Anisotropy signatures of dark matter annihilation Le Zhang (University of Wisconsin-Madison) Based on L. Zhang, G. Sigl; arxiv:0807.3429; JCAP09(2008)027 L. Zhang, F. Miniati, G. Sigl; arxiv:1008.1810 PASI2012 2012.3.7 1
DM Models thermal equilibrium A Weakly Interactive Massive Particles (WIMPs)P <σν>n x << H freeze out The present relic density requires <σv>! 3 10-26 cm 3 s -1 (0.1/" χ h 2 )!fairly agree with electroweak cross sections if m x! 100 GeV!Candidates: Neutralino, Gravitino... Kolb, Turner DM can be observable in CRs, e.g. photons, leptons, antimatter... 2 e + e - propagate in random B fields and ISRF, emitting synchrotron radiations and ICS γ-rays.
DM halo and substructure CDM cosmology structure forms hierarchically (small form firstly, larger halos form later) After kinetic decoupling, free-streaming erases density fluctuations (Loeb et al.) SUSY- CDM predicts halo down to 10-6 M (Green et al.). First objects from at z=60 If survive, multitude of substructure halos in galaxies (Diemand et al) Enhance annihilation rate ρ 2 (clumping factor) Diemand et al. (2005) Diemand et al. (2008) The earth-like halos to form in Universe at z!60. DM subhalos in MW 3
Radio Signatures from Extragalactic DM J(ν, Ω) Y e[> E c (ν)] σv m 2 X 1 Bν dz H(z) Annihilation (1 + z) 3/2 δ 2 (z,ω) 1+ u op u B + u CMB u B (1 + z) 4 S cut = 10 mjy χχ"e + e - B fields photons S cut =0.1 mjy DM signature: flatter power spectrum at 200<l<3000 than std. origins foreground have less contamination at small scales 4 Current radio observations at 2GHz open a new window to detect DM annihilation signals
ICS gamma-rays from Galactic DM subhalos prompt emissions (no diffusion) ICS emissions M min =10 6 M Monte Carlo realization λ D 1 kpc (~5 o ) E e = 1 TeV;E γ =10GeV log(i) 5
Angular power spectrum No Diffusion 1. E e = 1 TeV 2. M min =10-6 M E γ =10GeV E γ =100GeV E γ =1GeV Key point: Due to e + e - diffusion, exponential suppression at l 10, which determined by diffusion length l "d/λ D 6
Conclusions For radio signatures: Extragalactic DM halos produce flat Cl at GHz, which may allow to distinguish DM signals from std., relevant for future surveys such as LOFAR and SKA For γ-ray signatures: Pure IC produces a new feature in Cl with exponential suppression at l#10, which could be detected by Fermi after foreground removal. 7
Backup 8
Dependance on annihilation channels ww ττ μμ preliminary results 1 GeV 10 GeV 100 GeV 9
Latest Hints e + excess (PAMELA) Harder e + +e - spectrum (Fermi-LAT, HESS, ATIC) Antiproton well reproduced by astrophysical models (PAMELA) #various dark matter models have been fitted to the excesses, require ~ TeV DM particles mostly annihilate or decay into leptons [Cirelli et al 2009] Nature 458, 607 (2009) PRL.102:181101,2009 Fermi-LAT PRL.105:121101,2009 PAMELA 10
Optimal frequency An optimal window at frequency ~ 2 GHz Can we test the signal of DM annihilation more powerfully? (remove foreground) Diffuse background from DM and std. sources b >20 o if foreground have a smooth directional dependence, less contamination in anisotropies A b =10, B=10 μg, m x =100GeV, Ye=10, <σv>=3 10-26 cm 3 /s 30
e + e - propagation in the MW Escaping r T. Delahaye The diffusion zone usually assumed to be a cylinder with half-height L of a few kpc and a radius R 20 kpc Particles confined by randomly oriented B fields 12
Galactic propagation of charged particles is described by solving the Transport equation: Galactic DM decays n t Dn = Q(r,p) D n(r,z,e) phase space density Dn = (D xx n V c n)+ p D ( p 2 D pp p n p 2 Q ± (r,e 0 )= ρ X(r) dn ± m X τ X de 0 ) p [ ṗn p 3 ( V cn) ] Spatial diffusion D xx (E) = D 0 E δ Energy loss syn+ic+brems+... Reacceleration (Alfvèn waves) D pp E 2 v A2 /D xx Convection V c (stellar winds) Relative abundance of elements (B/C, antiprotons, radiative nuclei C, O...) determines propagation parameters 13
Propagation Models Model δ 1 D 0 R L V c dv c /dz V a h reac [kpc 2 /Myr] [kpc] [kpc] [km/s] km/s/kpc [km/s] [kpc] MIN 0.85/0.85 0.0016 20 1 13.5 0 22.4 0.1 MED 0.70/0.70 0.0112 20 4 12 0 52.9 0.1 MAX 0.46/0.46 0.0765 20 15 5 0 117.6 0.1 DC 0/0.55 0.0829 30 4 0 6 0 4 DR 0.34/0.34 0.1823 30 4 0 0 32 4 L1 0.5/0.5 0.1523 20 4 0 0 10 4 Strong & Moskalenko; Donato et al; D. Bernardo et al B/C imposing D 0 /L!const., increasing L implies more e + e - in diffusion zone, translating to enhanced flux. 14
(1) Radio anisotropy signatures Motivation: radio background is ten times smaller than gamma-ray if predicted fluxes are comparable, extracting DM signals at radio band Extragalactic radiation background 15
