Constraints on
late-decaying dark-matter models
Kallia Petraki
University of Melbourne
(in collaboration with N. Bell and A. Galea)
Dark-matter decay
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Information about particle nature of dark matter.
Identity of DM particles may explain
cosmological observations
e.g. dark-matter decay responsible for the
observed structure of the universe
CDM and structure formation
Cold Dark Matter:
Matter “Bottom-up” hierarchical
formation of structure, small-scale structure
favoured.
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At large scales, ΛCDM simulations reproduce
observed structure successfully.
At small scales, some CDM predictions do not
match observations.
CDM problems
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Galactic density profiles: central cusps rather
than cores [Gilmore, Wyse; Strigari et al.]
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Overprediction of satellite galaxies [Klypin; Moore]
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No pure-disk galaxies predicted [Governato et al.;
Kormendy et al.]
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Overprediction of halos in low-density voids
[Peebles]
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The angular momentum problem: gas condenses
early and looses too much angular momentum
[Dolgov]
Possible solutions
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Gastrophysics: complicated astrophysical
solutions for individual problems.
Warm Dark Matter: free-streaming suppresses
structure at small scales.
Cold Dark Matter: modification of the standard
structure-formation picture → DM decay.
Decay mode
where
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DM energy density approximately conserved
very little energy dissipated into relativistic
particles: limits from CMB anisotropies relaxed
recoil velocities of DM particles affect galactic
structure, if decays occur at appropriate time:
solve some of the CDM problems
Dark-matter decay &
structure formation
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Early decays, before recombination: WDM.
WDM
Constraints from free-streaming and phase
packing [Cembranos, et al. (2005); Kaplinghat (2005)]
Late decays: affect evolution of structure inside
haloes.
haloes Semi-analytical methods and simulations
needed. [Sanchez-Salcedo (2003); Abdelqader and
Melia (2008); Peter, et al. (2010)]
Dark-matter decay &
structure formation
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Early decays, before recombination: WDM.
WDM
Constraints from free-streaming and phase
packing [Cembranos, et al. (2005); Kaplinghat (2005)]
Late decays: affect evolution of structure inside
haloes.
haloes Semi-analytical methods and simulations
needed. [Sanchez-Salcedo (2003); Abdelqader and
Melia (2008); Peter, et al. (2010)]
+
Galactic and cosmic radiation backgrounds can
constrain emission of relativistic SM particles.
Dark-matter decay &
structure formation
Effect depends on:
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energy imparted to daughter DM particles
(recoil velocity)
time-scale of decay
Dark-matter decay &
structure formation
Interesting region:
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Constraints from:
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Halo massconcentration
relation.
Galaxy-cluster
mass function.
[Peter, Moody, Kamionkowski (2010)]
Dark-matter decay &
structure formation: simulations
[Peter, Moody, Kamionkowski (2010)]
Dark-matter decay &
structure formation
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Missing satellites: DM decay heats smaller
haloes, causes them to expand, and reduces
their circular velocity.
This can account for the deficit of galaxies with
vc ~ 10-20 km/s. [Abdelqader and Melia (2008)]
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Cusps vs Cores: Heating causes central cusps
to expand, and form cores [Sanchez-Salcedo
(2003)]
Decay channels
What is ? Derive constraints on decay rate using
radiation backgrounds, if belongs to the SM.
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[Yuksel, Kistler (2007)]
[Bell, Galea, KP (in preparation)]
photons: most stringent limits
neutrinos: least stringent limits
→ taken together, they delineate the range of
lifetime bounds for these models.
Decay channels
What is ? Derive constraints on decay rate using
radiation backgrounds, if belongs to the SM.
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[Yuksel, Kistler (2007)]
[Bell, Galea, KP (in preparation)]
[Bell, Galea, KP (in preparation)]
Constraints on DM decay
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Energy of relativistic decay products is
The flux is bounded by the observed
backgrounds:
(
: age of the universe)
Radiation backgrounds yield lower bounds on:
Backgrounds
atmospheric neutrino flux.
positron flux; photon backgrounds.
Photoproduction by
in the galaxy occurs via:
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internal bremsstrahlung,
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positron annihilations, while relativistic and at-rest,
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bremsstrahlung,
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inverse Compton effect,
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synchrotron emission.
Constraints
[Yuksel, Kistler (2007); Bell, Galea, KP (in preparation)]
Constraints
Syn
IC+B
IB
IA
511 keV
[Yuksel, Kistler (2007); Bell, Galea, KP (in preparation)]
Constraints
Syn
IC+B
IB
IA
511 keV
[Yuksel, Kistler (2007); Bell, Galea, KP (in preparation)]
Applying the constraints
on late-decaying DM models.
Do the constraints allow for dark-matter decay to
occur, such that:
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enough energy is released into the galaxies to
affect the formation of structure?
the energy release takes place at time scales
that can affect the evolution of the galaxy?
Applying the constraints
on late-decaying DM models.
Constraints from radiation backgrounds satisfied if:
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decay rate is sufficiently small: lower bound on
or
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decay fast enough, abundance of unstable DM
particles is suppressed today: upper bound on
→ excluded band of lifetimes.
[Bell, Galea, KP (in preparation)]
DM decay and structure formation
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Parameter space for DM decay to affect the
formation of structure is quite constrained,
when SM particles are produced in the decay.
Interesting regions remain open:
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decay into neutrinos, for
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decay into photons or
, for
Upper limit of the excluded band may be
improved with more sensitive observations.
This is not true for the lower limit.