: • primordial magnetic fields requires extremely efficient amplification • galactic evolution injection by with (early) starbursts (Kronberg et al. 1999; Bertone et al. 2006) AGN 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 5
• FR I/FR II radio galaxies: P 1. 4 GHz(FRI/II) 1024. 7 W Hz-1 • starburst dwarf galaxies: 1020. 5 W Hz-1 P 1. 4 GHz(dwg) P 1. 4 GHz(FRI/II) 15000 · P 1. 4 GHz(dwg) CDM helps … lifetime of radio galaxies (Bird et al. 2008): life 1. 5· 107 yr duty cycle: act 2· 108 yr 0. 015 H duty 8· 108 yr 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 6
Dwarf galaxies: deficiency of synchroton radiation (at GHz frequencies) II Zw 70 Klein, Weiland, Brinks (1991) Skillman & Klein (1988) high thermal fraction function of galaxy mass 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 7
nearby template: NGC 4449 1 kpc Mtot = 5· 1010 M D = 3. 7 Mpc • “open” B-field structures • synchrotron halo Chyży et al. (2000) Klein et al. (1996) 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 8
NGC 1569: D = 2. 2 Mpc, Mtot 1010 M Kepley et al. (2008) 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 9
Israel & de Bruyn (1988) - break in synchrotron spectrum: cease of SF several Myr ago Lisenfeld et al. (2004) - radio halo - radial magnetic field synchrotron and IC aging (fast in BCDGs!) log S uwind 30 · umag - should be detectable with LOFAR! time b 18 September, 2008 Astrophysics with E-LOFAR, Hamburg log 10
assume relativistic particles have left the dwarf galaxy by wind, diffusion with Alfvèn speed B = 1 G, ne = 0. 001 cm-3 b = 150 MHz 0. 40 Gyr Lprop 30 kpc at D 100 Mpc (Coma Cluster) ~ 2′ angular extent 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 11
can be detected with LOFAR: • e. g. synchrotron halo of NGC 4449: • WSRT @ 0. 61 GHz: B 1. 5 m. Jy/b. a. , 28″ 44″ beam • spectral index = -1. 5 (S ~ ) • expected brightness (5″ beam): • B 0. 3 m. Jy/b. a. @ 150 MHz feasible! 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 12
Inferences • expect a lot of low-frequency radio continuum halos wrapped around dwarf galaxies • all dwarf galaxies in which star formation has ceased no more than about 1 Gyr ago • detectable with LOFAR out to 100 Mpc • total energy deposited in such halos contains information on past starburst activity • their sizes and morphologies shed light on the question which processes/objects magnetized the IGM 18 September, 2008 Astrophysics with E-LOFAR, Hamburg 13
18 September, 2008 Astrophysics with E-LOFAR, Hamburg 14
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