Some Astrophysical Implications from Dark Matter Neutralino Annihilation 戸谷 友則 Tomo Totani KIAS-APCTP-DMRC workshop on Dark Side of the Universe May 24-26, KIAS, Seoul

Outline Neutralinos as the heating source: the solution to the “cooling flow problem” of galaxy clusters?  Totani 2004, Phys. Rev. Lett. 92, earth-mass sized microhalos and their contribution to the galactic/extragalactic background  Oda, Totani, & Nagashima 2005, astro-ph/050496

X-rays from galaxy clusters thermal bremsstrahlung of hot gas:  kT ~ 5-10 keV  n ~ cm -3 (virial)  n~10 -1 cm -3 (central)  M tot ~10 15 M sun M gas /M tot ~ Ω B /Ω M ~10% M star /M gas ~10% D=52 Mpc (1Mpc x 1Mpc) (50 kpc x 50 kpc)

the Cooling Flow Problem of Galaxy Clusters “Cooling flow clusters”  Central gas cooling time < the Hubble Time (~10 10 yr)  Theory predicts cooling flow: ~ M sun / yr Voigt & Fabian 03

the Cooling Flow Problem of Galaxy Clusters (2) No evidence for strong cooling flows from latest X-ray observations  A heating source required.  Required heating rate: ~10 45 erg/s during yr for a rich cluster AGNs? heat conduction? no satisfactory explanation yet…. Fabian ‘02 Peterson et al. ‘03 Green: STD CF model

Possible heat sources to solve the cooling flow problem of galaxy clusters Heat conduction  Effective if κ~0.3 Spitzer value  Useful for stabilizing intracluster gas  A fine tuning necessary, and central regions cannot be heated sufficiently by conduction (e.g. Bregman & David 98; Zakamska & Narayan ’03; Voigt & Fabian ‘04) AGNs  most cooling clusters have cD galaxies, X-ray cavities, and radio bubbles  Efficiency must be high (>~10% of BH rest mass to heat!)  Stability? AGNs generally episodic, intermittent t E ~10 7 yr, L E >> erg/s  Actual heat process unclear (jet? buoyant bubbles?)

Heat conduction and AGN to heat the intracluster gas Voigt et al Voigt & Fabian ’04

Clusters seen in optical and X-rays Courtesy from K. Masai

SUSY Dark Matter (WIMPs, Neutralinos) The most popular theoretical candidate for the dark matter  SUperSYmmetry: well motivated from particle physics  Lightest SUSY partners (LSPs) are stable by R-parity  Neutralinos (l.c. of SUSY partners of photon, Z, and neutral Higgs): the most likely LSP  Predicted relic abundance depends on annihilation cross section in the early universe (freeze out T~m χ /20), and is close to the critical density of the universe Constraint on the neutralino mass  50 GeV ~< m χ <~ 10 TeV  Lower bound from accelerator experiments  Upper bound from cosmic overabundance

Annihilation Signals Line gamma-rays:  χ χ  ２ γ  χ χ  Zγ  (σv) line ~ cm 3 s -1 << (σv) cont ~ cm 3 s -1 Continuum gamma-rays, e ±, p, p-bar, ν’s Search:  Line/continuum gamma-rays from GC, nearby galaxies, galaxy clusters  Positron/antiproton excess in cosmic-rays  …

Annihilation yields by hadron jets Annihilation energy goes to gammas, e ±,p, p-bars, neutrinos as: ~1/4, 1/6, 1/15, 1/2 Particle energy peaks at: 0.05, 0.05, 0.1, 0.05 m χ c 2.  (From DarkSUSY package, Gondolo et al. 2001)  Most energy is carried by ~1-10 GeV particles for m χ ~100GeV  photon flux peak at ~70 MeV (~m π /2) An example for yield Spectra from DarkSUSY Line γ

Neutralino Annihilation as a Heat Source Dark matter density profile for a rich cluster: Annihilation （∝ ρ 2 ) from the center is not important if γ<1.5 Contribution to heating is negligible if γ=1 (NFW). C.f erg/s required for cluster heating

Density spike by SMBH formation? Density “spike” can be formed by the growth of supermassive black hole (SMBH) mass at the center.  Young (1980); for stellar density cusps in elliptical galaxies  Gondolo & Silk (1999); for DM cusps  “Adiabatic” = growth time scale > orbital period at r s  Annihilation rate divergent with r → 0 since γ>1.5

Annihilation energy from the density spike at the cluster center Density maximum determined by annihilation itself: The annihilation luminosity from r<r c : A factor of about 10 enhancement by r>r c and time average Steady energy production after turned on ρ r rsrs rcrc

Does density spike really form? The Galactic Center  If it is the case, a part of SUSY parameter space can be rejected (Gondolo & Silk 1999)  It is not necessarily likely (Ullio et al. 2001; Merritt et al.2002) The GC is baryon dominated. Is SMBH at the DM center? Disturbed by baryonic processes, e.g., starbursts and supernovae scatter by stars Merger of SMBHs destroys the spike and cusps The cooling-flow clusters of galaxies:  A giant cD galaxiy always at the dynamical center  DM dominates baryons to the center (e.g., Lewis et al. 2003)  Adiabatic BH mass growth happens as a feed back to the cooling flow in the past

The cluster density profiles from X-ray observations Abell 2029 Lewis et al Moore NFW stars gas

Heating Processes: AGN vs. Neutralinos clusters with short cooling times have giant cD galaxies at the center, X-ray cavities, radio bubbles --- indicating feedback from the cluster center.  AGNs or Neutralinos? heating process?  relativistic particle production  bubble formation  mechanical/shock heating by bubble expansion and dissipation  energy loss of cosmic-ray particles stability?  neutralino annihilation is roughly constant once the density spike forms, while AGNs generally sporadic Blanton et al. ’03 Abell 2052 X-ray image + radio contour Birzan+ ‘04

