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J. Astrophys. Astr. (xxxx) xx, 000–000
LOFAR and APERTIF surveys of the radio sky:
probing shocks and magnetic fields in galaxy
clusters
Huub R¨ottgering
1∗
,Jose Afonso
2
,Peter Barthel
3
,
Fabien Batejat
4
,Philip Best
5
,Annalisa Bonafede
6
,
Marcus Br¨uggen
6
,Gianfranco Brunetti
7
,Krzysztof Chy˙zy
8
,
John Conway
9
,Francesco De Gasperin
10
,Chiara Ferrari
11
,
Marijke Haverkorn
1,12,17
,George Heald
12
,Matthias Hoeft
13
,
Neal Jackson
14
,Matt Jarvis
15
,Louise Ker
5
,
Matt Lehnert
16
,Giulia Macario
7
,John McKean
12
,
George Miley
1
,Raffaella Morganti
3,12
,Tom Oosterloo
3,12
,
Emanuela Orr`u
17
,Roberto Pizzo
12
,David Rafferty
1
,
Alexander Shulevski
3
,Cyril Tasse
16
,Ilse van Bemmel
12
,
Bas van der Tol
1
,Reinout van Weeren
1
,Marc Verheijen
3
,
Glenn White
18
, Michael Wise
12
, on behalf of the LOFAR collaboration
1
Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
2
Observat´orio Astron´omico de Lisboa, Faculdade de Ciˆencias,
Universidade de Lisboa, Tapada da Ajuda, 1349-018 Lisbon, Portugal
3
Kapteyn Instituut, Landleven 12, 9747 AD Groningen, The Netherlands
4
Chalmers University of Technology, Onsala Space Observatory, SE 439 92 Onsala, Sweden
5
Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
6
Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
7
INAF, Istituto di Radioastronomia, Via P Gobetti 101, IT 40129, Bologna, Italy
8
Jagiellonian University, ul. Orla 171 30-244 Krak´ow POLAND
9
Chalmers University of Technology, Onsala Space Observatory, SE 439 92 Onsala, Sweden
10
Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschildstraße 1, 85741 Garching, Germany
11
UNS, CNRS UMR 6202 Cassiop´ee, Observatoire de la Cote d’Azur, Nice, France
12
ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
13
Th¨uringer Landessternwarte, Tautenburg
14
Jodrell Bank Centre for Astrophysics, University of Manchester,
Turing Building, Oxford Road, Manchester M13 9PL, UK
15
Centre for Astrophysics, University of Hertfordshire, Hatfield, Herts, UK
16
Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France
17
Radboud University Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
18
Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA,
Space Science and Technology Department,
STFC Rutherford Appleton Laboratory, Chilton, OX11 0QX, UK
Received xxx; accepted xxx
1
arXiv:1107.1606v1 [astro-ph.CO] 8 Jul 2011
2 Huub R¨ottgering
Abstract. At very low frequencies, the new pan-European
radio telescope LOFAR is opening the last unexplored window
of the electromagnetic spectrum for astrophysical studies. The
revolutionary APERTIF phased arrays that are about to be in-
stalled on the Westerbork radio telescope (WSRT) will dramati-
cally increase the survey speed for the WSRT. Combined surveys
with these two facilities will deeply chart the northern sky over
almost two decades in radio frequency from ∼ 15 up to 1400
MHz. Here we briefly describe some of the capabilities of these
new facilities and what radio surveys are planned to study fun-
damental issues related the formation and evolution of galaxies
and clusters of galaxies. In the second part we briefly review
some recent observational results directly showing that diffuse
radio emission in clusters traces shocks due to cluster mergers.
As these diffuse radio sources are relatively bright at low frequen-
cies, LOFAR should be able to detect thousands of such sources
up to the epoch of cluster formation. This will allow address-
ing many question about the origin and evolution of shocks and
magnetic fields in clusters. At the end we briefly review some of
the first and very preliminary LOFAR results on clusters.
