Shock Waves in the Large-Scale Structure of the Universe
Dongsu Ryu, Hyesung Kang
备注：
Submitted to Astrophysics and Space Science (Special Issue for the proceedings of International Conference on HEDP/HEDLA-08). Pdf with full resolution Figure 1 can be downloaded from http://canopus.cnu.ac.kr/ryu/rk.pdf
Cosmological shock waves are induced during hierarchical formation of large-scale structure in the universe. Like most astrophysical shocks, they are collisionless, since they form in the tenuous intergalactic medium through electromagnetic viscosities. The gravitational energy released during structure formation is transferred by these shocks to the intergalactic gas as heat, cosmic-rays, turbulence, and magnetic fields. Here we briefly describe the properties and consequences of the shock waves in the context of the large-scale structure of the universe.
Cosmological shock waves result from supersonic flow motions induced by hierarchical clustering of nonlinear structures in the universe. These shocks govern the nature of cosmic plasma through thermalization of gas and acceleration of nonthermal, cosmic-ray (CR) particles. We study the statistics and energetics of shocks formed in cosmological simulations of a concordance $Λ$CDM universe, with a special emphasis on the effects of non-gravitational processes such as radiative cooling, photoionization/heating, and galactic superwind feedbacks. Adopting an improved model for gas thermalization an…
In the standard theory of the large scale structure formation, matter accretes onto high density perturbations via gravitational instability. Collisionless dark matter forms caustics around such structures, while collisional baryonic matter forms accretion shocks which then halt and heat the infalling gas. Here, we discuss the characteristics, roles, and observational consequences of these accretion shocks.
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…
We study the properties of cosmological shock waves identified in high-resolution, N-body/hydrodynamic simulations of a $Λ$CDM universe and their role on thermalization of gas and acceleration of nonthermal, cosmic ray (CR) particles. External shocks form around sheets, filaments and knots of mass distribution when the gas in void regions accretes onto them. Within those nonlinear structures, internal shocks are produced by infall of previously shocked gas to filaments and knots, and during subclump mergers, as well as by chaotic flow motions. Due to the low temperature of the accreting gas, t…
Cosmological hydrodynamic simulations of large scale structure in the universe have shown that accretion shocks and merger shocks form due to flow motions associated with the gravitational collapse of nonlinear structures. Estimated speed and curvature radius of these shocks could be as large as a few 1000 km/s and several Mpc, respectively. According to the diffusive shock acceleration theory, populations of cosmic-ray particles can be injected and accelerated to very high energy by astrophysical shocks in tenuous plasmas. In order to explore the cosmic ray acceleration at the cosmic shocks, …
We investigate the dynamical importance of a newly recognized possible source of significant feedback generated during structure formation; namely cosmic ray (CR) pressure. We present evidence for the existence of numerous shocks in the hot gas of galaxy clusters (GCs). We employ for the first time an explicit numerical treatment of CR acceleration and transport in hydro simulations of structure formation. According to our results, CRs provide an important fraction of the total pressure inside GCs, up to several tenths. This was true even at high redshift (z=2), meaning that such non-thermal c…
Cosmological shock waves play a major role in heating baryonic gas in the large scale structure of the universe. In this contribution we study the shock-heated gas in connection with shocks themselves. The distributions of shock speed and temperature of shock-hated gas both should reflect the depth of gravitational potential of associated nonlinear structures, so their morphology. From their distributions we find, in addition to hot gas in and around clusters/groups and warm-hot intergalactic medium (WHIM) with T=10^5-10^7 K mostly in filaments, there is a significant amount of low temperature…
We investigate the destruction of dust grains by sputtering in the high-velocity interstellar shocks driven by supernovae (SNe) in the early universe to reveal the dependence of the time-scale of dust destruction on the gas density $n_{{\rm H}, 0}$ in the interstellar medium (ISM) as well as on the progenitor mass $M_{\rm pr}$ and explosion energy $E_{\rm 51}$ of SN. The sputtering yields for the combinations of dust and ion species of interest to us are evaluated by applying the so-called universal relation with a slight modification. The dynamics of dust grains and their destruction by sputt…
