Imprint of Intergalactic Shocks on the Radio Sky

Strong intergalactic shocks are a natural consequence of structure formationin the universe. They are expected to deposit large fractions of their energyin relativistic electrons (xi_e~0.05 according to SNR observations) andmagnetic fields (xi_B~0.01 according to cluster halo observations). Wecalculate the synchrotron emission from such shocks using an analytical model,calibrated with a hydrodynamical LCDM simulation. The resulting signal composesa large fraction of the extragalactic radio background (ERB) below 500 MHz. Theassociated angular fluctuations dominate the sky for frequencies nu<10 GHz andangular scales arcmin-deg (after a modest removal of point sources), providedthat xi_e*xi_B>3*10^-4. The fluctuating signal is most pronounced for nu<500MHz, dominating the sky even for xi_e*xi_B=5*10^-5. The signal will be easilyobservable by next generation radio telescopes such as LOFAR and SKA, and ismarginally observable with present telescopes. It may be identified usingcross-correlations with tracers of large scale structure, possibly even inexisting <10 GHz CMB anisotropy maps and high resolution ~1 GHz radio surveys.Detection of the signal will provide the first identification of intergalacticshocks and of the WHIM, and gauge the unknown intergalactic magnetic field. Weshow that existing observations of the diffuse <500 MHz radio background arewell fit by a simple, double-disk Galactic model, precluding a directidentification of the diffuse ERB. Modelling the frequency-dependent anisotropypattern observed at very low (1-10 MHz) frequencies can disentangle thedistributions of Galactic cosmic-rays, ionized gas and magnetic fields. Spacemissions such as ALFA will thus provide important insight into the structureand composition of our Galaxy (abridged).Comment: Accepted for publication in ApJ. Presentation improved and references adde