Non-Thermal Radiation from Intergalactic ShocksUri Keshet1, Eli Waxman1, Abraham Loeb2, Volker Springel3, and Lars Hernquist2

1) Weizmann Institute of Science; 2) Harvard University; 3) Max Planck Institute for Astrophysics

1. “Cosmic γ-ray background from structure formation in the intergalactic medium”, Loeb and Waxman 2000, Nature, 405, 156-1582. “Fluctuations in the radio background from intergalactic synchrotron emission”, Waxman and Loeb 2000, The Astrophysical Journal, 545, L11-L143. “Gamma rays from intergalactic shocks”; Keshet, Waxman, Loeb, Springel and Hernquist 2003, The Astrophysical Journal, 585, 128-1504. “The case for a low extragalactic gamma-ray background”; Keshet, Waxman and Loeb 2004, Journal of Cosmology and Astrophysics, 04 (006)5. “Imprint of Intergalactic shocks on the radio sky”; Keshet, Waxman and Loeb 2004, submitted to ApJ [astro-ph/0402320]6. “A statistical detection of γ-ray emission from galaxy clusters…”, Scharf & Mukherjee 2002, The Astrophysical Journal 580, 154-1637. “EGRET observations of the extragalactic gamma-ray emission”, Sreekumar et al. 1998, The Astrophysical Journal 494, 523-534

Approximations:1) Large scale structure (LSS) composed of halos2) Halo mass distribution: isothermal sphere3) Strong shocks only

CONCEPT

ANALYTICAL MODEL

TREE-SPH COSMOLOGICAL SIMULATION

BIBLIOGRAPHY

Max Planck Institute Max Planck Institute for Astrophysicsfor Astrophysics

Converging flows during structure formation (SF)

Electron acceleration

Inverse-Compton (of CMB photons) and synchrotron emission

Collisionless shocks

Intergalactic shocks emit radiation in the following process:

M,T

rsh

Me-e-

γ

γ

γ

γ

Halo Dimensional Analysis:

Halo Parameters:M - massT - temperaturersh - shock radius

- mass accretion rateM

)(

),(

5

2 3

zGH

zMM

facc G

zMzMM

3),(),(

fT2),(),( zMmzMTk pB

fr)(

),(

5

2),(

zH

zMzMrsh

facc

fT

fr

unknown dimensionless

parameters

Relativistic Electron Distribution

Strong shocks2d

dne

2

/

sh

Lacc

crt

HtT

B

yr102 1keV

7

74

1

34

eCMBT

ecool u

cmt

yr102.1 1

20010

2/1

77

max 1.010103.3

G

B

K

T

),( zMLsyn ),(8),( 2

zMLu

zMB IC

cmb

),(),(2

3zMTkzMN Bb

maxln2

1

e),( zMLIC

energy cutoff γmax:cooling limited

121 srcmskeV05.0/6.1 eTacciC ffI

1212212 srcmserg01.0/05.0/105 BerTaccsyn fffI

syn

log 10

M/M

⊙

z

IC

z

log 10

M/M

⊙

Emitted Radiation:

electron energy ξe≈5% (2.5%-7.5%)

magnetic energy ξB≈1% (0.05%-2%)

Parameterization (no complete model):

halo luminosity:

Integration over halo abundance:

(images: the integrands in the mass-redshift plane)

% out of shock thermal energy

Cosmological modelΩΛ vacuum energy 0.7Ωdm dark matter energy 0.26Ωb baryon energy 0.04h Hubble coefficient 0.67n fluctuation power 1σ8 fluctuation normalization 0.9

Simulation parametersNb number of SPH particles 2243

Ndm # of dark matter particles 2243

L simulation box size 200 MpcMSPH SPH particle mass 3.6 10ⅹ 11M⊙

Mres mass resolution ~1011M⊙

Z0 initial redshift 50

Cooling is inefficient in the relevant regions (panel 1) Adiabatic simulations suffice Entropy changes (panel 2) of SPH particles trace the shocks (panel 4)

SHOCK IDENTIFICATION

n[cm-3]

Simulated sky

Δθ=42’

1-2-1-

6

sr cm s

101.1 J

Δθ=12’

1-2-1-

9

sr cm s

102.8 J

INTEGRATED EMISSION

South

North

Previous estimate05.045.1 XI

1215 srcms ph10

:units MeV,100(Sreekumar et al. 1998)

