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Waves and Solitons in Multi-component Self-gravitating
Systems
Kinwah Wu Mullard Space Science Laboratory
University College London
Curtis Saxton (MSSL, UCL)Ignacio Ferreras (P&A, UCL)
Outline
• Collective oscillations, non-linear waves and solitons (a brief overview)
• Multi-component self-gravitating systems• Two studies: results and some thoughts - “tsunami” & “quakes” in galaxy clusters - solitons in self-gravitating sheets and 1D infinite media
Collective phenonmona in multi-component systems
- Coupled oscillators: different oscillation normal modes and dissipation
processes non-linear mode coupling resonance - Two-stream instability - Landau damping
cf. electron-ion plasma wave-wave interaction particle-particle interaction wave-particle interaction
Solitons as non-linear wave packets
• Non-linear, non-dispersive waves: The nonlinearity which leads to wave steeping
counteracts the wave dispersion. • Interact with one another so to keep their basic
identity -- particle liked• Linear superposition often not applicable• Propagation speed proportional to pulse height • An example soliton pulse profile:
pulse height propagation speed
Situations that give rise to solitons
- fluids with surface tension - ion acoustic oscillations in plasmas - etc
Korteweg-de Vries (KdV) Equation
Kadomstev-Petviasgvili (KP) Equation
mathematical methods developed to solve various soliton equations e.g. Baecklund transformation, inverse scattering method, Zakharov-Shabat method ……
Mutiple solitons
- development of “soliton trains” (Zabusky & Kruskal 1965) kdV equation with a cosine-like function as initial condition component solitions with larger pulse height travel faster a train of solitions lined up with the tallest leading the way
=> “big” solitions are more likely to find each other
depending on dimension
- resonance and phase shift
time
before collision
after collision
resonant states: momentum exchange phase shift
Multi-component self-gravitating systems
The fluid description (non-rotational case)
Conservation equations:
Poisson equation:
Quakes and Tsunami in Galaxy Clusters
Galaxy clusters as “spherical”2-component coupled oscillators
• DM - momentum carrier • Gas - coolant (dissipater)
inflowgas cooling
Solve to obtain the stationary structure
Perturbative analysis
Lagrange perturbation for the DM and gas components:
plus 6 coupled perturbed hydrodynamic equations
the perturbed Poisson equation
Diagonise the matrixDefine the boundary conditions Numerically integrate the DEs
algebraic functions of hydrodynamic variables
Normal mode oscillations (the DM component)
- oscillation modes depend on BCs- high order modes not damped - different stability properties for even and odd modes in some cases
eigen-mode
- similar eigen-planes can be generated for the gas component
mass inflow rate = 100 M_sun per year(size ~ 1 Mpc; kT_max ~ 10 keV)
Quakes and tsunami in galaxy clusters: dark-matter oscillations and gas
dissipation- close proximity between clusters --> excitation of DM oscillations, ie. cluster quakes - high-order modes are also fast growing --> oscillations may occur in a wide range of scales - oscillations in DM coupled with oscillations in gas - cooler interior of gas (due to radiative loss) --> slower sound speeds in the inner cluster region (“cooling flow” core) --> waves piled up when propagating inward, ie. cluster tsunami - mode cascade --> inducing turbulence and hence heating of the cluster throughout
cf. “original” cluster tsunami model of Fujita, Suzaki & Wada (2004) and Fujita, Matsumoto & Wada (2005) stationary DM provide background potential (ie. no quake), waves in gas piled up when propagating inward (“self-excited” tsunami only)
Solitons in self-gravitating sheets and 1D infinite media
Planar systems: two component infinite self-gravitating sheets
Suppose: 1. inertia of one component is unimportant 2. the component is approximately isothermal 3. polytropic EOS
“quasi-1D” approximation
Perturbative expansion
Use two new variables:
constant yet to be determined
Formation of solitons
KdV equation --> soliton solution
rescaling variables
effective sound speed
cf. Solitons in single-component self-gravitating systems (Semekin et al. 2001)
Some thoughts and questions:
2 colliding DM solitons
resonant state
Suppose the resonant half life
Q1: Are ridge solitons manifested as filaments in cosmic sheets? Q2: Can soliton collisions make globular clusters?
Summary
1. Collective and non-linear oscillations, which may not be present in single component self-gravitating systems, could be important in multi-component systems. 2. DM and gas play different roles in exciting and sustaining oscillations in astrophysical systems. 3. Galaxy clusters can be considered as couple oscillators with DM as the mode resonant medium and gas as the energy dissipater. Gas in clusters can be compression heated by acoustic coupling with the DM oscillations. 4. Solitons can be excited in DM/gas sheets and infinite self-gravitating systems, and they could lead to “bright” structure formation, provided that certain dynamical conditions are satisfied.