Key words: Galaxies: clusters: general, intracluster medium;
Radio continuum: galaxies; Radio telescopes
1. Introduction
At low frequencies, the new pan-European radio telescope LOFAR is opening
the last unexplored window of the electromagnetic spectrum for astrophys-
ical studies. The revolutionary APERTIF phased arrays that are about to
be installed on the Westerbork radio telescope (WSRT) will dramatically
increase the survey speed for the WSRT. The resulting vast area of new ob-
servational parameter space will be fully exploited for many studies directly
related to the formation of massive black holes, galaxies, and clusters. Par-
ticularly important are three research areas that are driving the design of
several surveys that are planned to be carried out with these new facilities.
These areas are: (i) forming massive galaxies at the epoch of reionisation, (ii)
magnetic fields and shocked hot gas associated with the first bound clusters
of galaxies, and (iii) star formation processes in distant galaxies. Further-
more, a most exciting aspect of LOFAR is that its enormous instantaneous
field of view coupled with its unprecedented sensitivity at low frequencies
∗
e-mail:rottgering@strw.leidenuniv.nl
Northern radio surveys and shocks and magnetic fields in clusters 3
equips LOFAR for the discovery of new classes of rare extreme-spectrum
sources.
In this contribution, we will first briefly describe LOFAR and APERTIF.
For a more extended description of LOFAR we refer to the contribution of
George Heald that extensively describes LOFAR and the way the data will
be handled to form deep images at low frequencies. Second, we will discuss
how the prime science drivers led to the definition of the planned contin-
uum surveys with LOFAR and APERTIF. Third, we briefly review some
of the work we have been carrying out to understand diffuse radio emis-
sion associated with merging clusters. We will mainly concentrate on some
of the statistical results obtained for a partly new sample of relics showing
correlations between their sizes, spectral indices and distances from the clus-
ters centres. In the contribution of Reinout van Weeren, he will high-light
recent results for newly discovered individual relics, including the recently
discovered spectacular double relics in the cluster CIZA J2252.8+5301 (van
Weeren et al. 2010). Finally, a few preliminary results from LOFAR obser-
vations mainly related to clusters are briefly presented. These results show
the enormous potential that LOFAR has for studying shocks and magnetic
fields in clusters.
2. LOFAR
LOFAR, the Low Frequency Radio Array, is a pan-European radio telescope
that is currently being commissioned. Its revolutionary design makes use of
phased array technology. This replaces the traditional and expensive me-
chanical dishes by a combination of simple receivers and modern computing
equipment. LOFAR has two types of antennas, one optimised for the 30 - 80
MHz range and one for the 110 - 240 MHz range. The antennas are grouped
together in stations the size of soccer fields. The signals from the antennas
will be digitised so that many beams on the sky can be formed. This makes
LOFAR an extremely efficient instrument to survey large areas of sky. The
Dutch part of the array will be finished in 2011 and will comprise 40 stations
distributed over an area of diameter of 100 km. In addition, in 2011 eight
stations in a number of European countries (Germany, UK, Sweden, and
France) are planned to be operational. Currently many functional elements
of the LOFAR imaging system are in place. These elements include: (i) sta-
tion beam formation, (ii) high speed data transport, (iii) software correlator
to produce visibilities, (iv) calibration algorithms, and (v) wide field map
making. Although a significant amount of both continued commissioning
and technical research will be needed to obtain maps with the theoretical
noise levels, the maps that are currently produced already are the deepest
ever at these low frequencies.
4 Huub R¨ottgering
With its unique design, LOFAR will provide enormous improvements
over previous facilities in the following three regions of parameter space:
• Very Low Frequencies, with 2 - 3 orders of magnitude improvement in
both sensitivity and angular resolution. This is a mostly unexplored
spectral region that is uniquely sensitive to ultra-steep spectrum z > 6
radio galaxies, diffuse emission from clusters and the oldest ‘fossil’
synchrotron electrons.
