Probing Dark Matter With Radio Observations

Probing Dark Matter With Radio Observations

Chorng-Yuan Hwang

黃崇源

Why do we need Dark Matter (DM)?

Rotation curve of Milky Way and galaxies

Mass of galaxy clusters Large scale structures of the Universe Cosmological model Modified Newtonian Dynamics (MOND)

might explain rotation curves but not large scale structures and CMB

Dark matter in galaxy

About 90% of the mass of galaxies is in the form of dark matter, which can not be observed.

The flat rotation curve of the Milky Way suggests that the dark halo of the Milky might distribute:

Total M r (r) r-2

Rotational Curve of Milky Way

Rotation Curves of Four Spirals

Clusters in Optical

Dark Matter In Galaxy Clusters

Dark Matter in Large-Scale Structures

Dark Matter in the Universe (Hinshaw et al 2009) (WMAP)

Fluctuations of CMB (Hinshaw et al 2009)

(WMAP)

Other Models/Theories

Modified Newtonian Dynamics (MOND) This might explain the rotation curves of

galaxies but is difficult to explain the large scale structures and CMB

It is not excluded that both dark matter and MOND are necessary.

Dark Matter Distribution

N-body simulation of dark matter halo structure:

NFW profile:

Cusp profile (Diemand 2007):

What is the Dark Matter (DM)? DM must be non-baryonic DM must be cold A viable candidate for the DM is the

Weakly Interacting Massive Particles (WIMPs).

The most favorable WIMPs are the neutralino predicated in the supersymmetric (SUSY) extension of the standard model and Kaluza-Klein particles.

Neutralino

A linear combination of two neutral higgsinos and two gauginos.

= B + W + H1 + H2

The most likely mass of is between ~ 50 GeV to 1 TeV

Self-annihilation of will decay into fermion pairs or gauge boson pairs and will finally become gamma ray, electrons and positrons.

Kaluza-Klein particles

Particles from compact extra dimension. the mass of the lightest Kaluza–Klein

particle is expected to be greater than 300 GeV

The annihilation of Kaluza–Klein particles can proceed through direct production of electron-positron pairs resulting in a source spectrum that is dominated by a delta function at the particle mass.

Prediction and Observations of DM: Difficult to detect directly

(CDMSII Collaboration 2010)

Relativistic electrons positrons decayed from DM If is the relic particle from the hot big bang

and constitute the DM, then the self-annihilating cross section is related to the abundance of dark matter

mh2 =h2 = 310-27 cm2 s-1/<v> From WMAP, mh2 =0.127, so the self-

annihilation cross section of is about <v> = 2.36 10-26 cm2 s-1

We might observe the resulting electrons and -rays that can be compared with models!

Where to Observe the DM

Simulations suggest that DM is clumped into sub-halos down to the Earth mass with a size of the solar system.

DM mass in clumps ~ 50% of total mass in halos

dN/dM ~ M-2

The source function is proportional to n2

Even small DM sub-halos can produce significant relativistic electrons and -rays!

Diemand et al 2007

Missing Subhalos?

Dark halo mass distribution (Diemand et al 2005)

Illuminating Dark Halos?

Electron/positron spectra

Source function:

Evolution of electrons:

Stationary spectra:

Equilibrium positron spectra in Milky Way from the annihilation of 100GeV using clumpy model of Diemand et al 2005

Equilibrium positron spectra in Milky Way from the annihilation of 1TeV using clumpy model of Diemand et al 2005

Spectrum of cosmic-ray electrons

ATIC results (2008): Kaluza-Klein particles?

Kaluza-Klein particles with mass of 620 GeV? (Chang et al 2008; ATIC)

FERMI Results (Abdo et al 2009)

Radio emission of electrons and positrons from decayed DM

Most of the astronomical objects have magnetic fields.

Relativistic electrons in magnetic fields can produce radio emission.

We can estimate the resulting radio emission and compare with radio observations of galaxies and galaxy clusters.

Radio Halos

Some clusters show radio halos from synchrotron radiation of relativistic electrons of unknown origins.

large scale and steep spectrum. 1/3 of clusters with mass > 1015 solar mass

show radio halos Magnetic fields in all clusters are ~ 5-

10G

A2163

Feretti (2003)

Models for Emission of DM Halo from Nearby galaxy Clusters and galaxies

Select several nearby rich clusters with measured X-ray profile and mass

Assume B=5 G and steady state NFW profile <v> = 2.36 10-26 cm2 s-1

n = mass density/m

m =100GeV – several TeV

subhalo mass: ~ 1012-10-4 M

Source functions of Coma main halo for 2TeV , solid line for fermion channels and dashed line for boson channels

Source functions of Coma halo for 100GeV , solid line for fermion channels and dashed line for boson channels

Self-Annihilation flux from CDM of Coma cluster

Self-Annihilation flux from CDM of A754 cluster

Self-Annihilation flux from CDM of A85 cluster

Self-Annihilation flux from a 1011 M DM halos at z=0.02

Conclusion and Summary

The predicted radio halo emission from the self-annihilation of DM neutralinos could be detectable.

The non-detection of radio halos for some massive clusters with high magnetic fields might be used to constrain the properties of the DM neutralinos or/and to constrain the structure formation models of the universe.

The End