Radio halos and relics in galaxy clusters

NGC315: giant (~ 1.3 Mpc) radio galaxy with odd radio lobe (Mack 1996; Mack et al. 1998). precessing jets (Bridle et al. 1976), but western one with peculiar bend towards the host galaxy - unusually flat radio spectrum in western lobe: first steepens (as expected), then flattens to  high  0.7 (S ~ -  ). - strong linear polarization: p  30%. Cosmological shock waves at intersecting filaments of galaxies Enßlin et al. (2000): originally symmetric radio galaxy “falling” into an intergalactic shock wave, along with its environment. compression  reacceleration of particles  strong alignment of magnetic field & increased synchrotron emissivity origin of large-scale gas flow and shock wave?

from theory of shocks (Landau & Lifschitz 1966)  temperature jump T 1 /T 2  3.3 ···· 20 compression ratio R  2.9 ···· 3.8 pressure jump P 1 /P 2  9.6 ···· 75 O’Drury (1983):    0.54 ···· 0.79 expected  S ~  NGC315 located within Pisces-Perseus Supercluster Enßlin et al. (2000) identify filaments of galaxies.with rather different velocity dispersions (redshifts from CfA survey, Huchra et al. 1990, 1992, 1995): - filament I:  v  400 km s -1 - filaments II - V:  v  90 ···· 220 km s -1 if gas has comparable  v, this translates into k ·T I  280 eV k ·T II-V  15 ···· 85 eV ‘view from above’ IIIIIIIVV NGC315

Coma Cluster

galaxy clusters contain: dark matter hot gas galaxies relativistic plasma work and reviews: Feretti & Giovannini

lifetime of radio structures: Thierbach, Klein, Wielebinski (2003) ν b = 0.44 GHz τ = 5.1 · 10 7 yr

“Halos“ and “relics“:

A2256 Röttgering et al. (1994) Thierbach (2000)

A3667 Röttgering et al. (1999)

Weather in Perseus Sijbring (1999)

Nature of radio structures radio halos (e.g. Coma): rich clusters with -  10% spirals -  v ~ 1000 km s -1 - L X ~  erg s -1 - no cooling flows central, smooth, extended, 1 Mpc in size unpolarized steep radio spectra, break in GHz range   -1.0, S ν ~ ν  origin: injection of relativistic e - by AGN, starbursts re-acceleration by galactic wakes secondary electrons from inelastic collisions between relativistic p and thermal ions (theory: Enßlin)

radio relics (e.g. A2256): clusters with substructure peripheral strongly polarized (p  20%) steep radio spectra  ~ -0.7  -1.0 origin: particles deposited by active radio galaxies shock waves in cluster mergers revive these (modelling & reviews on theory: Enßlin) ‘mini halos‘ (e.g. M87): surround dominant central radio galaxies clusters with cooling flows - particles delivered by radio galaxy - powered by magneto-hydrodynamic turbulence in cooling core

Filamentary polarized emission in a radio halo (Govoni et al. 2005) A2255 (z = ) high angular resolution filaments strongly polarized (20% - 40%) ordered on scales of ~ 400 kpc B parallel to relic, but perpendicular to filaments! not a simple picture …

XMM Newton observations of the Coma Cluster relic (Govoni et al. 2005) O'Drury 1983: spectral index  = (M 2 + 3) / 2 · (M 2 - 1) temp. ratio T1/T2 = (5 M 2 – 1) · (M 2 + 3) / 16 M 2 observed  = 1.18  T1/T2 = 2.1 no temperature increase in region of relic seen (X-ray spectra)  radio emission of relic not induced by a shock, but by turbulence?

LOFAR will disclose numerous nonthermal structures in galaxy clusters  shed light on structure formation in local universe help to understand magnetization of ICM -halos around AGN (radio galaxies, quasars, Seyferts) -halos around starburst galaxies (dwarfs!!!)

p + p  p + n + π + p + p  p + p + π - + π + π +  μ + + ν μ +  e + + ν e + ν μ π -  μ - + ν μ +  e - + ν e + ν μ n  p+ e - + ν e