arXiv:astro-ph/0212154v1 6 Dec 2002
PKS B1400−33: an unusual radio relic in a poor cluster
Australia Telescope National Facility, CSIRO, Locked bag 194, Narrabri, NSW 2390,
A. J. Beasley
Owens Valley Radio Observatory, California Institute of Technology, Pasadena, CA 91125
W. M. Goss and K. Golap
National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801
R. W. Hunstead
School of Physics, University of Sydney, NSW 2006, Australia
We present new arcminute resolution radio images of the low surface bright-
ness radio source PKS B1400−33 that is located in the poor cluster Abell S753.
The observations consist of 330 MHz VLA, 843 MHz MOST and 1398 and 2378
MHz ATCA data. These new images, with higher surface brightness sensitiv-
ity than previous observations, reveal that the large scale structure consists of
extended filamentary emission bounded by edge-brightened rims. The source is
offset on one side of symmetrically distributed X-ray emission that is centered on
the dominant cluster galaxy NGC 5419. PKS B1400−33 is a rare example of a
relic in a poor cluster with radio properties unlike those of most relics and halos
observed in cluster environments.
The diffuse source appears to have had an unusual origin and we discuss
possible mechanisms. We examine whether the source could be re-energized relic
radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC
5419. The more exciting prospect is that the source is relic plasma preserved
in the cluster gaseous environment following the chance injection of a radio lobe
into the ICM as a result of activity in a galaxy at the periphery of the cluster.
Subject headings: galaxies: clusters — radio continuum: galaxies — radio sources:
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Extended emission structures with steep radio spectral indices, which are not obviously
associated with any optical galaxy, are sometimes observed in rich cluster environments.
These sources are broadly separated into cluster wide ‘halos’ that appear relaxed and sym-
metric with respect to the X-ray intracluster gas, and the peripheral arc-like structures
referred to as ‘relics’. Both types are predominantly observed to be associated with hot
(TX ≥ 6 keV), X-ray luminous (LX > 4 × 1044erg s−1) rich compact clusters (Schuecker
and B¨ ohringer 1999); however, the clusters that have peripheral relics may have somewhat
lower temperatures (Feretti 1999). A standard hypothesis is that these sources are relic
synchrotron plasma that has been revived.
PKS B1400−33 is an extended radio source, with an 85 MHz flux density of 57 Jy (Mills,
Slee and Hill 1960), and has the lowest surface brightness of any source in the Parkes catalog.
PKS B1400−33 appears to be associated with the poor cluster S753 of Abell richness class
0; however, the source not been conclusively identified with any optical galaxy. Previous low
frequency images of the source using the VLA and the MOST (Goss et al.
a relatively bright rim along the NE edge and low surface brightness filamentary emission
trailing off to the SW. The source was observed to have a steep spectrum with spectral index
α (Sν∝ ν−α) in the range 1.2-2.4.
The unusual nature of this possible relic radio source and its potential value as a probe
of the gas dynamics and evolution of poor cluster environments has led us to carry out new
radio observations of the source. We have made images with improved surface brightness
sensitivity at 330 MHz using the Very Large Array (VLA), at 843 MHz with the Molonglo
Observatory Synthesis Telescope (MOST) and at 1398 and 2378 MHz using the Australia
Telescope Compact Array (ATCA). Table 1 is a summary of the new radio observations.
These are presented here followed by discussions on the origin of this source in the light of
the current understanding of the phenomenology and formation of relics in the intracluster
2.The radio continuum images
PKS B1400−33 was observed at 330 MHz with the VLA separately in the CnB, C,
and DnC array configurations in order to image the extended emission in this low surface
brightness source. Hybrid arrays were included because of the southern declination. The
data were self calibrated and imaged in aips using three-dimensional imaging routines. The
VLA 330 MHz image of the source is shown in Fig. 1. The image has been made combining
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data from all the configurations with a beam of FWHM 77 × 53 arcsec2at a P.A. of −24◦;
the r.m.s. noise is 2.5 mJy beam−1. This radio image, as well as all the higher frequency
radio images presented here, has been corrected for the primary beam attenuation.
Because of its steep spectrum, the total extent of the diffuse source is best defined by
the 330 MHz VLA image. The low surface brightness source is bounded to the NE by a
relatively bright rim of emission (marked NE rim in Fig. 1), while the source is bounded
on the opposite SW side by a second fainter ridge of emission (marked SW rim in Fig. 1).
Both the bounding rims are concave outwards. The emission that lies between the bounding
ridges is filamentary and decreases in surface brightness from NE towards the SW. The bright
compact source that is observed in the image close to the NW edge of the extended source,
at RA 14h03m38.s7, DEC −33◦58′39′′(J2000.0), has a flux density of 0.95 Jy at 330 MHz; the
compact source appears slightly resolved and has a deconvolved size of about 25 arcsec. As
discussed below, this compact component is identified with the galaxy NGC 5419. There is
evidence for a protrusion towards the SW beyond the bounding rim; this extension, marked
P in Fig. 1, is observed in the 330 MHz image at a level of ∼ 20 mJy beam−1. The total
flux density of the extended radio source PKS B1400−33 is 8.5 Jy at 330 MHz, excluding
the emission from NGC 5419 and the source 5.′5 to its south, marked B in Figs. 1-4, that
is presumably an unrelated confusing source. The error in the absolute flux density scale at