• Size of the Instantaneous Field of View, of many tens of square de-
grees. This will deliver a transformational increase in speed to survey
the radio sky, crucially important for the quest for rare objects such
as distant clusters, proto-clusters and z > 6 radio galaxies and rare
transient phenomena.
• Low-Frequency Radio Spectroscopy, enabling studies of redshifted neu-
tral hydrogen at the Epoch of Reionisation.
The design of LOFAR is very versatile and has led to the development of 6
key science projects, related to cosmic rays, epoch of reionisation, transients
and pulsars, cosmic magnetism, the Sun, and extragalactic surveys. In this
contribution, we will focus on LOFAR and APERTIF surveys to probe the
extragalactic sky.
3. APERTIF
APERTIF, the new Phased Array Feed receiver system for the Westerbork
Synthesis Radio Telescope (WSRT) will dramatically enlarge the instanta-
neous Field-of-View of the WSRT (see Oosterloo et al 2010 for a detailed
description). This is done by replacing the current single Frontend Feeds
with Phased Array Feeds (PAFS). Each of the PAFs consists of 121 Vivaldi
elements and will detect the radiation field (in dual polarisation) in the focal
plane of each dish over an area of about one square meter. Because of this,
many beams can be formed simultaneously for each dish making it possible
to image an area of about 8 square degree on the sky, which is an increase
of about a factor 30 compared to the current WSRT. Its large 300 MHz
bandwidth will not only cater for sensitive continuum imaging, but is also
crucial for efficient HI and OH emission surveys and for studies of polarised
emission from large areas.
4. Transformational radio surveys
4.1 LOFAR
The three fundamental areas of astrophysics that have driven the design of
the planned LOFAR surveys are: (i) forming massive galaxies at the epoch
Northern radio surveys and shocks and magnetic fields in clusters 5
of reionisation, (ii) magnetic fields and shocked hot gas associated with the
first bound clusters of galaxies, and (iii) star formation processes in distant
galaxies. The areas, depths and frequencies of the surveys have been chosen
so that they would contain: (i) 100 powerful radio galaxies close to or at
the epoch of reionisation, (ii) 100 radio halos at the epoch when the first
massive bound galaxy clusters appeared, and (iii) 100 proto-clusters. The
resulting survey parameters are based on estimates of luminosity functions
for powerful radio galaxies by Wilman et al. 2008, for radio halos by Enßlin &
R¨ottgering 2002 and Cassano et al. 2010, and for proto-clusters by Venemans
et al. 2007. To achieve the goals of the LOFAR surveys, a three-tiered
approach has been adopted (for details see R¨ottgering et al. 2010). Tier-1
represents the all sky survey at frequencies 15, 30, 60 and 120 MHz. Tier-
2 are the medium deep surveys over 1000 sqr deg at 30, 60, 120 and 200
MHz, while Tier-3 encompasses about 100 sqr degrees down to an extreme
depth of 6 µJy rms at 150 MHz. The resulting depth versus frequency is
given in Fig. 1. In addition, very deep data will be taken on a selected
sample of 60 nearby clusters. Another important motivation of LOFAR
is to provide the entire international astronomical community with unique
surveys of the radio sky that have a long-lasting legacy value for a broad
range of astrophysical research. The international LOFAR survey team has
identified a range of fundamental astrophysical research topics on which
LOFAR surveys will have an important impact. These topics include (i)
the formation and evolution of large scale structure of the Universe, (ii)
the physics of the origin, evolution and end-stages of radio sources, (iii) the
magnetic field and interstellar medium in nearby galaxies, and (iv) Galactic
sources such as supernova remnants, HII regions, exoplanets and pulsars.
4.2 WODAN
The extremely large field of view of APERTIF would enable the WODAN
(Westerbork Observations of the Deep APERTIF Northern-Sky) project.
