A&A 373, 106-112 (2001)
DOI: 10.1051/0004-6361:20010581
L. Feretti1 - R. Fusco-Femiano2 - G. Giovannini1,3 - F. Govoni1,4
1 -
Istituto di Radioastronomia - CNR, via P. Gobetti 101, 40129
Bologna, Italy
2 - Istituto di Astrofisica Spaziale CNR, via del Fosso del Cavaliere,
00133 Roma, Italy
3 - Dipartimento di Fisica, Univ. Bologna,
Via B. Pichat 6/2, 40127 Bologna, Italy
4 - Dipartimento di Astronomia, Univ. Bologna, via Ranzani 1,
40127 Bologna, Italy
Received 7 February 2001 / Accepted 11 April 2001
Abstract
New radio data is presented for the rich cluster Abell 2163.
The cluster radio emission is characterized by the presence of a
radio halo, which is one
of the most powerful and extended halos known so far.
In the NE peripheral cluster region, we also detect diffuse elongated
emission, which we classify as a cluster relic.
The cluster A2163 is very hot and luminous in X-ray. Its central region
is probably in a highly non relaxed state, suggesting that this
cluster is likely to be a recent merger. The existence of a radio halo
in this cluster confirms that halos are associated with hot massive
clusters, and confirms the connection between radio halos and
cluster merger processes.
The comparison between the radio emission of the halo and
the cluster X-ray emission shows a close structural similarity.
A power law correlation is found
between the radio and X-ray brightness, with index =0.64.
We also report the upper limit to the hard X-ray emission,
obtained from a BeppoSAX observation.
We discuss the implications of our results.
Key words: radio continuum: general - galaxies: clusters: general - galaxies: clusters: individual: A2163 - intergalactic medium - X-rays: galaxies: clusters
Recent observations of clusters of galaxies have revealed a new and complex scenario in the structure of the intergalactic medium. The clusters are not simple relaxed structures, but are still forming at the present epoch. Substructures, commonly observed in the X-ray distribution of a high number of rich clusters (Henry & Briel 1993; Burns et al. 1994), are evidence of the hierarchic growth of clusters from the merger of poorer subclusters.
An important problem in cluster phenomenology involves cluster-wide radio
halos, whose prototype is Coma C (Giovannini et al. 1993, and references
therein). These are extended diffuse radio sources located at the
cluster centers, with typical sizes of
> 1
h50-1 Mpc, regular shape, steep radio spectra
and no significant polarization.
Diffuse radio sources, named relics, have also been detected
in peripheral regions of the clusters. They are irregular in shape and
generally highly polarized. The
origin and properties of halos and relics are still being
debated. According to
recent suggestions, the cluster merger process
may play a crucial role in
the formation and energetics of these sources
(see Feretti & Giovannini 1996 and references therein, Feretti 1999).
Abell 2163
is a distant (z=0.203, Struble & Rood 1999),
rich cluster, and is one of the hottest
(mean
kT = 12-15 keV, Elbaz et al. 1995; Markevitch et al. 1996),
and most X-ray luminous (
= 6
1045 erg s-1, Arnaud et al. 1992) among known clusters. A BeppoSAX observation
measures a temperature of 10-11 keV out to
,
with a marginally
significant (
)
rise in temperature at larger radii
(Irwin & Bregman 2000).
According to the morphological study of Elbaz et al. (1995),
based on ROSAT data, and the spectroscopic analysis
of Markevitch et al. (1996), based on ASCA data,
the cluster is likely to be a recent merger. A highly nonrelaxed state
in the cluster inner region is confirmed
by the recent Chandra results (Markevitch et al. 2000).
A strong Sunyaev-Zel'dovich (SZ) effect has been reported
in this cluster (Holzapfel et al. 1997; Désert et al. 1998).
We present here the radio data of A2163 obtained with the Very Large Array (VLA). In particular, we analyze in detail the diffuse radio emission whose presence was reported by Herbig & Birkinshaw (1994). We also give data for the extended radio galaxies. In addition, we report the results obtained by the Phoswich Detector System (PDS) on board BeppoSAX to search for hard X-ray radiation.
We adopt H0=50 km s-1 Mpc-1 and q0 = 0.5. With these values, 1 arcsec corresponds to 4.27 kpc at the distance of A2163.
Radio observations were obtained with the VLA in different configurations (see Table 1) at 20 cm, and with the D array at 6 cm. The data were calibrated and reduced with the Astronomical Image Processing System (AIPS), following the standard procedure (Fourier inversion, CLEAN and RESTORE, selfcalibration).
