Cosmic -ray Background from Dark Matter Annihilation in the Large-s

cale StructureEiichiro KomatsuUniversity of Texas at AustinCosmology Seminar@MPAAugust 21, 2007

K. Ahn & EK, PRD, 71, 021303R (2005); 72, 061301R (2005)S. Ando & EK, PRD, 73, 023521 (2006)S. Ando, EK, T. Narumoto & T. Totani, MNRAS, 376, 1635 (2007)S. Ando, EK, T. Narumoto & T. Totani, PRD, 75, 063519 (2007)

What Is Out There?WMAP 94GHz

What Is Out There?

Deciphering Gamma-ray Sky AstrophysicalAstrophysical: Galactic vs Extra-galactic

Galactic origin (diffuse)• E.g., Decay of neutral pions produced by cosmic-rays interacting with th

e interstellar medium. Extra-galactic origin (discrete sources)

• Active Galactic Nuclei (AGNs)• Blazars• Gamma-ray bursts

Exotic: Galactic vs Extra-galactic Galactic Origin

• Dark matter annihilation in the Galactic Center• Dark matter annihilation in the sub-halos within the Galaxy

Extra-galactic Origin• Dark matter annihilation in the other galaxies

Blazars Blazars = A population of AGNs whose relativistic j

ets are directed towards us. Inverse Compton scattering of relativistic particles in jet

s off photons -> gamma-rays, detected up to TeV

How many are there? EGRET found ~60 blazars (out of ~100 identified source

s) GLAST is expected to find thousands of blazars.

• GLAST’s point source sensitivity (>0.1GeV) is 2 x 10-9 cm-2 s-1

• AMS-2’s equivalent (>0.1GeV) point source sensitivity is about 10 times larger, ~ 10-8 cm-2 s-1 (G. Lamanna 2002)

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Blazar Luminosity Function Update

Luminosity-Dependent Density Evolution (LDDE) model fits the EGRET counts very well. This model has been derived from

X-ray AGN observations, including the soft X-ray backgroundCorrelation between blazars and radio sourcesLDDE predicts that GLAST should detect ~3000 blazars.This implies that AMS-2 would detect a few hundred blazars.

Narumoto & Totani, ApJ, 643, 81 (2006)

LDDE

Redshift distribution of blazars that would be detected by GLAST

•LDDE1: The best-fitting model, which accounts for ~1/4 of the gamma-ray background.

•LDDE2: A more aggressive model that accounts for 100% of the gamma-ray background.

•It is assumed that blazars are brighter than 1041 erg/s at 0.1 GeV.Ando et al. (2007)

-ray Background Un-resolved Blazars that are b

elow the point-source sensitivity will contribute to the diffuse diffuse backgroundbackground.

EGRET has measured the diffuse background above the Galactic plane.

LDDE predicts that only ~1/4 of the diffuse light is due to blazars! AMS-2 will do MUCH better than

EGRET in the diffuse background

(G. Lamanna 2002)

Ando et al. (2007)

Dark matter (WIMP) annihilation

WIMP dark matter annihilates into gamma-ray photons.

The dominant mode: jets

•Branching ratios for line emission (two gamma & gamma+Z0) are small.WIMP mass is likely

around GeV–TeV, if WIMP is neutralino-like.

Can GLAST or AMS-2 see this?

GeV-γ

Ando et al. (2007)

DM Annihilation in MWDiemand, Khlen & Madau, ApJ, 657, 262 (2007)

•Simulated map of gamma-ray flux by Diemand et al., as seen from 8kpc away from the center.

Why MW? Look Outside!WIMP dark matter particles

are annihilating everywhere.Why focus only on MW? There are so many dark matter halos in the universe.We can’t see them

individually, but we can see them as the background light.

We might have seen this already in the background light: the real question is, “how can we tell, for sure, that the signal is indeed coming from dark matter?”

Gamma-ray Anisotropy

Dark matter halos trace the large-scale Dark matter halos trace the large-scale structure of the universe.structure of the universe.

The distribution of gamma-rays from these sources mustmust be inhomogeneous, with a well defined angular power spectrumangular power spectrum.

If dark matter annihilation contributes >30%, it should be detectable by GLAST in anisotropy.A smoking gun for dark matter annihilation. It would be very interesting to study if AMS-2 would

be able to detect anisotropy signal --- remember, the mean intensity will be measured by AMS-2 very well!

Ando & EK (2006); Ando, EK, Narumoto & Totani (2007)

“WMAP” for Gamma-rays? WMAP 94GHz

Why Anisotropy? The shape of the power spectrum is determined by the

structure formation, which is well known. Schematically, we have:

((Anisotropy in Gamma-ray SkyAnisotropy in Gamma-ray Sky))= (= (MEAN INTENSITYMEAN INTENSITY) x ) x

The mean intensity depends on particle physics: annihilation cross-section and dark matter mass.