330 MHz is believed to be about 2 %.
The 843 MHz MOST image of PKS B1400−33 is shown in Fig. 2. This image, obtained
using the most sensitive 23′field of view, better reproduces the extended structure than the
843 MHz image in Goss et al. (1987). The image has been made with a beam of FWHM
77×43 arcsec2at a P.A. of 0◦and has an r.m.s. noise of 1 mJy beam−1. The 843 MHz image of
the extended source shows the relatively bright rim along the NE, the filamentary structures
trailing from this ridge towards the SW and the curved rim that defines the boundary of
the diffuse source at the SW end. The flux density of the extended source (again excluding
NGC 5419 and the embedded backgound source B) is 1.3 Jy at 843 MHz. NGC 5419 has a
flux density of 0.46 Jy at 843 MHz. Additionally, the 843 MHz image clearly reveals a local
emission peak embedded within the diffuse emission at RA 14h03m55.s9 DEC −34◦06′03′′
(J2000.0); we hereinafter refer to this feature as component C. It has a flux density of 13
mJy beam−1at 843 MHz. The flux density scale in the MOST image is accurate to 5 %.
PKS B1400−33 was observed using the 375 m and the 750B 750 m array configurations
of the ATCA during 1995 Jan and June. Observations were made in two 128 MHz wide
bands centred at 1398 and 2378 MHz; the bands were covered using 13 channels. The
extended source was covered with a nine-pointings mosaic with the pointings spaced 8′apart
in RA and DEC. Continuum images at the two frequencies were made separately using
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the channel data, adopting a bandwidth synthesis approach to avoid bandwidth smearing
effects; the mosaic imaging was carried out in aips++. Deconvolution used the multi-scale
CLEAN algorithm: the bright compact source at the edge of the extended emission was
first removed from the ‘dirty’ image using a ‘box’ CLEAN and subsequently a multi-scale
CLEAN was performed on the residual image. The final images at the two frequencies were
then convolved to identical beams of FWHM 70 × 45 arcsec2at a P.A. of 0◦. Consequently,
different weightings were adopted at the two ATCA frequencies during the imaging step;
however, the convolution to identical final beams implies that the images presented here
effectively have the same visibility range. ATCA mosaic images of the PKS B1400−33 field
at 1398 and 2378 MHz are shown in Figs. 3 and 4 respectively. The r.m.s. noise is 0.5
mJy beam−1in both images. The relatively bright ridge along the NE and the filamentary
emission trailing towards SW have been imaged at 1398; however, at 2378 MHz, the ridge
appears non-uniform and clumpy and the filamentary emission is undetected, possibly due
to the poorer signal-to-noise ratio. The bounding ridge at the SW of the extended source is
not detected in either of these higher frequency ATCA images. The wide-field mosaic images
show several continuum sources, presumably unrelated, in the field. Based on the ATCA
data, the flux density of NGC 5419 is 0.30 and 0.23 Jy at 1398 and 2378 MHz respectively.
The total flux density of the extended source PKS B1400−33 is 0.46 and 0.10 Jy at 1398
and 2378 MHz. The central component C is detected in both the ATCA images with flux
density 5 and 2 mJy beam−1at 1398 and 2378 MHz respectively. The absolute flux density
scale in the ATCA observations was set using observations of PKS B1934−638 whose flux
density is known, relative to sources in the northern hemisphere, with an uncertainty of 2
The parameters of the radio images presented here are in Table 2. PKS B1400−33 has
an angular size of approximately 24′×14′. Assuming that the source is at the distance of the
cluster S753, which is 40h−1Mpc from the Sun (h = H◦/100, where H◦is the Hubble constant
in km s−1Mpc−1), the linear dimensions of the extended source are approximately 280h−1
kpc × 160h−1kpc. The 1.4 GHz radio luminosity of the extended source is 2.2h−2×1022W
3. The distribution of the radio spectral index
The radio spectra of the extended emission and of the compact source associated with
NGC 5419 are shown in Fig. 5. Apart from the measurements at 330, 843, 1398 and 2378
MHz reported here, the plot includes previous estimates of the flux densities of the compact
and extended sources in Goss et al. (1987) as well as our estimates of the 85 and 408 MHz
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Fig. 5.— Radio spectra of the extended emission (upper panel) and the compact source
associated with NGC 5419 (lower panel). The open circles represent previously published
measurements, while the star symbols show flux density measurements derived from obser-
vations presented in this paper.
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Fig. 6.— Distribution of the spectral index α between 330 and 843 MHz as computed from
images made with a beam of FWHM 80′′. The 1 − σ uncertainty is ±0.06 in α. Contours
are at α values of 2.2, 2.0, 1.8, 1.6, 1.4, 1.2 and 1.0.
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Fig. 7.— 330 MHz radio contours overlaid on a DSS digitization of the UK Schmidt SERC-J
survey image of the field. Contours are at 10 mJy beam−1× (−1, 1, 2, 4, 8, 16, 32, 64).
The compact radio source identified with NGC 5419 is indicated; the candidate host galaxy
discussed in section 7.3 is at the center of the box marked H.
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Fig. 8.— Contours of the ROSAT PSPC broad band X-ray image of the S753 cluster field
overlaid on a greyscale representation of the 330 MHz VLA image shown in Fig. 1. The
X-ray image has been smoothed to a resolution of 2.′2 FWHM and the contours are at 8, 12,
16, 20, 30, 40, 50, 60, 70, 80 and 90 % of the peak.