The Origin and Acceleration of Cosmic Rays in Clusters of Galaxies HWANG, Chorng-Yuan 黃崇源 Graduate Institute of Astronomy NCU Taiwan

Outline  Clusters of Galaxies  Cosmic-Ray Electrons in Clusters  Conventional Sources of CRs  Cosmic rays from dark matter  Models and Results  Summary

Clusters of Galaxies  Largest gravitational bound systems in the Universe  Thousands of galaxiesgalaxies  Collapse of primordial density peaks  Mass ~ solar mass  Baryon mass ~ 10% (galaxies and ICM)ICM  mostly dark matter of unknown nature  All might contribute to CRs

Clusters in Optical

Hot Intra-cluster Medium Temperature ~ 10 8 K n e ~ cm -3 Mass ~ solar mass Thermal X-ray emission ~ erg s -1 Energy ~ erg Cooling time ~ s Coma (Chandra)

Evidences of Non-thermal Energy in Galaxy Clusters 4 Radio halos and relicshalos –Cosmic rays and magnetic fields 4 Radio Bubbles in X-ray ImagesBubbles –Interaction of cosmic rays and magnetic field with hot ICM –Non-thermal energy is important 4 Magnetic fields: 5-10  G from Faraday rotation measurements (e.g. Clarke 2001)

Radio Halo and Relic of Coma (Feretti 2003)

Mini Radio Halo in Perseus (Gitti 2003)

Other Evidences of Cosmic Rays in Galaxy Clusters 4 Hard X-ray Excess Emission (?) Hard X-ray –IC scattered of CMB by  ~ 10 4 electrons –  Bremsstrahlung of supra-thermal electrons (?X)X –  Point sources (?) 4 EUV and Soft X-ray Excess Emission (?) EUV –IC scattered of CMB by  ~ 300 electrons –Only Coma and Virgo Clusters –Other SXE sources are correlated with SXB and must be wrong (Bregman & Lloyd-Davies 2006) 4 Evidences of CRs from HXR/EUV Excess in clusters are not indisputable.

Hard X-ray Excess of Coma (Fusco-Femiano 2003)

SZ effect caused by superathermal model for hard X-ray excess

EUV Excess of Virgo (Berghöfer 2003)

Conventional Sources of CRs 4 Shocks during the formation and evolution of Clusters –Accretion –Mergers 4 Stars: –Normal and starburst galaxies 4 Massive black holes: –Radio galaxies, –Jets of AGNs

Origins of CR Electrons 4 Observationally, we only see CR electrons 4 Since the CR electrons are short-lived, they must be newly (re-)generated. 4 Primary Electrons –Injected from conventional sources: –(Re-)accelerated by shocks 4 Secondary Electrons –Pion decays –Knockon electrons

Problems of CR Electrons 4 Scale size of radio halos >> V diffusion  t life –Large-scale sources or re-acceleration 4 The magnetic fields –derived from ICS for EUV/hard X-ray excess ~ 0.4  G –observed with Faraday rotation ~ a few  G 4 Life time of radio halos/relics? 4 Primary or Secondary?

Re-acceleration Models 4 CR electrons are injected by the merger shocks and re-accelerated by ensuing violent turbulence. 4 HXR are ICS of the CMB photons. 4 Try to fit the spectral index distribution. 4 High magnetic fields 4 HXR emission is mainly from low filed regions

Reacceleration Model for Coma (Kuo, Hwang, Ip 2003)

Properties EUV emission 4 CR electrons of the IC EUV:  ~ I EUV  I X-ray 4 EUV emission from Coma might be due to secondary electrons (Bowyer et al 2004)

A Secondary Model 4 Charged pion decays and knockon electrons 4 Cooling mechanisms: synchrotron, ICS of CMB, ionization & bremsstrahlung. 4 Steady state 4 Magnetic Fields ~ 5  G 4 Observed beta model for thermal protons 4 CR proton density?

Assumption of CR protons 4 n CRp   CRp -p 4 p=2.5 and min(  CRp ) ~ 2 4 Total energy density of CR protons: –~ thermal energy density –~ 1% of thermal energy density ( ~5  G ) –~ 0.01% of thermal energy density (~0.4  G )

B=5  G, CR Energy density = 1, 0.01, thermal energy density, EUV

The Cooling Time for EUV electrons are long! (B=5  G )

One big injection followed by continuum small injections of cosmic-ray electrons can fit the observed EUV and radio data (Tsay, Hwang, Bowyer 2002).

Results for cosmic-ray electrons from conventional sources 4 Successful re-acceleration models of primary electrons for radio/HXE/EUV. 4 EUV-CR electrons might be relic CR electrons and are independent from radio-CR electrons. 4 Secondary models for the EUV emission will overproduce the radio emission. 4 For B=5  G the energy density of CR protons must be less than 1% of the thermal energy density in order not to avoid overproducing the radio emission.

DM origins for Cosmic Rays 4 What is dark matter? 4 A viable candidate for the DM is the Weakly Interacting Massive Particles (WIMPs). 4 The most favorable WIMP for DM is the neutralino predicated in the supersymmetric extension of the standard model.

Neutralino  4 A linear combination of two neutral higgsinos and two gauginos. –  =  B +  W +  H 1 +  H 2 4 The most likely mass of  is between ~ 50 GeV to 1 TeV 4 Annihilation of  will decay into fermion pairs or gauge boson pairs and will finally become electrons or positrons. 4 Is the resulting relativistic electrons observable?

 as the Dark Matter 4 If  is the relic particle from the hot big bang and constitute the DM, then 4  m h 2 =   h 2 = 3  cm 2 s -1 / 4 From WMAP,  m h 2 =0.127, we can fix = 2.36  cm 2 s -1 4 We can estimate the resulting electrons and compare with observations of radio halos in galaxy clusters.

Models for Radio Halo Emission from Dark Matter 4 Select several massive clusters with measured B field (5-10  G) 4 assume B=5  G and steady state 4 NFW profile 4 = 2.36  cm 2 s -1 4 m  =50 GeV - 1 TeV 4 n  = cluster mass/volume/m  4 Production rate  n  2

Cluster Sample 4 Coma (NCF, halo) 4 A754 (NCF, halo) 4 A85 (CF, relic) 4 A119 (no radio emission)

Source functions for 1TeV , solid line for fermion channels and dashed line for boson channels (Coma)

Equilibrium electron spectra in cluster halos from the annihilation of 1TeV  (Coma)

Radio power in cluster halos from the annihilation of 100GeV  (Coma)

Radio power in cluster halos from the annihilation of 1TeV  (Coma)

Radio halo flux of Coma compared with radio flux from the annihilation of 100GeV 

Radio halo flux of Abell 754 compared with the radio flux from the annihilation of 100GeV 

Radio relic flux of Abell 85 compared with the radio flux from the annihilation of 100GeV 

Radio flux of Coma compared with the theoretical flux of Abell 119 from the annihilation of 100GeV 

Radio flux of Coma compared with the theoretical flux of Abell 119 from the annihilation of 1TeV 

Results for DM CRs 4 The predicted radio halo emission from the neutralinos annihilation should be detectable. 4 The non-detection of radio halos for some massive clusters with high magnetic fields can be used to constrain the composition and mass of the DM neutralinos.

Thank you!