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Nonthermal Radiation from Merging Clusters of Galaxies

Nonthermal Radiation from Merging Clusters of Galaxies. Robert C. Berrington (NRL) Chuck Dermer (NRL) Penn State U., May 27 th , 2003. (ApJ, in press, 2003, astro/ph 0209436). Outline. Shocks are formed during structure formation in merging clusters of galaxies

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Nonthermal Radiation from Merging Clusters of Galaxies

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  1. Nonthermal Radiation from Merging Clusters of Galaxies Robert C. Berrington (NRL) Chuck Dermer (NRL) Penn State U., May 27th, 2003 (ApJ, in press, 2003, astro/ph 0209436)

  2. Outline • Shocks are formed during structure formation in merging clusters of galaxies • Nonthermal particle and photon production: • radio, UV, X-ray, g-ray emission • Contributions to unidentified EGRET g-ray sources and diffuse background radiations • UHECR acceleration • Nonthermal probe of CDM distribution

  3. Structure Formation • Density fluctuations cause region to collapse. • Magnitude of the density fluctuation determines the formation time • Larger structures form by accreting smaller clumps--hierarchical merging • Lumpy, continuous accretion • Mergers (virialized clumps) Major mergers (disrupts internal dynamics of dominant cluster; >~ 60% relative masses) Minor mergers (~10-60%) Accretion processes (<10%) (Fujita and Sarazin 2001) B. Moore, www.nbody.net

  4. Rich Clusters Contain thousands of Galaxies Masses ~1015 Msun Poor Clusters Contain hundreds of Galaxies Masses ~1014 MSun Physical Properties of Galaxy Clusters Coma MKW4

  5. Regular and Irregular Clusters Berrington, Lugger, and Cohn 2002 • Measure velocity distributions • Subcluster structure in Abell 2256

  6. Galaxy Clusters at X-ray Energies • Hot gas ~5-15% of total mass of cluster • Hot ICM emits in X-ray via thermal bremsstrahlung (free-free) • Rich clusters: ~ 5-10 keV • Poor clusters: ~1-5 keV • Lx ~ 1043-1045 ergs s-1 • Tx ~ 2-10 keV

  7. b-model density distributions • Inferred from surface brightness density profile • Profile of bulge/elliptical galaxy and large-scale galaxy cluster structure • Spherically symmetric, isothermal beta-model profile:

  8. Model Cluster (Bode et al. 1994) • N-body simulation • CDM cluster and individual galaxy halo King-model distributions • 80000 particles per cluster; 40000 in DM halo; 40000 in galaxies of which 10% in normal matter

  9. The Physics of Cluster Mergers • Total gravitational energy available:

  10. Cluster Merger Simulation • Zero impact parameter • Cluster galaxies evolutionary • History: morphology of disturbed system (e.g., cD galaxies ) • Merger rate of supermassive black holes • Merger trees

  11. Semi-Analytic Cluster Dynamics • Cluster infall velocity • Assume matter follows an isothermal  or NFW model • Calculate merging cluster infall velocity and center-of-mass position by solving: M(r) is the mass interior to radius r, m is subcluster mass. For b-model:

  12. Merging Speeds

  13. Shocks in Merging Clusters • Shocks form in ICM at boundary • if vs exceeds sound speed of ICM • Thermal particles swept up in shock • Accelerated via first-order Fermi • Sizes: ~1 Mpc are possible • May also reaccelerate particles • Head-tail radio galaxies nearby

  14. Hydrodynamical Simulations • Show development of shocks both before and after center-of-mass passage. Ricker and Sarazin (ApJ, 561, 621, 2001)

  15. Cluster Radio Emission • Nonthermal Synchrotron radiation • Luminosity: ~1040-1042 ergs s-1 • Steep spectra • Radio galaxies • Sources of extended radio emission • Radio Halos • No optical counterpart; mimic X-ray morphology • Typically with sizes ~1 Mpc • Occur in most massive and X-ray luminous rich clusters • Unpolarized • Radio Relics  Peripheral Halos • Irregularly shaped; cluster periphery • Linearly polarized from shock compression • Found in clusters with evidence of a recent or ongoing merger Coma A2256

  16. Nonthermal Cluster Emissions • Nonthermal X-ray Emission • B-SAX, RXTE obs. of A1656 (Coma), A2256, A3667 (~1043-1044 ergs s-1) • Compton-scattered CMB • Nonthermal UV Emission • EUVE observations (~60-250 eV) of Virgo, Coma, Fornax, A2199, … • Either Compton-scattered CMB or thermal tail emission • > 100 MeV g-ray Emission • Association with unidentified EGRET sources Coma (Fusco-Femiano et al. 1999)

  17. Shock Dynamics • Solve shock jump conditions • Density follows Isothermal  model for both clusters • Dominant cluster rc = 250 kpc, R1=1.5 Mpc,  = 0.75, n0 = 10-3 cm-3 • Merging cluster rc = 150 kpc, R2=0.75 Mpc,  = 0.75, n0 = 10-3 cm-3 • Energy density equal in forward and reverse shock fluid Shocked fluid velocities

  18. Shock Speeds • Forward and reverse Mach numbers: • Compression ratio: •  = 5/3 is the ratio of specific heats for an ideal gas

  19. Fermi Acceleration at Shocks • First-order (shock) more important than second-order (stochastic) in nonrelativistic shocks • Produces power law distribution • Index determined from compression ratio

  20. Evolution of Compression Ratio, Spectral Index Reverse Shock Forward Shock

  21. Nonthermal Particle Evolution • Fokker-Plank equation Coulomb diffusion term Energy loss rate Energy gain rate from stochastic acceleration Source term Catastrophic loss from p-p, p-, and diffusion out of the system

