Radio Relics in clusters of Galaxies
Gabriele Giovannini
Dipartimento di Astronomia, Bologna University
Istituto di Radioastronomia, CNR/INAF Bologna
In collaboration with: M. Tugnoli, A. Bonafede, L. Feretti,
F. Govoni, M. Murgia, V. Vacca
Relics
Extended synchrotron sources in a peripheral cluster region (not
at the cluster center), not identified with the activity of a single
galaxy; connected to cluster merger activity
In most cases Relics show an elongated structure
The spectra are quite steep
Frequently found to be linearly polarized at a level of about 10-30%
We have identified in literature + our data 34 clusters with at least
1 Relic source. In 11 clusters we have also a radio H, in 10 we see a
Double R (in 4 cases Double R + H).
Relics: important to study properties in cluster peripheral regions:
ICM physics (shocks) & large scale magnetic fields in low
density regions
Problems:
Many Relics have not been studied or observed in detail, deep
detailed images are still missing
==> size and radio flux density possibly under-estimated
Most of radio spectra have been obtained with flux densities at
only 2 different frequencies. Very few spectral index images
Relics are at the cluster periphery, in many cases even outside
X-Ray images: comparison radio-X not easy.
Difficult to find in present radio surveys (sensitivity and angular
resolution)
Positive: new recent results, e.g.: CIZA J2242.8+5301 (van Weeren et al.
2010), 0217+70 (Brown et al. 2011), ZwCl0008.8+5215(van Weeren et al. 2011)…...
Upcoming sensitive all-sky radio surveys will improve our
knowledge on physical properties in external cluster regions
Morphology
Elongated and peripheral classic relics: 27/34 (79%) [radio gischts]
Diffusive shock acceleration
Extended-diffuse: 7/34 (21%) [radio phoenices]
shock reacceleration of old radio emitting regions
Elongated peripheral relic sources
-- mostly extended
-- asymmetric transversal profile (+spectral index)
-- no dominant internal substructures
-- steep – straight spectrum
-- polarized
VLA C+D data
Giovannini et al. 1991
CIZA J2242.8+5301 “sausage cluster”:
Fig. 1 WSRT radio image at 1.4 GHz.
Colors represent intensity of radio emission; red contours (linearly spaced)
represent the x-ray emission from ROSAT showing the hot ICM.
Published by AAAS
R J van Weeren et al. Science 2010;330:347-349
Not elongated Relics
Rcc
A 13
A548b
200
500
430
A2063
46
Size
250
260
310
40
Rcc
A 85
530
AS753 135
A1664 1350
A4038
45
Size
360
280
1400
60
circular shape
smaller size and distance with respect to the cluster
center (but see A1664)
steep (curved) spectrum
Substructure: filaments, polarizazion
In some clusters cD at the cluster center (connection?)
A85
VLA 90 cm
z=0.0555
Size 360 kpc
Dcc= 530 kpc
Giovannini & Feretti 2000
A1664 z=0.1276 2.3 kpc/”
Color: X-Ray
Contour: VLA D-array
1.4 GHz
See Govoni et al. 2001
VLA – C array
A 548b
From Feretti et al. 2006
Radio (contour) + X-ray (color)
Diffuse emission 0917+75
Old diffuse emission from a dead radio galaxy?
Relic radio source?
Radio emission from a galaxy filament?
Studied in detail by:
Dewdney et al. 1991 (radio halo)
Harris et al. 1993 (Relic: shape + polarization)
Giovannini & Feretti 2000, peculiar, very far (>4Mpc)
from the nearest rich cluster
New data here
VLA 1.4 GHz D array
Size: 1.7 Mpc
Heq = 0.5-0.7 10-6 G
logP1.4 = 24.61 W/Hz
Not detected by ROSAT
and XMM
H > 0.99 10-6 G
(Chen et al. 2008)
VLA 5 GHZ
1.5
0.8
1.8
1.3
1.3
Spectral index 1.4 – 5 GHz
A748
VLA 1.4 GHz
A786
300 kpc
Filament of galaxies in the region of the
Radio emission: zphot is 0.138 (Girardi pc)
This filament could connect A786 and
A748 nearby clusters at the same
distance
<%> 17%
Preliminary result
Radio halos
Relics
From Giovannini
et al. 2009
CONCLUSIONS
• The number of known Relics is increasing however better data
are necessary for many of them
• Different morphology and distance from the cluster center could
be correlated with different properties
• Elongated peripheral Relics have been found in 11 clusters where
also a halo source is present. In 9 clusters two similar Relics,
located symmetrically with respect to the cluster center, are
present. But in A548b: two extended Relics on the same side
• Relics located near the FRG have a smaller size and very steep
curved radio spectrum. They show a round shape with evident
filaments, but A1664 and A548b shows a round extended shape
in a peripheral region
• There is increasing evidence that Relics are tracers of shock
waves in merger events
• Relic radio Power shows a correlation with the X-Ray Luminosity
similar to the correlation found for Halo sources. Relics have a
higher Radio power than Halos with the same X-ray Luminosity
(shocks are more efficient than turbulence?)
• The next generation of radio telescopes (SKA + pathfinders) are
necessary to study Relics and properties of the medium in low
density regions
• Relics are important to understand the presence of large scale
magnetic fields in low density regions
THANK YOU