Hunting for Dark Matter in Anisotropies of Gamma-ray Sky: Theory - - PowerPoint PPT Presentation
Hunting for Dark Matter in Anisotropies of Gamma-ray Sky: Theory and First Observational Results from Fermi-LAT Eiichiro Komatsu (Texas Cosmology Center, Univ. of Texas at Austin) MPA Seminar, September 14, 2011 Motivation How can we see
Eiichiro Komatsu (Texas Cosmology Center, Univ. of Texas at Austin) MPA Seminar, September 14, 2011
dark matter particles?
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the diffuse emission (after removing the known Galactic emission) is unknown!
>90% of the diffuse emission is unknown!
Hoeflich 2005)
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Blazars = A population of AGNs whose relativistic
Inverse Compton scattering of relativistic particles in jets
How many are there? (They are rare.)
EGRET found ~70 blazars (out of ~100 associated
sources) over the full sky
Fermi-LAT found ~570 blazars (out of ~820 associated
sources) over the full sky (LAT 1FGL catalog)
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Fermi-LAT Collaboration, ApJ, 720, 435 (2010) S–2.5 S
S
S–1.6
The integral converges! A convincing detection of a break in dN/dS
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Flux
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all blazars BL Lac Flat-spectrum radio quasars Fermi-LAT Collaboration, ApJ, 720, 435 (2010)
the rest?
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for most of the MeV gamma-ray background. It turns out that the measured supernova rate is too small for that! The origin of the MeV background is unknown. Ahn, Komatsu & Hoeflich (2005)
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number of photons averaged over the sky), and look for spectral features. However, dark matter is not the only source of gamma-ray photons. How can we distinguish between dark matter signatures and astrophysical sources?
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E.g., for supernovae:
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E.g., for dark matter annihilation:
Diemand, Khlen & Madau, ApJ, 657, 262 (2007)
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Dark matter particles are
annihilating (or decaying) everywhere in the Universe!
While we cannot resolve individual
dark matter halos, the collective signals can be detected in the diffuse gamma-ray background.
How can we detect such
signatures unambiguously?
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Dark matter halos trace the large-scale structure Therefore, the gamma-ray background must be
anisotropy must be there.
And, their spatial distribution can be calculated within the
framework of Lambda-CDM model (using analytical calculations or numerical simulations)
Ando & EK (2006); Ando, EK, Narumoto & Totani (2007)
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WMAP 94GHz
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Fermi-LAT 1–2 GeV
Astrophysical: Galactic vs Extra-galactic
Galactic origin (diffuse)
medium.
Extra-galactic origin (discrete sources)
Exotic: Galactic vs Extra-galactic
Galactic Origin
Extra-galactic Origin
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from the Fermi-LAT map.
function of energies.
unresolved sources (below the detection threshold) and truly diffuse component (if any).
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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 Δ
The mean intensity depends on particle physics: annihilation cross-section and dark
matter mass.
The fluctuation power, Δ, depends on structure formation.
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power spectrum:
measured by WMAP .
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where
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where
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2-point function of δ2 = 4-point function
To compute the power
spectrum of anisotropy from dark matter annihilation, we need three ingredients:
mass,
and
θ (= π / l)
Dark matter halo
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in collaboration with
V. Vitale (on behalf of Fermi-LAT Team)
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events)
measure the power spectrum!
measured by WMAP .
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Mask |b|<30 degrees
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Mask |b|<30 degrees
Mask |b|<30 degrees
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Mask |b|<30 degrees
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WMAP maps
is the “photon noise” (Poisson statistics) in the power spectrum, which we must subtract.
given by the mean number density of photons over the sky (which is precisely calculable).
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spectrum multiplied by the harmonic transform of the “point spread function” (PSF). (It is called the “beam transfer function” in the WMAP analysis.)
versions of Fermi-LAT instrument response functions and compute PSF.
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Photon noise has been subtracted
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Photon noise has been subtracted
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Photon noise has been subtracted
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Photon noise has been subtracted
multipoles (larger angular scales).
noise.
multipoles, indicating the contribution from unclustered point sources (more later)
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DATA: CLEANED = Galactic Model Map Subtracted
DATA: CLEANED = Galactic Model Map Subtracted
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DATA: CLEANED = Galactic Model Map Subtracted
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DATA: CLEANED = Galactic Model Map Subtracted
see at l<150 is largely coming from the Galactic foreground.
affected by the foreground, and thus is usable for investigating the extra-galactic gamma-ray background.
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single power-law: Cl ~ ln Therefore, we will find the best-fitting constant power, CP. (“P” stands for “Poisson contribution”)
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Consistent with a single power-law. For CP~E–2Γ,
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(statistical errors only)
sources) agrees with that of detected blazars. Fermi-LAT Collaboration, ApJ, 720, 435 (2010)
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Distribution of energy spectrum indices of detected blazars
as
intensity as
interpretation:
blazars.
fact that the same unresolved blazars contribute
background.
consistent picture of the gamma-ray sky.
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dividing CP by the measured mean intensity squared:
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(statistical errors only)
matter properties.
the constant power at l>150 is
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factor of l(l+1).
gives ~l2
at l>150
Ando & EK (2006); Ando, EK, Narumoto & Totani (2007)
/<I>2 DM ann. Blazars
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Dark matter predictions are still consistent with data, but not so far away!
the gamma-ray background.
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diffuse gamma-ray background from Fermi-LAT 22mo maps.
contribution from unresolved blazars
mystery
far away from the current limit. Wait for results from the future Fermi analysis (3 to 7 more years to go!)
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