  22. Electrons Synchrotron: Bremsstrahlung: Compton: First-order Coulomb: Protons Coulomb: Physical Processes/Energy Loss Rates • Coulomb diffusion coefficient:

  23. Particle Injection • Power law distribution with exponential cutoff • Occurs only if M  1.0 • Occurs only during lifetime of shock • Normalization • Where e,p is an efficiency factor, and is set to 5%. • Typical values are Etot1063-64 ergs

  24. Maximum Particle Energies • Acceleration time constraint • Energy loss constraint • for electrons • for protons • Size-scale limitation

  25. Maximum Particle Energies B = 0.1 mG

  26. Nuclear Losses • Pion-production event results in a proton loosing 1/3 of its energy. Treat as a loss with time scale • Secondary electrons produced by proton-proton interactions

  27. Photopion Losses • p- interaction result in a proton losing ~1/2 to ~1/5 of its energy. • Treated with a loss timescale • Adopted cross sections are • p (Ep) = 380 mb for 200 MeV < Ep < 500 MeV (K = 0.2) • p (Ep) = 120 mb for Ep > 500 MeV (K = 0.5) (Atoyan and Dermer 2003)

  28. Escape • Particles diffuse away from the host cluster on a time scale • Clusters are storage volumes for < 1018 eV cosmic rays Rcl is the radius of the cluster, and Bohm is Bohm diffusion coefficient Larmor radius: (Berezinsky, Blasi,Ptuskin 1997)

  29. Particle Energy Spectra zi=0.3; B=1.0 mG

  30. with metric Redshift Evolution and Energy Loss • (0, R, ) (mass, curvature, and dark energy) • We adopt (0.3, 0.0, 0.7) • Redshift of cluster: • Total Energy Dependence • Thermal Bremsstrahlung redshift dependence • Cosmic Microwave Background (CMBR) dependence • UCMBR(z) = UCMBR(z=0) (1 + z)4 • UCMBR(z=0) = 2.5 x 10-7 MeV cm-3 • Particle energy loss processes

  31. Particle Energy-Loss Timescale n=10-3 cm-3; B=1.0 mG n=10-6 cm-3; B=0.1 mG

  32. Nonthermal Photon Spectra zi=0.3, z=0.22 z=0

  33. Nonthermal Photon Spectra B = 1.0G

  34. Nonthermal Photon Spectra B = 0.1G

  35. Nonthermal Particle Luminosity Evolution B= 1 mG zi=0.3 B= 0.1 mG Expect to detect tens to hundreds of galaxy clusters with LOFAR

  36. Minimum Spectral Index

  37. Unidentified EGRET sources? • Statistical test associating Abell clusters with unidentified EGRET sources. (Colafrancesco 2001; Kitayama and Totani 2002) • Previous studies have assumed an unusually hard particle distribution • If we assume softer spectra—power law slopes ~2.4 • Need luminosities >>1043 ergs s-1 for >100 MeV at a distance of ~100 Mpc

  38. Detection with GLAST • Expect several to tens of clusters of galaxies to be observed with GLAST

  39. Diffuse Extragalactic -ray Background • Structure formation shocks could produce 50% of g-ray background (Loeb and Waxman 2000) • > 10 MeV power-law with slope: 2.100.03 • Central cusps in dark matter profiles will harden spectral indices • Supported by numerical simulations, and gravitational lensing • Observational evidence questionable Sreekumar et al. 1998

  40. Photon Spectra when Nonthermal Particles are Hadronically Dominated

  41. NFW Profile • Cuspy central density profile

  42. Effect of NFW profile • If CDM distribution is traced by normal matter distribution, clusters of galaxies are • weak and soft -ray sources • make small (~1%) contribution to diffuse extragalactic -ray background • do not accelerate particles to ultra-high (> 1019 eV) energies • If CDM distribution is described by NFW profile, clusters of galaxies are • Slightly harder -ray sources • Could make stronger contribution to diffuse extragalactic -ray background

  43. Nonthermal Particle Pressure • Each cluster merger event will contribute a population of nonthermal cosmic rays • Most clusters experience several mergers • Proton distribution is cumulative • Applies pressure in addition to the thermal gas of the system • Nonthermal proton Coulomb processes provide additional heating source

  44. Nonthermal Emission from Cluster Merger Shocks: Summary • Unidentified EGRET sources • Diffuse Extragalactic -ray Background • Nonthermal Particle Pressure • Detectability with GLAST and LOFAR • Should be able to detect these features with the next generation of -ray observatories • Possible indicator of the dark matter profiles • Not a dominant contributor to the Diffuse Extragalactic -ray Background • Will significantly alter thermal X-ray emission

  45. Backup Slides

  46. Structure Formation • Density fluctuations cause region to collapse. • Magnitude of the density fluctuation determines the formation time • Larger structures form by accreting smaller clumps--hierarchical merging • Lumpy, continuous accretion

  47. Press-Schecter Formalism • Gives mass distribution of galaxy clusters assuming linear perturbation theory • Structure is built up by hierarchical merging • Fits computer models of structure formation

  48. Structure Formation • Lacey-Cole (LC) function • Gives rate of mergers versus redshift and accreting body mass • Similar to PS formalism

  49. Lacey-Cole Formalism • Probability of Mergers of Structures with different masses with cosmic time

  50. External Pressure Accretion shock Nonthermal Particle Pressure • Accretion vs. Cluster merger shocks • Accretion acts as an external pressure Infalling matter

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