The fluctuation power, , depends on structure formation. The hardest part is the prediction for the mean intensity.

However… Remember that the mean intensity has been measured already! The prediction for anisotropy is robust. All we need is a

fraction of the mean intensity that is due to DM annihilation.

Blazars account for ~1/4 of the mean intensity. What about dark matter annihilation?

A Simple Route to the Angular Power Spectrum

To compute the power spectrum of anisotropy from dark matter annihilation, we need three ingredients:

1. Number of halos as a function of mass,

2. Clustering of dark matter halos, and

3. Substructure inside of each halo.

θ (= π / l)

Dark matter halo

Astrophysical Background: Anisotropy from Blazars Blazars also trace the large-scale structure.

The observed anisotropy may be described as the sum of blazars and dark matter annihilation.

Again, three ingredients are necessary:1. Luminosity function of blazars,2. Clustering of dark matter halos, and3. “Bias” of blazars: the extent to which blazars trace the und

erlying matter distribution.• This turns out to be unimportant (next slide)

Is the blazar power spectrum different sufficiently from the dark matter annihilation power spectrum?

Predicted Power SpectrumAndo, Komatsu, Narumoto & Totani (2007)

At 10 GeV for 2-yr observations of GLAST

Blazars (red curves) easily discriminated from the DM signal --- the blazar power spectrum is nearly Poissonian.

The error blows up at small angular scales due to angular resolution (~0.1 deg) & blazar contribution.

39% DM 61% DM

80% DM 97% DM

What If Substructures Were Disrupted…

39% DM 61% DM

97% DM80% DM

• S/N goes down as more subhalos are disrupted in massive parent halos.

• In this particular example, the number of subhalos per halo is proportinal to M0.7, where M is the parent halo mass.

• If no disruption occurred, the number of subhalos per halo should be proportional to M.

“No Substructure” or “Smooth Halo” Limit

39% DM 61% DM

97% DM80% DM

Our Best Estimate:Our Best Estimate:

“If dark matter annihilation contributes > 30% of the mean intensity, GLAST should be able to detect anisotropy.”

• A similar analysis can be done for AMS-2.

Positron-electron Annihilation in the Galactic Center

Jean et al. (2003); Knoedlseder et al. (2005);Weidenspointner et al. (2006)

INTEGRAL/SPI has detected a significant line emission at 511 keV from the G.C. Extended over the bulge -- inconsi

stent with a point source! Flux ~ 10-3 ph cm-2 s-1

Continuum emission indicates that more than 90% of annihilation takes place in positronium.

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INTEGRAL/SPI Spectrum Ortho-positronium co

ntinuum is clearly seen (blue line)

Best-fit positronium fraction = (96 +- 4)%

Where do these positrons come from?

Churazov et al. (2005)

Light Dark Matter Annihilation Light (~MeV) dark matter particles can produce non-re

lativistic positrons, which would produce line emission at 511keV. The required (S-wave) annihilation cross section (~a few x 10-26 cm3 s-1) is indeed reasonable! Boehm et al., PRL, 92, 101301 (2004) Hooper et al., PRL, 93, 161302 (2004)

The fact that we see a line sets an upper limit on the positron initial energy of ~3 MeV. Beacom & Yuksel, PRL, 97, 071102 (2006)

Continuum gamma-ray is also produced via the “internal bremsstrahlung”, XX -> e+e- Beacom, Bell & Bertone, PRL, 94, 171301 (2005)

How about the extra-galactic background light?How about the extra-galactic background light?

AGNs, Supernovae, and Dark Matter Annihilation…

The extra-galactic background in 1-20MeV region is a superposition of AGNs, SNe, and possibly DM annihilation.

SNe cannot explain the background.

AGNs cut off at ~1MeV. ~20 MeV DM fits the d

ata very well.

Ahn & EK, PRD, 71, 021303R; 71, 121301R; 72, 061301R (05)

COMPTEL

SMM

HEAO-1

AGNs

SNe

DM

SummaryConvincing evidence for gamma-rays from DM will have a huge impact on particle physics and cosmology.

The Galactic Center may not be the best place to look. The extrThe extra-galactic gamma-ray backgrounda-galactic gamma-ray background, which has been measured by EGRET and will be measured more precisely by AMS-2 and GLAST, may hold the key.

The mean intensity is not enough: the power spectrum of cosmic gamma-rthe power spectrum of cosmic gamma-ray anisotropy is a very powerful probeay anisotropy is a very powerful probe.If >30% of the mean intensity comes from dark matter annihilation (at 10 GeV), GLAST will detect it in two years. Prospects for detecting it in AMS-2 data remain to be seen.A possibility of MeV dark matter is very intriguing.