1. Using the -ray Background as a Path to Mapping Dark Matter Clustering in the Universe
Eiichiro Komatsu
University of Texas at Austin
A&M, May 18, 2007
K. Ahn & EK, PRD, 71, R (2005); 72, R (2005)
S. Ando & EK, PRD, 73, (2006)
S. Ando, EK, T. Narumoto & T. Totani, MNRAS, 376, 1635 (2007)
S. Ando, EK, T. Narumoto & T. Totani, PRD, 75, (2007)

2. What Is Out There?
WMAP 94GHz

3. What Is Out There?

4. 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

5. Blazars
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)

6. Blazar Luminosity Function Update
Narumoto & Totani, ApJ, 643, 81 (2006)
LDDE
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.

7. 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)

8. -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)

9. Dark matter (WIMP) annihilation
GeV-γ
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)

11. 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.

13. 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?”

14. 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!

15. “WMAP” for Gamma-rays?
WMAP 94GHz

16. (Anisotropy in Gamma-ray Sky)
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)
= (MEAN INTENSITY) 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?

17. A Simple Route to the Angular Power Spectrum
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.
Dark matter halo
θ (= π / l)

18. 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?

19. Predicted Power Spectrum
Ando, 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

20. What If Substructures Were Disrupted…
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.
39% DM
61% DM
80% DM
97% DM

21. “No Substructure” or “Smooth Halo” Limit
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.
39% DM
61% DM
80% DM
97% DM

22. 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 ~ ph cm-2 s-1
Continuum emission indicates that more than 90% of annihilation takes place in positronium.

23. INTEGRAL/SPI Spectrum
Churazov et al. (2005)
INTEGRAL/SPI Spectrum
Ortho-positronium continuum is clearly seen (blue line)
Best-fit positronium fraction = ( )%
Where do these positrons come from?

24. 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 cm3 s-1) is indeed reasonable!
Boehm et al., PRL, 92, (2004)
Hooper et al., PRL, 93, (2004)
The fact that we see a line sets an upper limit on the positron initial energy of ~3 MeV.
Beacom & Yuksel, PRL, 97, (2006)
Continuum gamma-ray is also produced via the “internal bremsstrahlung”, XX -> e+e-
Beacom, Bell & Bertone, PRL, 94, (2005)
How about the extra-galactic background light?

25. AGNs, Supernovae, and Dark Matter Annihilation…
Ahn & EK, PRD, 71, R; 71, R; 72, R (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.
HEAO-1 DM
AGNs
SMM
COMPTEL
SNe

26. Summary
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.