Shock Waves Of The Large-Scale Structure Formation In The Universe

Abstract

Abstract. Simulations of structure formation in the Universe predict accretion shock waves at the boundaries of the large-scale structures as sheets, filaments, and clusters of galaxies. If magnetic fields are present at these shocks, particle acceleration should take place, and could contribute to the observed cosmic rays of high energies. When the radio plasma of an old invisible radio lobe is dragged into such a shock wave, the relativistic electron population will be reaccelerated and the plasma becomes radio-luminous again. Such tracers of the accretion shock waves are observed at the boundaries of some clusters of galaxies: the so-called cluster radio relics, which are large regions of diffuse radio emission, without any parent galaxy nearby. The observed properties of the cluster radio relics are naturally explained by accretion shock waves. Radio relics therefore give the first evidence for the existence of accretion shocks of the large-scale structure formation and they allow investigations of the shock properties. Shock Waves of the Large-Scale Structure FormationThe large-scale structure of the Universe, seen in the structured galaxy distribution, is still forming. Matter is flowing out of the cosmic voids onto sheets and filaments. Within the filaments the matter flows to the density cusps frequently located at the intersection points of filaments: the clusters of galaxies. Whenever the flow passes from one structure into another, its velocity suddenly changes and several Mpc sized shock waves occur. At these shock waves the kinetic energy of the gravitationally accelerated gas is

Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000

“…We propose here that the large-scale radio emission and magnetic field detected in ZwCl 2341.1+0000 are strong evidences for diffusive shock-acceleration (Bell 1978, Blandford & Ostriker 1978 of particles in structure formation flows (Enßlin et al 1998, Miniati et al 2001b. A natural consequence of this process would be a power-law spectrum of the momentum distribution.…”

VLA Radio Observations of the HST Frontier Fields Cluster Abell 2744: The Discovery of New Radio Relics

“…The leading theory for relics is that they trace particles (re) accelerated at shocks (e.g., Enßlin et al 1998;Markevitch et al 2005;Kang & Ryu 2011;van Weeren et al 2017a). The radio spectral index is related to the slope of the underlying electron…”

“…Following Enßlin et al (1998), the observed polarization fraction of relics can be used to determine a lower limit on the viewing angle of the relic. The alignment of unordered magnetic fields at the shock front should be caused by shock compression, with the degree of alignment dependent on the compression factor.…”

“…Assuming a weak magnetic field (i.e., the magnetic pressure of the relic is small compared to the internal gas pressure, see Enßlin et al 1998), the observed polarization fraction of the relic is related to the viewing angle via the formula where δ is the slope of the electron energy distribution (see Section 5.2). Figure 13 plots Equation (10) for all viewing angles and polarization fractions for our sources R1 to R4.…”

“…Giant radio relics are arc-like sources of synchrotron radiation of megaparsec size. The leading theory behind their formation is that of shock acceleration (Enßlin et al 1998;Drury 1983) of either thermal or preaccelerated fossil electrons from AGNs or other radio galaxy activity (e.g., Markevitch et al 2005;Giacintucci et al 2008;Kang & Ryu 2011;Pinzke et al 2013;van Weeren et al 2017a), although other models exist (Fujita et al 2015).…”