Dark Matter Annihilation

Dark Matter Annihilation
Marcel Zemp (UM)
with Jürg Diemand (UCSC), Mike Kuhlen
(IAS), Piero Madau (UCSC), Ben Moore
(UZH), Doug Potter (UZH) & Joachim
Stadel (UZH)
Extreme Star Formation in Dwarf Galaxies
Ann Arbor, 29. July 2009
Dark Matter Candidate
z Prime Dark Matter candidate ⇒ WIMP
(Weakly Interacting Massive Particle)
z Interacts only via gravity and weak
nuclear force ⇒ neutral
z Motivated by WIMP miracle
z Supersymmetry ⇒ LSP
(Lightest Supersymmetric Particle)
z If WIMPs are Majorana particles
⇒ colliding WIMPs self-annihilate
Dark Matter Annihilation
z Annihilation channels:
- fermion-antifermion pairs (tree level)
- gauge boson pairs (tree level)
- Higgs bosons (tree level)
- photons (loop level)
z After decay and hadronisation:
- high energy neutrinos
− +
- relativistic e , e & p, p̄
- gamma rays
Gamma Rays
0
arXiv:0906.1822
π → γγ
Internal Bremsstrahlung:
Final State Radiation (FSR)
Virtual Internal Bremsstrahlung (VIB)
χχ → γγ
χχ → γZ
Annihilation Luminosity
z Annihilation is a two-body process
L≡
2
ρ dV
z For cusped profiles
√
3 2
3
L ∝ rs ρs ∝ Vmax cV
z Luminosity is concentrated
L(rs )/Ltot ∼ 90%
r200b = 402 kpc
800 kpc
Via Lactea II
2
ρ
Total Annihilation Signal
ApJ, 2008, 686, 262
Subhalo Annihilation Signal
ApJ, 2008, 686, 262
Central Flux Corrections
γ=
ApJ, 2008, 686, 262
γ=
1.2
1.0
VLII
VLI
Via Lactea II Subhaloes
z 40000 subhaloes within 400 kpc
z 2000 subhaloes within 50 kpc
z 20 subhaloes within 8 kpc
z Subhaloes locally at 8 kpc that looked
smooth in previous simulations
z Subhaloes within subhaloes etc.
⇒ Subsubhaloes ⇒ Sub2haloes
⇒ Subnhaloes
rtidal
Sub2haloes
Mtidal = 1.97 × 109 Mo
Mtidal = 5.09 × 109 Mo
Subnhalo Abundance
r2
Nature, 2008, 454, 735
00
=
10
50
0
40
2
kp
kp
c
kp
c
c
−3
N (> Vmax ) ∝ Vmax
Subhalo Spatial Distribution
n(r) ∝ ρ(r)M(r)
−1
Vmax > 3 km s
Nature, 2008, 454, 735
Subhalo Concentrations
Mass dependence from Bullock
MNRAS, 2001, 321, 559
cV ≡ ρ̄(rVmax )/ρcrit,0 ∝ (Vmax /rVmax )2
Nature, 2008, 454, 735
Boost Factor
z Small subhaloes contribute more than
large ones
z Total resolved subhalo contribution is
97% of host halo in Via Lactea II
⇒ boost factor B = 1.97
z Extrapolation to smaller masses can
lead to B = O(10)
z Tidal debris ⇒ B = O(1)
Corrected Total Signal
ApJ, 2008, 686, 262
Diffuse Background
z Isotropic extragalactic component
⇒ measured by EGRET
z Galactic component
⇒ modelled with GALPROP
z Undetectable subhaloes and smooth
host halo
⇒ modelled from simulation
z Detector sensibility
z Calculate Signal-to-Noise S
Detectable Subhaloes
< σv >= 3 × 10−26 cm3 s−1
Mχ = 100 GeV/c2
ApJ, 2008, 686, 262
S>5
Signal-to-Noise
Results
z a few subhaloes should be detectable
z 95 % are extended sources
⇒ discrimination against pointlike
sources like pulsars
z Distribution on sky is consistent with
isotropy
z High S ⇒ massive subhaloes with
median Vmax = 24 km s-1
z D ~ 10 – 100 kpc