This project aims to chart the entire accessible northern sky at 1400 MHz
down to 10 µJy rms and about 1000 deg
2
down to 5 µJy. WODAN will be
an important compliment to the EMU (Evolutionary Map of the Universe)
project. EMU will use the phased-array feed (PAF) mounted on the Aus-
tralian SKA Pathfinder (ASKAP, De-boer et al. 2009) to chart the entire sky
south of δ = 30
◦
to a similar depth as WODAN. For a detailed description
of EMU we will refer to the contribution of Ray Norris to this conference.
WODAN and EMU have an enormous synergy with the LOFAR surveys:
virtually all the 5 × 10
7
radio sources from the LOFAR all sky surveys will
have their flux density at 1400 MHz measured. It will yield radio data
for all radio loud AGN, and most luminous starbursts up to z = 2. The
resulting densely populated radio color-color diagrams will be a powerful
tool to spectrally discriminate between very rare radio sources with extreme
6 Huub R¨ottgering
Figure 1. Flux limits (5 sigma) of the proposed LOFAR and APERTIF surveys
compared to other existing radio surveys. The triangle represent existing surveys:
HDF (VLA Richards et al. 2000; WSRT Garrett et al. 2000), WENSS, NVSS, 6C,
VLSS and 8C. The lines represent different power-laws (S ∼ ν
α
, with α = −1.6
and −0.8) to illustrate how, depending on the spectral indices of the sources, the
LOFAR surveys will compare to other surveys.
radio spectra such as diffuse emission from clusters and very distant radio
galaxies. For nearby resolved sources it will instantly yield spectral index
and spectral curvature maps, a very rich source of information to constrain
many physical parameters. As the combined surveys will cover the entire sky,
measurements of the Integrated Sachs-Wolfe effect, galaxy auto-correlation
functions and cosmic magnification will significantly tighten cosmological
model parameters (Raccanelli et al. submitted).
5. LOFAR and diffuse radio emission from clusters of galax-
ies
Clusters of galaxies are large ensembles of hundreds of galaxies embedded in
hot gas and held together by gravity. Besides the hot thermal gas observed
in X-rays, the intra-cluster-medium (ICM) contains relativistic electrons (E
≈ Gev) and magnetic fields (1 − 10µG), which have been detected via syn-
chrotron emission in the radio band. LOFAR is uniquely suited to probe
Northern radio surveys and shocks and magnetic fields in clusters 7
these synchrotron emitting regions and will address many questions related
to the large-scale magnetic fields and relativistic particles mixed with the
thermal ICM. These questions include: What are the strengths and topolo-
gies of the magnetic fields? When and how were the first magnetic fields gen-
erated? How were magnetic fields subsequently amplified and maintained?
Furthermore, diffuse radio sources in galaxy clusters are likely to be di-
rect signatures of huge shock waves caused by massive cluster mergers. These
shocks have a crucial impact on the energetics and detailed temperature dis-
tribution of the cluster gas. LOFAR observations are therefore very relevant
for studies of the evolution of the energy content of both the thermal and
non-thermal gas in the cluster. Some of the most prominent nearby clusters
of galaxies host such diffuse synchrotron emitting radio sources. Classical
examples of spectacularly large (∼ 1 Mpc) diffuse cluster emission have
been found for the Coma cluster, Abell 2256 and Abell 3667. The proper-
ties of the associated clusters are extreme: they are very X-ray luminous,
have high temperatures (kT > 7 keV), large masses (> 10
15
M
), and high
galaxy velocity dispersions. The overall properties are indicative of the vio-
lent merging of sub-clusters, an important process in the assembly of massive
clusters. Diffuse radio emission associated with clusters of galaxies has been
classified into three groups: relics, halos and phoenixes (e.g. Giovannini &
Feretti 2004).
Cluster relics are large elongated diffuse structures at the periphery of
clusters. Recently we have discovered a spectacularly long and narrow relic
with a size of 2.0 Mpc × 50 kpc, located at a distance of 1 Mpc from the
centre of the merging cluster CIZA J2242.8+5301 (van Weeren et al. 2010).
