Exploring the magnetized cosmic web with low frequency radio

Exploring the magnetized cosmic web with
low frequency radio emission
Nicholas Battaglia (University of Toronto, CITA), Christoph Pfrommer(CITA), Jonathan Sievers(CITA),
J. Richard Bond (CITA), Torsten Enβlin(MPA)
Methodology & Results
Recent improvements in the capabilities of low frequency radio
telescopes (GMRT, LOFAR, MWA, LWA) provide a unique
opportunity to study thermal and non-thermal properties of the
cosmic web. We argue that the diffuse, polarized emission from
giant radio relics (gischt) traces structure formation shocks. It can
thus provide insight into:
•  the strength/coherence of large scale magnetic fields
•  the process of diffusive shock acceleration of electrons
•  the properties of the warm-hot intergalactic medium (WHIM)
•  the exploration of observables beyond the thermal cluster
emission which are sensitive to the dynamical state of the cluster.
The associated radio observables enable us to extract the
aforementioned physical properties in clusters. We predict that
these next generation radio telescopes have the potential to
discover a substantially larger sample of radio relics and that
future experiments, such as SKA, should enable us to probe
fundamental parameters of plasma physics in clusters.
We model the synchrotron emission associated with the radio gischt
by calculating the primary shock-accelerated electron population
developed by Pfrommer et al. (2008). We use a simple
parametrization for the magnetic field that scales with the thermal
energy density. We search for spatially correlated synchrotron
emission from formation shocks, which form our relic sample and
study the properties of these relics in the clusters (Fig. 1).
Through studying the radio gischt observables we conclude:
•  The luminosity functions are sensitive to the cluster mass,
dynamical state and the magnetic field parametrization (Fig. 2).
•  The RM and power spectra have the potential to infer the
magnetic pressure support and discriminate the nature of MHD
turbulence in clusters (Fig. 3).
•  The spectral index maps are sensitive to the spectrum of
relativistic electrons and the shock strength (Mach number). This
enables us to infer hydrodynamical properties of the WHIM (Fig.
4).
-1
0
y [ h-1 Mpc ]
y [ h-1 Mpc ]
0
0
10-1
-1
-1
0
-1
-1
10-2
-2
-2
-2
-2
-2
-2
-2
-2
-2
-1
0
-1
0
x [ h-1 Mpc ]
1
2
1
2
(Battaglia et al. 2008)
-1
0
-1
0
x [ h-1 Mpc ]
1
2
10-3
1
2
(Battaglia et al. 2008)
Figure 1. Left: Three-colour image of a large merging cluster. The
energy dissipation rate at structure formation shocks is shown by the
colours red and yellow. Radio synchrotron emission at 150 MHz from
shock-accelerated relativistic electrons is shown by the colour blue
and emphasized by the contours. This radio gischt emission traces
the shocks, highlights the intermittent nature of mass accretion in
galaxy clusters, and illuminates magnetic fields. Right: Surface
brightness emission map for radio relics found in the same cluster.
Our relic finder groups SPH particles using a friends-of-friends
algorithm; we additionally require these particles to exceed an
emissivity threshold.
5
75
0.2
0.0
0
0
-0.2
-75
-5
-0.4
-0.2
0.0
x [ Mpc ]
0.2
10-6
1
-150
-0.4
P[RM] (k)
P[Bz] (k)
10
0.4
(Battaglia et al. 2008)
10-7
MFR1
MFR2
MFR3
10
1
1
10
k [h Mpc-1]
100
(Battaglia et al. 2008)
1.5
(Battaglia et al. 2008)
0.4
1.4
0.2
1.3
0.0
1.2
-0.2
1.1
-0.4
References
Battaglia N., Pfrommer C., Sievers J. L., Bond J. R., Enβlin T. A., 2008, MNRAS Submitted, arXiv:0806.3272
Pfrommer C., Enßlin T. A., Springel V., 2008, MNRAS, 385, 1211
150
0.4
P[RM](k) k2 [rad2 m-4]
1
5
RM [ rad m-2 ]
1
Figure 3. Left: Faraday rotation measure map of the largest
relic in a merging cluster, if placed at z ≈ 0.05 (mimicking
A2256). The contours represent the surface brightness at
1.4 GHz. Top right: power spectrum of the RM map P[RM](k)
and the line-of-sight component of the magnetic field P[Bz
](k). The excess power in P[RM](k) at large angular scales
comes from fluctuations in ne. Bottom right: power spectra
of RM maps for different magnetic field realizations. All RM
power spectra recover the shape and characteristic scale of
their magnetic input power spectra.
1
1
0
Rotation measure and power spectra
1
y [ h-1 Mpc ]
1
arcmin
0
-5
10
1
(Battaglia et al. 2008)
2
22
2.
Top:
Luminosity
functions for a
sample of 4 clusters
with
masses
ranging 2 orders of
magnitude. Bottom:
Luminosity
functions for a
varying magnetic
decline (αB). With
both panels we
show the luminosity
functions
dependence on
cluster
mass,
dynamical state and
the magnetic field
parameters.
P[RM](k) k2 [rad2 m-4]
1
Figure
arcmin
0
!",2D
-1
S! [ mJy arcmin-2 ]
2-2
y [ Mpc ]
2
2
Luminosity
functions
(Battaglia et al. 2008)
P[Bz](k) k2 [µG2 Mpc]
Abstract
1
-0.4
-0.2
0.0
x [ h-1 Mpc ]
0.2
0.4
(Battaglia et al. 2008)
Spectral index
Figure 4. Left: Spectral index map between 150 MHz and 1.4 GHz of the
total emission map around the large relic. The contours show orders of
magnitude in surface brightness at 1.4 GHz. In bright synchrotron
emitting regions, the spectral index is uniform across the relic implying
that this relic traces a single shock. Right: Radial profiles of the large
merging cluster restricted to the solid angle subtended by this relic for
the density and pressure. The shocked region is marked by the red
diamonds. We can determine a shock’s median Mach number from the
spectral index, then predict pre-shock values using the Rankine-Hugonoit
jump conditions assuming a fixed adiabatic index.