Comparison with Fermi-LAT observations Fermi data at 1-2, 10-20, 100-300 GeV with removal of 3-month 10- sigma bright sources (Dobler et al,2010) unbiased anti-biased Unbiased: 1. Strongest constraint (S/B! 1) on σv/m 2 by highest g-rays 2. S/B! 5, 3 for 1-2, 10-20 GeV Anti-biased: amplitude ~100 times smaller than unbiased, due to smaller mean intensity <I> m x =1TeV, <σv>=3 10-26 cm 3 /s, M min =10-6 M 16
17 Porter, et al. (2005) Subhalo and ISRF models ( ) 2 ρ(r) =ρ s exp exp α [ 2 ( ) r α ] α rs Einasto profile (1965), better to fit the profile of small clumps ρ NFW (r) = ρ s N anti (< ζ) x(1 + x) or 2 N tot = (1 + ac 200) ζ β 1+ac 200 ζ γ Radial distribution with respect to smooth component 1. unbiased: number density follows NFW 2. anti-biased: number density from Gao et al. (2004) N(> M) 64 ( M 10 8 M ) αm Cumulative number of subhalos above a given mass M by Via Lactea simulation from Diemand et al. (2007) Considered numerous earth-like clumps, the total annihilation rates dominated by subhalos rather than by the smooth host halo ISRF: superposition of three blackbody-like spectra with different T and normalizations?b@a E γ (GeV) SL (T p =3800 K) IR (T p =40.6 K) CMB (T p =2.73 K) 1 14 GeV (2.28 kpc) 136 GeV (1.1 kpc) 526 GeV (0.48 kpc) 10 44 GeV (1.76 kpc) 431 GeV (0.65 kpc) 1665 GeV 100 141 GeV (1.26 kpc) 1365 GeV 5267 GeV Table 2. The dependence of the characteristic electron energy E on the energy E γ of gamma-ray emission through inverse Compton scattering off the various blackbody components of the ISRF with temperatures T p. For the cases E<1TeV the corresponding diffusion length λ D (E) is also shown in braces. For 1 TeV mono-energetic injection, numerical results show: most of the gamma-ray intensity at 1 GeV produced by pairs with E 526 GeV scattering off the CMB; 10 GeV by pairs with E 431 GeV off IR; 100 GeV by pairs with E 141 GeV off SL
Extragalactic DM signatures Extragalactic synchrotron flux: J(ν, Ω) Y e[> E c (ν)] σv 1 m 2 X Bν dz H(z) (1 + z) 3/2 δ 2 (z,ω) 1+ u op u B + u CMB u B (1 + z) 4 Y e [>E c (ν)]: Multiplicity (typical ~ 10) per annihilation into e + e - with E> E c, where E c (ν) = 5.9(ν/GHz) 1/2 (B/6μG) -1/2 GeV δ 2 (z,#)clumping factor depend on 1. halo density profile (NFW) 2. mass distribution of halos (PS) 3. mass cut-off (dwarf galaxy ~ 10 6 M ) 4. substructures (A b =10) B=10μG somewhat larger than typical since most of annihilations occur in densest region corresponding to large B 18
The Galactic dark matter distribution Anisotropies from dark matter substruc Anisotropies from dark matter substructu host halo 1.Host halo: a smooth dark matter halo 2.Substructure halos: Abundance (boost <I>) total number N(>Msub) =109(Msub/M )-0.9 Anisotropies from dark matter substructure [Diemand et al. (2007)] Radial unbiased distribution (boost <I>, large scale features) Unbiased: number density follows smooth halo Anti-biased: considering tide force [Gao et al. (2004)] anti-biased Figure 2. Gamma-ray intensity per K from substructure for the unbiased rad distribution with αm = 0.9 and minimum subhalo masses of Mmin = 107 M Figure 1. (Top panel) Gamma-ray intensity per K from the dark matter ha (top) and 10M! (bottom). Clustering of the subhalos in the direction of without substructure (assuming the smooth component accounts for 100% of th galactic center (the center of the map) is quite pronounced, particularly in dark matter density) as observed from our position. The map is centered on th Mmin = 10M! case. galactic center. (Bottom panel) Same as top panel, with the galactic emissio mask of [75] applied. total flux dominated by subhalos, host halo negligible!!! 19 scenario with an experiment having an angular resolution comparable to that of Fer the anti-biased case thethose subhalos isotropic. Theofanti-biased radial als The maps shown are fromappear the bottom panels figures 2 and 3 distribution convolved with results in beam roughly order lesstarget total angular flux for aresolution given minimum mat 0.1, Fermi s for E >subhalo 10 GeV. Gaussian of an width σbof=magnitude since scale the typical of subhalos much larger in thethat unbiased color is the distances same for both panels are of this figure andthan matches of the distributio smooth h (note that theincolor differs between the from mapsthe forscales the unbiased andoriginal anti-biase emission map figurescale 1, but is offset slightly used in the ma distributions). maps here radial are fordistribution, the αm = 0.9 mass function, but simila in figures 2 andthe 3. For theshown unbiased a strong dipole feature due