The relic displays highly aligned magnetic fields and a strong spectral index
gradient due to cooling of the synchrotron emitting particles in the post
shock region. We have argued that these observations provide conclusive
evidence that shocks in merging clusters produce extremely energetic cosmic
rays. Detailed modelling of the morphology, polarization properties and
variations of the radio spectrum, allowed us to determine the strength of
the magnetic field (5 µG) and the Mach number (4.6
+1.3
−0.9
) of the shock. Our
numerical simulations indicated that the impact parameter of the cluster
collision was about zero and the mass ratio of the colliding clusters was
roughly 2:1 (van Weeren et al. in prep.).
Cluster radio halos are located at the centres of clusters, their diffuse
morphologies following that of the X-ray emission. The origin of the halos
is not understood. Especially their enormous ∼ 1 Mpc sizes pose problems.
The radiative lifetimes of the synchrotron emitting electrons are so short
that the electrons need to have been accelerated to relativistic speeds close
to the place where they radiate. Although many explanations have been put
forward, a currently favoured one is that turbulence due to cluster mergers
is capable of accelerating electrons to relativistic speeds (e.g. Brunetti et al.
2001). Alternatively, relative electrons could be produced when relativistic
8 Huub R¨ottgering
protons from AGNs in the cluster collide with thermal protons within the
cluster gas (Dennison, 1980). A second important issue relates to the origin
of the magnetic fields (e.g. Dolag 2006). Are they primordial in origin and
have turbulent processes subsequently amplified the fields? Or have outflows
from active galaxies or starburst galaxies transported magnetic fields into
the inter-galactic medium?
Radio phoenixes are suggested to be due to shocks in the cluster gas
that would adiabatically compress old radio plasma ejected by former active
galaxies. The resulting diffuse objects would have an extremely steep radio
spectrum making them relatively bright at low radio frequencies (Enßlin and
Kopal-Krishna 2001). Simply considering the timescales related to the AGN
activity, synchrotron losses, and the presence of shocks we recently argued
that such sources could determine the general appearance of clusters in low
frequency LOFAR maps (van Weeren, et al. 2009a).
Because these radio sources associated with cluster wide shocks are dif-
fuse, have low luminosities and steep radio spectra, they are difficult to
detect with conventional radio observatories, such as Westerbork. As a re-
sult there are only about 50 cluster radio sources currently known. Due to
its extreme sensitivity at low radio frequencies, LOFAR will be the break-
through instrument for this field of research (Cassano et al. 2010). For the
first time, the occurrence and characteristics of diffuse cluster radio sources
will be measured as a function of cluster properties up to the epoch at which
the first massive clusters assembled (z ∼ 1). This will directly show the ef-
fects of shock waves on the evolution of the cluster gas and magnetic fields,
and test predictions that cluster merging is rampant at high redshift. De-
tailed LOFAR maps of rotation measures, polarization properties and radio
spectra of nearby halos will distinguish between the various physical models
for the origin of the diffuse radio emission. It also will probe radio AGN
activity over long time scales, important for studies of the radio feedback
processes in clusters. With the LOFAR observations, we will address the
following questions:
• What are the properties of the cluster-wide shocks (rate of occurrence,
volume filling, geometry, Mach numbers)? How do they accelerate
particles?
• What are the characteristics of the magnetic fields (strength, topol-
ogy)? And how do these relate to models of the origin of the fields?
• What is the total energy input into the cluster medium by radio loud
AGN?
• What are the properties of the merging clusters (mass ratios, impact
parameters) as can be directly deduced by the relic morphologies?
• How do the properties of merging clusters evolve over cosmic time?
Northern radio surveys and shocks and magnetic fields in clusters 9
6. Towards a sample of relics, a prelude to LOFAR
As discussed, detailed radio observations of individual relics clearly suggest
that relics originate in shocks induced by merging clusters. This scenario
can be further tested by studying larger samples of relics. From GMRT,
WSRT and VLA observations of a sample of diffuse radio sources from the
74 MHz VLSS survey with spectral indices α < −1.7, 5 new relics were
discovered. A comparison of the NVSS and WENSS radio catalogues with
the ROSAT all sky catalogue, 5 additional relics were found. Combined
with 17 known relics from the literature, the resulting sample was large
enough for a statistical study. For details we refer to van Weeren et al.
