Transcript Big Stuff from Gamma

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)
What Is Out There?
WMAP 94GHz
What Is Out There?
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

Blazars = A population of AGNs whose relativistic
jets are directed towards us.


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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)
Narumoto & Totani, ApJ, 643, 81 (2006)
Blazar Luminosity Function Update
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
This
predicts that GLAST should detect ~3000 blazars.
implies that AMS-2 would detect a few hundred blazars.
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.
Ando et al. (2007)
•It is assumed that
blazars are brighter than
1041 erg/s at 0.1 GeV.
-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 (G. Lamanna 2002)
Ando et al. (2007)
Dark matter (WIMP)
annihilation
dark matter
annihilates into gammaray photons.
The dominant mode:
jets
GeV-γ
WIMP
•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 AMS2 see this?
Ando et al. (2007)
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!
 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?”
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!
“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 Sky)
= (MEAN INTENSITY) x 



The mean intensity depends on particle physics: annihilation crosssection 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
Dark matter halo
To compute the power
spectrum of
anisotropy from dark
matter annihilation,
we need three
ingredients:

1.
2.
θ (= π / l)
3.
Number of halos as a
function of mass,
Clustering of dark
matter halos, and
Substructure inside of
each 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.
2.
3.
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?
Ando, Komatsu, Narumoto & Totani (2007)
Predicted Power Spectrum
 At
39% DM
61% DM
10 GeV for 2-yr
observations of
GLAST
 Blazars
80% DM
97% DM
(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.
What If Substructures Were
Disrupted…
39% DM
80% 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.
97% DM
“No Substructure”
or “Smooth Halo” Limit
39% DM
80% DM
61% DM
97% 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.
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.
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Churazov et al. (2005)
INTEGRAL/SPI Spectrum

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 (~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 s1) is indeed reasonable!



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-


Boehm et al., PRL, 92, 101301 (2004)
Hooper et al., PRL, 93, 161302 (2004)
Beacom, Bell & Bertone, PRL, 94, 171301 (2005)
How about the extra-galactic background light?
Ahn & EK, PRD, 71, 021303R; 71, 121301R; 72, 061301R (05)
AGNs, Supernovae, and
Dark Matter Annihilation…




The extra-galactic
background in 120MeV region is a
HEAO-1
superposition of AGNs,
SNe, and possibly DM
AGNs
annihilation.
SNe cannot explain the
background.
AGNs cut off at ~1MeV.
~20 MeV DM fits the
data very well.
DM
SMM
COMPTEL
SNe
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.