We present an analytical description of the energetics of the population of cosmic accretion shocks, for a concordance cosmology. We calculate how the shock-processed accretion power and mass current are distributed among different shock Mach numbers, and how they evolve with cosmic time. We calculate the cumulative energy input of cosmic accretion shocks of any Mach number to the intergalactic medium as a function of redshift, and we compare it with the energy output of supernova explosions as well as with the energy input required to reionize the universe. In addition, we investigate and qua…
Chandra has recently observed 1E0657-56, a hot merging system at z=0.3 (the ``bullet'' cluster), for 500 ks. I present some of the findings from this dataset. The cluster exhibits a prominent bow shock with M=3.0+-0.4 (one of only two known M>>1 shock fronts), which we use for a first test of the electron-ion equilibrium in an intergalactic plasma. The temperatures across the shock are consistent with instant shock-heating of the electrons; at 95% confidence, the equilibration timescale is much shorter than the collisional Spitzer value. Global properties of 1E0657-56 are also remarkable. Desp…
We use a 1-D Lagrangian code which follows both a gaseous and a dark component to study the radiative shocks that appear in the evolution of spherical scale-free perturbations in an Einstein-de Sitter Universe. The detailed behaviour of the shock depends on whether the radiative cooling is dominated by bremsstrahlung or line cooling. Bremsstrahlung is the main energy loss mechanism for systems with circular velocity $V_{c} > 100 km s^{-1}$. In this case, we can reproduce the kinematics of the shock and of the cooling wave to a high degree of accuracy with a simple analytic model. When line coo…
1. Introduction, 2. The (quasi-parallel) foreshock; Ion foreshock, Ion foreshock boundary region; Diffuse ions;Low-frequency upstream waves; Ion beam waves; The expected wave modes; Observations; Diffuse ion waves; Electron foreshock; Electron beams; Langmuir waves; stability of the electron beam; Electron foreshock boundary waves; Nature of electron foreshock waves; Radiation; Observations; Interpretation; 3. Quasi-parallel shock reformation; Low-Mach number quasi-parallel shocks; Turbulent reformation; Observations; Simulations of quasi-parallel shock reformation; Hybrid simulations in 1D; H…
It has been claimed that in stellar winds traversed by strong shocks the mechanism for driving the wind by sound wave pressure cannot operate because sound waves cannot propagate past the shocks. It is shown here that sound waves can propagate through shocks in one direction and that this is a sufficient condition for the sound wave pressure mechanism to work. A strong shock amplifies a sound wave passing through it and can drag the sound wave away from the star. It is immaterial for the sound wave pressure gradient that the sound wave vector points towards the star. Since the strong shocks dr…
Three-dimensional hydrodynamic simulations of large scale structure in the Universe have shown that accretion shocks form during the gravitational collapse of one-dimensional caustics, and that clusters of galaxies formed at intersections of the caustics are surrounded by these accretion shocks. Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about 5 Mpc, respectively, in the $Ω=1$ CDM universe. Assuming that energetic protons are accelerated by these accretion shocks via the first-order Fermi process and modeling particle transport around the shocks through Bohm diff…
The long-range energy deposition by heavy-ion beams makes new types of shock wave experiments possible in the laboratory. We have investigated a situation that is of relevance to supernova dynamics in astrophysics, where a shock wave is irradiated by a flux of neutrinos depositing energy throughout the shock wave and surrounding matter, thus changing the behaviour of the running shock. We have carried out fluid-dynamical simulations to study generic features of stimulated shock waves. First we consider an idealized case assuming uniform energy deposition into a planar shock wave propagating th…
Large-scale diffuse radio emission is observed in some clusters of galaxies. There is ample of evidence that the emission has its origin in synchrotron losses of relativistic electrons, accelerated in the course of clusters mergers. In a cosmological simulation we locate the structure formation shocks and estimate their radio emission. We proceed as follows: Introducing a novel approach to identify strong shock fronts in an SPH simulation, we determine the Mach number as well as the downstream density and temperature in the MareNostrum Universe simulation which has 2x1024^3 particles in a 500 …
Using the Monte Carlo simulations we apply a method of discrete small amplitude particle momentum scattering to reproduce highly anisotropic conditions at relativistic shocks. We discusse acceleration times scales in relativistic shocks. Next, we describe newly found acceleration mechanism at ultrarelativistic shocks. The universal spectral index and the acceleration time for the shocks are given. In the end reflections from ultrarelativistic shocks are considered.