2009b. For this sample, we found that larger relics are mostly located in the
cluster periphery, while smaller relics are found closer to the cluster center.
We also discovered an anti-correlation between the steepness of the spectral
index and the physical size of the relics. A likely explanation for these two
correlations is that the larger shock waves occur mainly in lower-density
regions. The larger shocks then have larger Mach numbers translating into
flatter radio spectra. As larger relics are also more luminous, this then also
explains that within this sample the more luminous radio relics have flatter
spectral indices. Finally, there is a tendency for the steep spectrum relics
to show more spectral curvature. This would provide evidence for spectral
ageing due to inverse compton and/or synchrotron losses. We note however
that some of the smallest relics might be due to the compression of fossil
AGN radio plasma. Their very steep and curved spectrum sources are also
consistent with this scenario.
6.1 LOFAR and cluster observations: the rich cluster of galaxies Abell 2256
Abell 2256 is a rich X-ray cluster at z = 0.058 that has undergone a merging
event estimated to have happened 0.3 Gyr ago (e.g. Miller et al 2003). Apart
from 9 tailed sources, it rather exceptionally contains three classes of diffuse
cluster radio sources: relics, halos and phoenixes. The northern relics have
been discovered a long time ago (Bridle and Fomalon 1976) and were studied
in detail by Clarke and Enßlin (2006). They also clearly showed that A2256
possesses a central halo with a luminosity following the X-ray - radio halo
luminosity relation (Liang et al. 2000). In very deep 325 MHz GMRT radio
maps van Weeren et al. (2009a) recently discovered three diffuse elongated
radio sources with extremely steep spectral indices located about 1 Mpc
from the cluster center. These properties indicate that these objects can be
classified as phoenixes.
As A2256 is one of the most luminous radio emitting clusters showing
so many intriguing characteristics, it was one of the prime candidates to be
observed during early commissioning of LOFAR (see also R¨ottgering et al.
2010; Heald et al. 2010). It was observed in the HBA band in May 2010
10 Huub R¨ottgering
for about 8 hours. The data were taken with 10 core stations and 5 remote
stations and the observed frequencies ranged from 115 to 165 MHz. An
image from 18 subbands covering a total of 4 MHz of bandwidth around 135
MHz was made (see Figure 2). The resolution of the image is 31 × 19 arcsec
and the noise is ∼ 5 mJy/beam. So far, the deepest images at low frequencies
have been obtained with the GMRT at 150 MHz (Intema 2009, Intema et
al. submitted, Kale and Dwarakanath 2010). The GMRT image clearly
shows the relic and several of the head tail galaxies that are also visible
on the LOFAR image. The GMRT image recovers the central part of the
halo emission. With LOFAR’s very sensitive central core, the full extent of
the halo is visible, showing LOFAR’s power to study diffuse steep spectrum
emission from clusters. Next steps in improving this image are reducing the
data from all the 256 sub-bands, and the application of more sophisticated
data reduction algorithms. These include proper wide-field imaging taking
the varying station beams into account, iteration of self-calibration/peeling
loops, and removal of ionospheric corrections following the “SPAM” method
(Intema et al. 2009). Finally, recently we have observed A2256 in the
lowband and produced images at 20, 30 and 49 MHz. A spectral map from a
combination of 49 MHz and the 350 MHz WSRT data (Brentjens et al. 2008)
very nicely spatially resolved the extremely streep spectrum central radio
halo from the flatter spectrum northern relics. We also recently obtained
data on Coma A and A2255. The commissioning team is working very hard
to obtain excellent images.
Acknowledgements C. Ferrari acknowledges financial support by the Agence
Nationale de la Recherche through grant ANR-09-JCJC-0001-01.
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