Searching for Intergalactic Shocks with the SKA

Abstract

Strong intergalactic shocks are a natural consequence of structure formation in the universe. These shocks are expected to deposit large fractions of their energy in relativistic electrons (xi_e~0.05 of the thermal energy according to supernova remnant observations) and magnetic fields (xi_B~0.01 according to cluster halo observations). We discuss the expected synchrotron emission from such shocks, and the observational consequences for next generation radio telescopes such as the Square Kilometer Array. We present an analytical model, calibrated and verified based on a hydrodynamical LCDM simulation. The resulting signal composes a large fraction (up to a few 10%) of the extragalactic radio background below 500 MHz. The associated angular fluctuations, e.g. delta T_l>260(xi_e*xi_B/5*10^-4)(nu/100 MHz)^-3 K for multipoles 400<l<2000, dominate the radio sky for frequencies<10 GHz and angular scales 1 arcmin-1 deg (after a modest removal of point sources), provided that xi_e*xi_B>3*10^-4. The fluctuating signal is most pronounced for nu<500 MHz, dominating the sky there even for xi_e*xi_B=5*10^-5. We find that the signal is easily observable by next generation telescopes such as the SKA, and marginally observable with present telescopes. The signal may be identified using cross-correlations with tracers of large-scale structure (e.g. gamma-ray emission from intergalactic shocks). Detection of the signal will provide the first identification of intergalactic shocks and of the warm-hot intergalactic medium (believed to contain most of the baryons in the low redshift universe), and gauge the unknown intergalactic magnetic field.