Supersonic turbulence generates distributions of shock waves. Here, we analyse the shock waves in three-dimensional numerical simulations of uniformly driven supersonic turbulence, with and without magnetohydrodynamics and self-gravity. We can identify the nature of the turbulence by measuring the distribution of the shock strengths. We find that uniformly driven turbulence possesses a power law distribution of fast shocks with the number of shocks inversely proportional to the square root of the shock jump speed. A tail of high speed shocks steeper than Gaussian results from the random superp…
The first-order cosmic ray acceleration at ultrarelativistic shocks is investigated using the Monte Carlo method. We apply a method of discrete particle momentum scattering as a model of particle pitch angle diffusion to reproduce highly anisotropic conditions at the shock wave. Shocks with Lorentz factors $γ$ up to 320 and varying magnetic field inclinations $ψ$ are considered. Values of diffusion coefficients upstream in the point where energy spectral indices stabilize to the limit 2.2 were calculated. The obtained acceleration time does not depend on shock conditions.
We use the one-dimensional steady version of the equations derived in paper I to compute the structure of shock waves. The agreement with experiment is good, especially when we retain the experimental value of the Prandtl number adopted in II.
Generalization of the self-similar solution for ultrarelativistic shock waves (Blandford & McKee, 1976) is obtained in presence of losses localized on the shock front or distributed in the downstream medium. It is shown that there are two qualitatively different regimes of shock deceleration, corresponding to small and large losses. We present the temperature, pressure and density distributions in the downstream fluid as well as Lorentz factor as a function of distance from the shock front.
We apply a semi-classical approach of handling waves as quasiparticle gas in a slowly varying flow -- analogous to ray tracing -- to calculate the Alfven wave transmission parameters, the resulting cross-helicity of the waves and the scattering-centre compression ratio, for cases where the shock thickness is large enough for the turbulent waves in the plasma to see the transition of the background flow parameters as smooth and slowly varying. For nonrelativistic shocks the wave transmission produces similar effects on the downstream turbulence and the scattering-centre compression ratio as doe…
We study the properties of the shock waves for a viscous accretion flow having low angular momentum in presence of synchrotron cooling. We present all possible accretion solutions in terms of flow parameters. We identify the region of the parameter space for steady and oscillating shocks and show the effect of various energy dissipation processes on it. We discuss the role of the shock waves while explaining the observations from black hole candidates.
Gamma-ray astronomy will play a crucial role in the investigation of nonthermal processes in the large scale structure of the universe. Particularly, galaxy clusters (GC) observations at this photon energy will help us understand the origin of radio emitting high energy particles, the possible level of cosmic-ray (CR) pressure in intracluster environment, and the strength of intracluster magnetic fields. In addition here we point out the importance of these observations for a possible detection of cluster accretion shocks and for constraining their CR acceleration efficiency. We model spatial …
In this paper, we study the collapse of a superhorizon-sized void in the early, radiation-dominated universe using an improved general relativistic code. We find that in general a relativistic or nonrelativistic void collapses via a shock at the speed of light. This is true even if the outward velocity of the void wall is enormous. As the wall thickness decreases, the shock strength increases and the collapse time decreases up to a limit set by the shock tube solution. In addition, as the outward velocity of the wall increases, the collapse time increases somewhat. When the collapse occurs in …
Self-similar solutions to the problem of a strong imploding relativistic shock wave are calculated. These solutions represent the relativistic generalisation of the Newtonian Gouderley-Landau-Stanyukovich problem of a strong imploding spherical shock wave converging to a centre. The solutions are found assuming that the pre-shocked flow has a uniform density, and are accurate for sufficiently large times after the formation of the shock wave.