Freq. | Bandwitdh | Array | Date | Duration |
MHz | MHz | min | ||
1365/1465 | 50 | D | 02 Feb. 1998 | 140 |
1365/1465 | 25 | C | 02 Jan. 1999 | 200 |
1365/1465 | 25 | A | 21 May 1998 | 50 |
4885/4835 | 50 | D | 26 Dec. 1997 | 120 |
At 20 cm, images with angular resolutions ranging from
to
were produced.
At the lowest resolution, maps
for the two observing frequencies were produced separately, to get some
information about the spectral index.
We also obtained maps of the polarized intensity in the standard way.
An image at 6 cm was produced with resolution
(
), and a rms noise level of 0.35 mJy/beam.
The radio image of this cluster is shown in
Fig. 1, where the crosses mark discrete unrelated sources
detected in the map at
resolution, with a
brightness higher than 0.2 mJy/beam.
The diffuse emission permeates the cluster center for a
total extent of
, corresponding
to
2.9 Mpc. With this size, the halo in A2163
is the largest known radio halo. It displays a very regular shape,
slightly elongated in the E-W direction. The low brightness
emission does not fade into the noise, but shows rather sharp boundaries.
This is an indication that the whole structure is properly imaged
here.
We made polarization maps of the cluster at 20 cm.
Some polarized flux is detected at a level of 2-4%. Because of the
noise, this should be considered as an upper limit.
At higher angular resolution (Fig. 2), only the central region
of the radio halo is detected. All the discrete sources embedded within
the diffuse emission are easily visible.
The high resolution image reveals the existence of diffuse features
at the periphery of the cluster (labeled D1, D2, D3 and D4 in Fig. 2).
The diffuse emission D1 seems to be coincident with a faint
galaxy. The northern source D2 could be related to a nearby
point-like
source. The two emissions D3 and D4 do not show any obvious
optical counterpart. If they
were both associated with the strong eastern radio
source at position
,
,
which coincides with a faint galaxy, they would form
a wide-angle-tailed (WAT) radio source of about 1.3 Mpc in
size (assuming it is a cluster
radio galaxy). This seems unlikely, since the size would
be quite large for
a WAT (e.g., O'Donoghue et al. 1990) and since
WAT sources are usually located at the cluster center.
Although the above arguments may be weak, we suggest the
reasonable possibility that at least the source D3 is a
diffuse peripheral relic, similar to the source 1253+275 in the Coma cluster
(Giovannini et al. 1991), while the source D4 could be connected either to the
mentioned
point source or to the relic. More observations are necessary to investigate
this point.
Halo | Relic (D3) | |
Flux density - 20 cm (mJy) | ![]() |
![]() |
Largest size (kpc) |
![]() |
![]() |
Spectral index | ![]() |
![]() |
Power - 20 cm (![]() |
![]() |
![]() |
Luminosity (![]() |
13.6 -7.8+26.9 | 10.6 -7.3+26.6 |
Volume (![]() |
![]() |
![]() |
![]() ![]() |
8.2 | 36 |
![]() ![]() |
2.3 | 0.34 |
![]() ![]() |
0.9 | 2.0 |
![]() |
Figure 3: Radio map at 20 cm of the diffuse emission, after subtraction of the discrete unrelated sources. The resolution is the same as in Fig. 1, whereas the noise level is 0.05 mJy/beam. Contour levels are -0.15, 0.15, 0.3, 0.5, 0.7, 1, 2, 3, 5 mJy/beam. |
We obtained a 20 cm image of the radio halo after subtraction of the discrete unrelated sources. To this aim, we produced an image of discrete sources, by selecting from the original data set only the data from the long baselines, which do not contain the extended halo emission. This image was then subtracted from the image of Fig. 1. The halo map obtained in this way is presented in Fig. 3. The diffuse emission surrounding the halo is due to the relic and to the other diffuse features, which are extended and therefore difficult to be subtracted entirely. The parameters for the central radio halo, after subtraction of the unrelated sources, and for the relic are presented in Table 2.
At 6 cm, only the strongest discrete sources within and around the radio halo are well detected. The radio halo is not detected because of missing short spacings. Also the other diffuse features are completely resolved at 6 cm.
Since the radio halo is not detected at 6 cm, we
attempted an estimate of its spectral index by comparing
the images obtained from the two separate
VLA frequencies at 20 cm. Although the frequencies are quite close,
the flux densities are slightly different, and the
estimated spectral index (
)
is
.
