Big Stuff from Gamma
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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.