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Using the  -ray Background as a Path to Mapping Dark Matter Clustering in the Universe . Eiichiro Komatsu University of Texas at Austin PPC07@Texas A&M, May 18, 2007. K. Ahn & EK, PRD, 71, 021303R (2005); 72, 061301R (2005) S. Ando & EK, PRD, 73, 023521 (2006)

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using the ray background as a path to mapping dark matter clustering in the universe

Using the -ray Background as a Path to Mapping Dark Matter Clustering in the Universe

Eiichiro Komatsu

University of Texas at Austin

PPC07@Texas A&M, May 18, 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)

deciphering gamma ray sky
Deciphering Gamma-ray Sky
  • Astrophysical: Galactic vs Extra-galactic
    • Galactic origin (diffuse)
      • E.g., Decay of neutral pions produced by cosmic-rays interacting with the 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 = A population of AGNs whose relativistic jets are directed towards us.
    • Inverse Compton scattering of relativistic particles in jets off photons -> gamma-rays, detected up to TeV
  • How many are there?
    • EGRET found ~60 blazars (out of ~100 identified sources)
    • 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)
blazar luminosity function update

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

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 background
    • Correlation between blazars and radio sources
  • LDDE predicts that GLAST should detect ~3000 blazars.
    • This implies that AMS-2 would detect a few hundred blazars.


redshift distribution of blazars that would be detected by glast
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
-ray Background
  • Un-resolved Blazars that are below the point-source sensitivity will contribute to the diffuse background.
  • 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

Ando et al. (2007)

(G. Lamanna 2002)

dark matter wimp annihilation
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?

Ando et al. (2007)

dm annihilation in mw

Diemand, Khlen & Madau, ApJ, 657, 262 (2007)

DM Annihilation in MW
  • Simulated map of gamma-ray flux by Diemand et al., as seen from 8kpc away from the center.
why mw look outside
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

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

Gamma-ray Anisotropy
  • Dark matter halos trace the large-scale structure of the universe.
  • The distribution of gamma-rays from these sources must be inhomogeneous, with a well defined angular 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!
why anisotropy
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 Sky)


    • 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
A Simple Route to the Angular Power Spectrum

Dark matter halo

  • To compute the power spectrum of anisotropy from dark matter annihilation, we need three ingredients:
    • Number of halos as a function of mass,
    • Clustering of dark matter halos, and
    • Substructure inside of each halo.

θ (= π / l)

astrophysical background anisotropy from blazars
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:
    • Luminosity function of blazars,
    • Clustering of dark matter halos, and
    • “Bias” of blazars: the extent to which blazars trace the underlying 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 spectrum

Ando, Komatsu, Narumoto & Totani (2007)

Predicted Power Spectrum

39% DM

61% DM

  • 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.

80% DM

97% DM

what if substructures were disrupted
What If Substructures Were Disrupted…

39% DM

61% 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.

80% DM

97% DM

no substructure or smooth halo limit
“No Substructure” or “Smooth Halo” Limit

39% DM

61% DM

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.

80% DM

97% DM

positron electron annihilation in the galactic center

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

Positron-electron Annihilation in the Galactic Center
  • INTEGRAL/SPI has detected a significant line emission at 511 keV from the G.C.
    • Extended over the bulge -- inconsistent 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.
integral spi spectrum

Churazov et al. (2005)

  • Ortho-positronium continuum is clearly seen (blue line)
  • Best-fit positronium fraction = (96 +- 4)%
  • Where do these positrons come from?
light dark matter annihilation
Light Dark Matter Annihilation
  • Light (~MeV) dark matter particles can produce non-relativistic 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?
agns supernovae and dark matter annihilation

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

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 data very well.







  • Convincing 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 extra-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-ray 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.