This steep spectrum is consistent
with the values obtained in halo sources (e.g. Feretti & Giovannini 1996).
The map of the spectral index is quite noisy, so it is
impossible to reveal any trend across the radio halo.
Radiogal. | Label | RA(J2000) | DEC | S20 | LLS |
h m s | ![]() ![]() ![]() |
mJy | kpc | ||
J1615-062 | T1 | 16 15 40.6 | -06 13 48 |
![]() |
630 |
J1615-061 | T2 | 16 15 41.3 | -06 09 08 | ![]() |
170 |
J1616-062 | T3 | 16 16 05.6 | -06 14 56 |
![]() |
380 |
Three radio sources in this cluster are found to show a tailed structure in the high resolution images. They are labeled T1, T2 and T3 in Fig. 2, and are listed in Table 3. They are all coincident with faint galaxies on the digitized Palomar Sky Survey images. Their radio images at high resolution are presented in Figs. 4-6, where a cross indicates the probable host galaxy. The total flux density and size given in Table 3 are derived from the maps at lower resolution, since some structure is missing in the high resolution images. The 6 cm images show the radio nucleus and only the beginning of the tail, confirming the tailed structure of these radiosources. It is worth noting that all 3 radio galaxies have the tails oriented in the same direction (West). This could be explained by the presence of merger-induced bulk motion of the intergalactic medium, as suggested by Bliton et al. (1998).
In Fig. 7 we show the overlay of the radio and X-ray images,
for a morphological comparison.
The radio image is the same as that presented in Fig. 1. The
X-ray image was obtained with the ROSAT
Position Sensitive Proportional Counter (PSPC)
with a total exposure time of 12 133 s
(Elbaz et al. 1995). The image presented here has been reprocessed with a
more accurate determination of the exposure map, and refers
to the 0.5-2 keV energy range, in order to have a lower background.
It has been corrected for the vignetting effect, and smoothed
with a Gaussian of
.
As noted by Elbaz et al. (1995), the cluster X-ray structure is clearly
non spherical and in particular the shape is
elongated in the E-W direction. There is also some evidence
that the axis of the elliptically shaped distribution
in the central region of the cluster has a different orientation, i.e.
inclined between the E-W and the NE-SW direction.
These morphological features are strickingly similar to those
found in the radio structure, confirming the connection
between hot and relativistic plasma, found in other clusters
(Deiss et al. 1997; Feretti 1999; Liang et al. 2000).
In addition, we note that there appears to be a compact X-ray source
at the location of the radio source D2.
![]() |
Figure 8:
Plot of the radio brightness versus the X-ray brightness,
obtained in cells of
![]() |
The cluster was observed in February 98 for a total exposure time of 109 ksec.
Spatially resolved spectroscopy
derived from the data obtained with the Medium-Energy
Concentrator Sectrometer (MECS) has been published by Irwin & Bregman (2000).
The average temperature between 1.65 and 10.5 keV is found to be
keV.
The azimuthally averaged radial temperature profile shows a rather constant
trend up to a distance of
from the cluster center. This result
agrees with the ASCA result of White (2000) but disagrees strongly with the
result from ASCA and ROSAT by Markevitch et al. (1996), who found a temperature
decrease at large distance.
We have analysed the data from the high energy detector
PDS, operating
in the 15-200 keV energy range, with a field of view of
(FWHM, hexagonal) to search for hard X-ray emission, as detected in the
Coma cluster (Fusco-Femiano et al. 1999; Rephaeli et al. 1999)
and A2256 (Fusco-Femiano et al. 2000) which both show extended radio regions.
The PDS instrument uses the rocking collimator technique for background
subtraction. The background level is the lowest obtained thus far
with high-energy instruments on board satellites and it is very stable,
thanks to the equatorial orbit. No modeling of the time variation of the
background is required (see Fusco-Femiano et al. 1999).
The data analysis gives evidence for hard X-ray emission
at a confidence level of
,
with a
count rate of
c s-1 in the PDS
energy range. The data is well fitted by a thermal component at the
average cluster temperature of 11 keV. For the non-thermal emission,
an X-ray flux upper
limit of
erg cm-2 s-1 in the 20-80 keV
energy range is derived.
This upper limit is about 4 and 2 times lower
than the non-thermal flux measurements obtained
in the same energy range for
Coma (Fusco-Femiano et al. 1999) and
A2256 (Fusco-Femiano et al. 2000), respectively.