Efficiency of heating/CR production: AGN versus Neutralinos AGN efficiency must be very high, L heat ~ ε (m ● c 2 /t age )  ε~1 for some clusters with t age ~ yr CR production from AGNs: L CR ~(outflow efficiency) x (shock acceleration efficiency) x (m ● c 2 /t age )  ε<~0.01 normally expected neutralino annihilation: direct energy conversion into relativistic particles Fujita&Reiprich ’04

gamma-ray detectability from clusters: test of this hypothesis Continuum gamma-rays at ~1-10 GeV (for m χ ~100GeV)  ~40 gamma-rays per annihilation  Very close to the EGRET upper limit for a 100Mpc  Many positional coincidence between clusters and un-ID EGRET sources (e.g. Reimer et al. 2003)  GLAST will likely detect Line gamma-rays  A few photons for a cluster with line = cm 3 s -1 for GLAST in ~5 yr operation.  Negligible background rate (~10 -3 ) within the energy and angular resolution  Air Cerenkov telescopes may also detect  superposition of many clusters will enhance the S/N

GC versus Clusters: quantitative comparison flux measure Mρ av / (4πD 2 ) in [GeV 2 cm -5 ]  MW as a whole d=8kpc 3.9x10 21  GC (within 0.1°or r<14pc), NFW: 1.9x10 18 Moore: 1.5x10 21 NFW+spike, r<r c : 5.4x10 21 M+spike, r<r c : 2.6x10 24  clusters (d=77Mpc, Lx=10 45 erg/s: Perseus) NFW 1.5 x CF with L χ =10 45 erg/s: 1.6 x 10 21

Conclusions: Part I neutralino annihilation can be a heat source to solve the cooling flow, if the density spike is formed by SMBH mass evolution in the cluster center Better than heating by AGNs in:  stability  efficiency to produce relativistic particles/ bubbles This hypothesis can be tested by GLAST / future ACTs

gamma-ray background from annihilation in the first cosmological objects

Gamma-rays from earth-mass subhalos!? density power-spectrum of the universe: SDSS 3D P(k), Tegmark et al. 04 δ= （ ρ- ρ av ） / ρ M sun ~10 -5 M sun green+, 04 cut off by neutralino free streaming

Gamma-rays from earth-mass subhalos!? earth mass (10 -6 Msun), 0.01 pc size objects forming z~60  Hofmann+, 01, Berezinsky+, 03, Diemand+, 04 ~50% of mass in the Galactic halo may be in the substructure n~500 pc -3 around Sun, encounter to the Earth for every 1,000 yr. controversial whether or not they are destroyed by tidal disruption at stellar z=26, 3kpc width (comoving) Diemand+, 04

the cosmic gamma-ray background Strong et al. ‘04

Contribution to the cosmic gamma-ray background Ullio et al ’02  substructures with M>~10 6 M sun  model1: M=171 GeV σv=4.5x cm 3 s -1 b 2gamma =5.2x10 -4  model2: M=76 GeV σv=6.1x cm 3 s -1 b 2gamma =6.1x10 -2 Assuming universal halo density profile, annihilation signal from GC exceeds if neutralinos have dominant contribution to EGRB (Ando 2005)

microhaloe contribution to EGRB flux from nearby microhalo is very difficult to detect (see also Pieri et al. ’05) When luminosity density is fixed, the flux from the nearest object scales as L 1/3  if background flux is the same, small objects are difficult to resolve microhalos form at very high z, with high internal density ∝ (1+z) 3 Even if microhaloes are destructed in centers of galactic haloes, total flux hardly affected （ mass within 10kpc is ~6% of MW halo ）  extragalactic background flux hardly affected. Oda, Totani, & Nagashima, astro-ph/

Microhalo formation in the Standard Structure Formation Theory

microhalo number density number density of all including those in sub(sub(sub…))structure number density around the Sun Simulation by Diemand et al. ： n~500 pc -3  f surv ~ 1 in their simulation.  ~5% of total mass in the form of microhaloes Berezinsky+ ’03  f surv ~ % (based on analytical treatment)  f surv should be higher for those with high ν the actual value of f surv is highly uncertain

internal density profile of microhaloes annihilation signal enhancement simulation:  concentration parameter c~1.6 in the simulation  （ α,β,γ)=(1, 3, 1.2)  f c = 6.1 γ~1.7

internal density profile of microhaloes. 2 γ~1.7 in the resolution limit of the simulation similar to halo density profile soon after major merger: a single power law with γ~1.5-2 (Diemand et al. ’05) γ>1.5  divergent annihilation luminosity  density core where annihilation time ~ yr γ=1.5  f c = 31 γ=1.7  f c = 190 γ=2.0  f c = 1.4x10 4 f c =6.1 is a conservative choice.

Flux from isolated objects the nearest microhaloes M31 (the Andromeda galaxy)  ~1.5x10 11 M sun within ~1° (13kpc)

Galactic and extragalactic gamma-ray background from microhaloes Oda et al. ‘05

Gamma-ray halo around the GC? Dixon et al. ‘98 excess from the standard CR interaction model of the Galactic gamma-ray background excess from the standard model x 1.2 gamma-ray halo? Flux level ~ EGRB Inverse Compton Model

Conclusions: Part II earth-mass sized microhaloes can be constrained by galactic/extragalactic background, much better than the flux from the nearest ones. If f surv ~ 1 (as suggested by simulations), microhaloes should have significant contribution to EGRB/GGRB, even with conservative choice of other parameters flux expected from M31 is close to the EGRET upper limit if f surv ~1  a good chance to detect by GLAST