C.L.%99@5.0XISouth

North

70% syn. tracer (22 GHz)30% gas tracer (21 cm)

EXTRAGALACTIC GAMMA-RAY BACKGROUND

GAMMA-RAY SIGNAL

n[cm-3]

MODEL CALIBRATION

CMB

Bremsstrahlung from Lyα clouds

IGM shocksGalactic synchrotron ℓ∝ -1/2

discrete sources ℓ∝

IGM fluctuation dominate on 1’-00.5 scales

~ const for 400<ℓ<4000

21 cm tomography

MHz

RADIO SKY

very dense, beyond shocks

Panel 1: phase space with tcool/tH contours

already cold

Translates Δ

S* to M

ach number M

Mpc

Mpc

Mpc

Panel 3: density slice (100x100x10 Mpc)

Panel 4: same density slice as shown in panel 3, but including the M>4 shocked particles only

M

103

102

101

104

100

10-1

10-2

10-3

10-4M=4

dN/dΔS*

∫dN/dΔS*

100 105Panel 2: histogram of entropy change ΔS*≡ ΔS/CV accumulated by SPH particles in the epoch 0<z<2

Particles with ΔS* above the cutoff trace all shocks of Mach number M>4

Shock fronts may be identified

>100 MeV

>10 GeV log10T

J/<J> Conclusions:

• >10 sources well resolved by GLAST (for ξe≈0.05)

• γ-ray clusters: targets for MAGIC, HESS, VERITAS

• γ-ray morphology: accretion rings with bright emission at filament intersections

J/<J>

Mpc

deg

deg Mpc

Model halo parameters (column 1) are calibrated with various features of the simulation (column 2). Agreement between the radiation fields extracted from the simulation and from the calibrated model (column 3) provides an independent check of the calibration scheme.

Param. Calibrated using Value (range)fT Mass average temperature: <T(z)>M ≈ 4X106e-0.9z K 0.5 (0.45-0.55)

facc Mass fraction processed by strong shocks, e.g. f(z<2) ≈ 41% 0.12 (0.08-0.17)

fr Typical size of bright emitting region (e.g. 2 Mpc for 1015M⊙) 0.1 (0.05-0.2)

ε2 dJ/

dε[k

eV s

-1 c

m-2 s

r-1]

10-3

10-2

10-1

100

keV GeVε[eV]

MeV

J ∝ ξesimulation

J ∝ ξe

J ∝ ξe facc fT

EGRET

F[ph s-1 cm-2]

N(>

F)

GLAST EGRETξe=5%non-calibrated

calibrated

Spectrum Source Number Count

non-calibrated

Conclusions:

• IGM ≥ 10% of EGRB flux

• > a dozen GLAST sources for ξe>0.03

• Cross-correlation with LSS (e.g. Scharf & Mukherjee 2002)

calibrated modelsimulation

CM

B

Galacticsynchrotron

IGM shocks

total

ERB estimates

ν [MHz]IGM shocks: significant fraction of the ERB

Radio sky brightness Intensity fluctuations δIℓ on 00.5 scales

modeled EGRB [ref 7] 408 MHz Previous EGRB estimates are:

• Non isotropic on large scale

• Correlated with Galactic tracers

• High (≈ total polar intensity)

EGRET >100 MeVFit EGRET latitude profile as a sum of 2 components; one linear in a Galactic gas tracer and the other linear in a synchrotron tracer:

Results:

Robust EGRB flux upper limit

(~1/3 of previous estimates)

CONCLUSIONS AND IMPLICATIONSConclusions

• γ-ray sources detectable by GLAST and Ćerenkov detectors

• Signal fluctuations dominate the radio sky on ~1’-0.50 scales

• Indirect detection, e.g. cross correlations with LSS tracers

• Extragalactic backgrounds: EGRB low, ERB unknown

• Calibrated analytical model, fast shock identification in SPH

Implications of signal detection

• First identification of intergalactic shocks

• Reconstruction of large-scale flows

• Tracer of warm-hot IGM (WHIM)

• Probe of intergalactic magnetic fields

ξe=5%

SNR observations

Bcluster ≈ 0.1μG

2

2

2

),()(

rG

zMr

CM 200)(

Cshr 3

50)(

isothermal sphere:

velocity dispersion

Hubble’s coefficient

effective mass

The emission from strong shocks dominates the radiation from the periphery of galaxy clusters and from galaxy filaments; traces LSS

e.g. according to the Press-Schechter halo mass function