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 GalaxiesCosmic-Ray Electrons in ClustersConventional Sources of CRsCosmic rays from dark matterModels and ResultsSummary

Clusters of Galaxies

Largest gravitational bound systems in the Universe

Thousands of galaxiesCollapse of primordial density peaksMass ~ 1015 solar mass Baryon mass ~ 10% (galaxies and ICM)mostly dark matter of unknown natureAll might contribute to CRs

Clusters in Optical

Hot Intra-cluster Medium

Temperature ~ 108 K

ne ~ 10-3 cm-3

Mass ~ 1014 solar mass

Thermal X-ray emission ~ 1044 erg s-1

Energy ~ 1062 erg

Cooling time ~ 1018 s Coma (Chandra)

Evidences of Non-thermal Energy in Galaxy Clusters

Radio halos and relics – Cosmic rays and magnetic fields

Radio Bubbles in X-ray Images– Interaction of cosmic rays and magnetic field

with hot ICM– Non-thermal energy is important

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

Hard X-ray Excess Emission (?)– IC scattered of CMB by ~ 104 electrons Bremsstrahlung of supra-thermal electrons (?X) Point sources (?)

EUV and Soft X-ray Excess Emission (?)– 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) 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

Shocks during the formation and evolution of Clusters– Accretion– Mergers

Stars: – Normal and starburst galaxies

Massive black holes: – Radio galaxies, – Jets of AGNs

Origins of CR Electrons

Observationally, we only see CR electrons Since the CR electrons are short-lived, they

must be newly (re-)generated. Primary Electrons

– Injected from conventional sources:– (Re-)accelerated by shocks

Secondary Electrons– Pion decays– Knockon electrons

Problems of CR Electrons

Scale size of radio halos >> Vdiffusion tlife

– Large-scale sources or re-acceleration

The magnetic fields– derived from ICS for EUV/hard X-ray

excess ~ 0.4 G– observed with Faraday rotation ~ a few G

Life time of radio halos/relics? Primary or Secondary?

Re-acceleration Models

CR electrons are injected by the merger shocks and re-accelerated by ensuing violent turbulence.

HXR are ICS of the CMB photons. Try to fit the spectral index distribution. High magnetic fields HXR emission is mainly from low filed

regions

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

Properties EUV emission

CR electrons of the IC EUV: ~ 300 IEUV IX-ray

EUV emission from Coma might be due to secondary electrons (Bowyer et al 2004)

A Secondary Model

Charged pion decays and knockon electrons

Cooling mechanisms: synchrotron, ICS of CMB, ionization & bremsstrahlung.

Steady state Magnetic Fields ~ 5 G Observed beta model for thermal protons CR proton density?

Assumption of CR protons

nCRp CRp-p

p=2.5 and min(CRp ) ~ 2

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, 0.0001 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

Successful re-acceleration models of primary electrons for radio/HXE/EUV.

EUV-CR electrons might be relic CR electrons and are independent from radio-CR electrons.

Secondary models for the EUV emission will overproduce the radio emission.

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

What is dark matter? A viable candidate for the DM is the

Weakly Interacting Massive Particles (WIMPs).

The most favorable WIMP for DM is the neutralino predicated in the supersymmetric extension of the standard model.

Neutralino

A linear combination of two neutral higgsinos and two gauginos. = B + W + H1 + H2

The most likely mass of is between ~ 50 GeV to 1 TeV

Annihilation of will decay into fermion pairs or gauge boson pairs and will finally become electrons or positrons.

Is the resulting relativistic electrons observable?

as the Dark Matter

If is the relic particle from the hot big bang and constitute the DM, then

mh2 =h2 = 310-27 cm2 s-1/<v> From WMAP, mh2 =0.127, we can fix

<v> = 2.36 10-26 cm2 s-1

We can estimate the resulting electrons and compare with observations of radio halos in galaxy clusters.

Models for Radio Halo Emission from

Dark Matter

Select several massive clusters with measured B field (5-10 G)

assume B=5 G and steady state NFW profile <v> = 2.36 10-26 cm2 s-1

m =50 GeV - 1 TeV

n = cluster mass/volume/m

Production rate n2 <v>

Cluster Sample

Coma (NCF, halo) A754 (NCF, halo) A85 (CF, relic) 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

The predicted radio halo emission from the neutralinos annihilation should be detectable.

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!