The diffuse halo in A2163 is more powerful and more extended than the
prototype halo source in the Coma cluster, Coma C. Like Coma C,
it is rather regular in shape.
The properties of radio halos and relics in clusters have been
recently reviewed by Feretti (2000) and Giovannini et al. (2000).
The formation of radio halos has been found to be strictly related
to the X-ray properties of the host clusters and to the presence
of cluster merger processes. In particular, the
percentage of clusters showing radio halos is much higher in
a sample of X-ray
luminous clusters (30% for
> 1045 erg s-1)
than in a complete cluster sample (
5%).
The clusters hosting a diffuse source have a
significantly higher X-ray luminosity than
clusters without a diffuse source.
Moreover, the radio properties of halos are linked to the properties
of the X-ray emitting gas through correlations (Feretti 2000; Liang et al.
2000): the halo monochromatic
radio power correlates with the X-ray luminosity, the X-ray temperature and
the cluster mass. A correlation is also found between the largest
radio size of halos and relics and the cluster X-ray luminosity,
with more X-ray luminous clusters hosting larger sources.
The properties of the radio halo in A2163 are fully consistent with the above suggestions. Indeed, this cluster is very hot and massive, and hosts the most powerful and extended halo known. From the temperature maps obtained with ASCA and recently with Chandra et al. (1996) and Markevitch et al. (2000) inferred that the cluster inner region is in a highly nonrelaxed state, with shocks or streams of shock-heated gas, probably due to a recent merger. This is also confirmed by Squires et al. (1997) from the study of the distribution of cluster light, thermal gas and dark matter. The merger process in this cluster is likely to provide the energy necessary for particle reacceleration, according to a model suggested by Brunetti et al. (2001) for the Coma cluster.
The radio structure of this halo shows close similarity to the structure of the X-ray emitting gas. This is evident from the overlay between radio and X-ray images, and from the correlation between radio and X-ray brightness. The connection between the hot and relativistic plasma could still reflect the connection between the maintenance of the radio halo and the energy provided by the cluster merger, and, when available for a large sample of clusters, could be used to discriminate between models of halo formation (see Govoni et al. 2001).
The presence of a relic in this cluster is very interesting. It is located at about 2.2 Mpc from the cluster center and is oriented roughly perpendicular to the cluster radial direction, like the other well know relics. The particles radiating in the relic could be reaccelerated in a shock produced by the cluster merger, as in A3667 (Roettiger et al. 1999). Unlike the cluster A3667, which has no radio halo at the center, the existence in the same cluster of both a central radio halo and a peripheral relic is not uncommon (Feretti 1999). This points in favor of a common origin for halos and relics, which could be investigated when detailed data on the cluster merger state and evolution will be available from the new generation X-ray telescopes.
Non-thermal hard X-ray radiation is expected in galaxy clusters with radio halos, because of inverse Compton (IC) scattering by the radio emitting relativistic electrons with the CMB photons. The alternative interpretations to the IC model proposed in the literature for the non-thermal emission detected in Coma (Enßlin et al. 1999; Sarazin & Kempner 2000; Blasi & Colafrancesco 1999; Dogiel 2000; Blasi 2000) show serious difficulties, as recently pointed out by Petrosian (2001).
From the PDS data analysis we derive evidence for hard X-ray emission
at a confidence level of
.
This can be
accounted for by the cluster thermal emission, whereas for the non-thermal
emission only an upper limit is derived.
The detection of non-thermal emission in A2163 appears problematic for the
great distance and high gas temperature of this cluster.
Within the framework of the IC model, it is
possible to derive, only using observables (see Rephaeli 1979),
a lower limit to the volume-averaged intracluster
magnetic field combining the synchrotron radio flux with the X-ray
flux upper limit for the non-thermal component (see Sect. 5).
Using the radio data reported in Table 2 for the halo, we
obtain
G. Assuming a radius of
1.4 Mpc for the radio emitting region, at the distance of the cluster,
we derive an upper limit to the electron energy density,
,
of
erg cm-3.
The figures obtained in this way are
consistent with the equipartition
values (see Table 2; note that the minimum energy density in the table
includes the contributions of both the radiating particles and the magnetic
field). The derived limits are not sufficient to establish
whether the radio halo is in equipartition conditions
or not.
Acknowledgements
We are indebted to Monique Arnaud for providing the X-ray ROSAT image, and for helpful discussions. The point-to-point radio-X-ray comparison was performed with the Synage++ package developed by Matteo Murgia. We thank Barry O'Connell for comments on the manuscript. The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation.