Some Astrophysical Implications from Dark Matter Neutralino Annihilation

1
Some Astrophysical Implications from Dark Matter
Neutralino Annihilation

• ?? ??
• Tomo Totani
• KIAS-APCTP-DMRC workshop on Dark Side of the
  Universe
• May 24-26, KIAS, Seoul

2
Outline

• Neutralinos as the heating source the solution
  to the cooling flow problem of galaxy clusters?
• Totani 2004, Phys. Rev. Lett. 92, 191301
• earth-mass sized microhalos and their
  contribution to the galactic/extragalactic
  background
• Oda, Totani, Nagashima 2005, astro-ph/050496

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X-rays from galaxy clusters

• thermal bremsstrahlung of hot gas
• kT 5-10 keV
• n 10-3 cm-3 (virial)
• n10-1 cm-3 (central)
• Mtot1015 Msun
• Mgas/Mtot OB/OM10
• Mstar/Mgas10

(1Mpc x 1Mpc)
D52 Mpc
(50 kpc x 50 kpc)
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the Cooling Flow Problem of Galaxy Clusters

• Cooling flow clusters
• Central gas cooling time lt the Hubble Time
  (1010yr)
• Theory predicts cooling flow 100-1000 Msun / yr

Voigt Fabian 03
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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 1045 erg/s during 1010yr
  for a rich cluster
• AGNs? heat conduction? no satisfactory
  explanation yet.

Green STD CF model
Fabian 02
Peterson et al. 03
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Possible heat sources to solve the cooling flow
problem of galaxy clusters

• Heat conduction
• Effective if ?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 (gt10 of BH rest mass to
  heat!)
• Stability?
• AGNs generally episodic, intermittent
• tE107 yr, LE gtgt 1045 erg/s
• Actual heat process unclear (jet? buoyant
  bubbles?)

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Heat conduction and AGN to heat the intracluster
gas
Voigt Fabian 04
Voigt et al. 2002
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Clusters seen in optical and X-rays
Courtesy from K. Masai
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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
  Tm?/20), and is close to the critical density of
  the universe
• Constraint on the neutralino mass
• 50 GeV lt m? lt 10 TeV
• Lower bound from accelerator experiments
• Upper bound from cosmic overabundance

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Annihilation Signals

• Line gamma-rays
• ? ?? 2?
• ? ?? Z?
• (sv)line 10-29 cm3s-1 ltlt (sv)cont 10-26
  cm3s-1
• Continuum gamma-rays, e, p, p-bar, ?s
• Search
• Line/continuum gamma-rays from GC, nearby
  galaxies, galaxy clusters
• Positron/antiproton excess in cosmic-rays

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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?c2.
• (From DarkSUSY package, Gondolo et al. 2001)
• Most energy is carried by 1-10 GeV particles for
  m?100GeV
• photon flux peak at 70 MeV (mp/2)

An example for yield Spectra from DarkSUSY
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Neutralino Annihilation as a Heat Source

• Dark matter density profile for a rich cluster
• Annihilation (??2) from the center is not
  important if ?lt1.5
• Contribution to heating is negligible if ?1
  (NFW).
• C.f. 1044-45 erg/s required for cluster heating

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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 gt orbital period
  at rs
• Annihilation rate divergent with r ? 0 since
  ?gt1.5

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Annihilation energy from the density spike at
the cluster center

• Density maximum determined by annihilation
  itself
• The annihilation luminosity from rltrc
• A factor of about 10 enhancement by rgtrc and time
  average
• Steady energy production after turned on

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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

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The cluster density profiles from X-ray
observations
Abell 2029 Lewis et al. 2003
NFW
Moore
stars
gas
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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

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Efficiency of heating/CR production AGN versus
Neutralinos

• AGN efficiency must be very high, Lheat e (m?
  c2/tage)
• e1 for some clusters with tage 1010 yr
• CR production from AGNs LCR(outflow efficiency)
  x (shock acceleration efficiency) x (m? c2 /tage)
• elt0.01 normally expected
• neutralino annihilation direct energy conversion
  into relativistic particles

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gamma-ray detectability from clusterstest of
this hypothesis

• Continuum gamma-rays at 1-10 GeV (for m?100GeV)
• 40 gamma-rays per annihilation
• Very close to the EGRET upper limit for a cluster
  _at_ 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 ltsvgtline 10-29
  cm3s-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

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GC versus Clusters quantitative comparison

• flux measure M?av / (4pD2) in GeV2 cm-5
• MW as a whole
• d8kpc
  3.9x1021
• GC (within 0.1or rlt14pc),
• NFW
  1.9x1018
• Moore
  1.5x1021
• NFWspike, rltrc 5.4x1021
• Mspike, rltrc
  2.6x1024
• clusters (d77Mpc, Lx1045 erg/s Perseus)
• NFW
  1.5 x 1016
• CF with L?1045erg/s 1.6 x 1021

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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

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gamma-ray background from annihilation in the
first cosmological objects
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Gamma-rays from earth-mass subhalos!?

• density power-spectrum of the universe

1015Msun
d(?- ?av)/ ?
SDSS 3D P(k), Tegmark et al. 04
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Gamma-rays from earth-mass subhalos!?
Diemand, 04
_at_ z26, 3kpc width (comoving)

• earth mass (10-6 Msun), 0.01 pc size objects
  forming z60
• Hofmann, 01, Berezinsky, 03, Diemand, 04
• 50 of mass in the Galactic halo may be in the
  substructure
• n500 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 encounters

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the cosmic gamma-ray background
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Contribution to the cosmic gamma-ray background

• Ullio et al 02
• substructures with Mgt106 Msun
• model1
• M171 GeV
• sv4.5x10-26 cm3s-1
• b2gamma5.2x10-4
• model2
• M76 GeV
• sv6.1x10-28 cm3s-1
• b2gamma6.1x10-2
• Assuming universal halo density profile,
  annihilation signal from GC exceeds if
  neutralinos have dominant contribution to EGRB
  (Ando 2005)

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microhaloe contribution to EGRB
Oda, Totani, Nagashima, astro-ph/0504096

• 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 L1/3
• if background flux is the same, small objects are
  difficult to resolve
• microhalos form at very high z, with high
  internal density ?(1z)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.

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Microhalo formation in the Standard Structure
Formation Theory
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microhalo number density

• number density of all including those in
  sub(sub(sub))structure
• number density around the Sun
• Simulation by Diemand et al. n500 pc-3
• fsurv 1 in their simulation.
• 5 of total mass in the form of microhaloes
• Berezinsky 03
• fsurv 0.1-0.5 (based on analytical treatment)
• fsurv should be higher for those with high ?
• the actual value of fsurv is highly uncertain

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internal density profile of microhaloes

• annihilation signal enhancement
• simulation
• concentration parameter c1.6 in the simulation
• (a,ß,?)(1, 3, 1.2)
• fc 6.1

?1.7
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internal density profile of microhaloes. 2

• ?1.7 in the resolution limit of the simulation
• similar to halo density profile soon after major
  merger a single power law with ?1.5-2 (Diemand
  et al. 05)
• ?gt1.5 ? divergent annihilation luminosity
• density core where annihilation time 1010 yr
• ?1.5 ? fc 31
• ?1.7 ? fc 190
• ?2.0 ? fc 1.4x104
• fc6.1 is a conservative choice.

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Flux from isolated objects

• the nearest microhaloes
• M31 (the Andromeda galaxy)
• 1.5x1011Msun within 1 (13kpc)

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Galactic and extragalactic gamma-ray background
from microhaloes
Oda et al. 05
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Gamma-ray halo around the GC?
Dixon et al. 98
excess from the standard model x 1.2
excess from the standard CR interaction model of
the Galactic gamma-ray background
gamma-ray halo? Flux level EGRB
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Conclusions Part II

• earth-mass sized microhaloes can be constrained
  by galactic/extragalactic background, much better
  than the flux from the nearest ones.
• If fsurv 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 fsurv1
• a good chance to detect by GLAST