DIFFUSE RADIO SOURCES in GROUPS and POOR CLUSTERS

1
DIFFUSE RADIO SOURCES in GROUPS and POOR CLUSTERS
Kisha Delain and Lawrence Rudnick, University of
Minnesota
080939 z0.04, P1.4 1023.1 W/Hz, Lx ?
1040.4 and Lx ? 1040.6 erg/s for the two pieces
(plotted separately below). Top Unfiltered
WENSS in red, DSS galaxies in blue background
AGN from FIRST in yellow. Bottom Right XMM
greyscale with WENSS contours no clear X-ray
detection is seen. Bottom Left DSS image with
diamonds at z0.04, stars at z0.075, circles at
z0.01 and crosses at z0.02.
ABSTRACT We have discovered new diffuse radio
sources likely associated with groups of galaxies
at low redshift (0.01-0.04) and without apparent
AGN by using the WENSS and WISH catalogs to
perform an unbiased survey. These sources
resemble the radio halos and 'relics' of rich
clusters, which are thought to be powered by
shocks and turbulence from infall into their deep
potential wells. Our detection of similar
sources within the shallow potential wells of
groups of galaxies challenges this model. Their
radio luminosities are approximately two orders
of magnitude higher than expected from the
extrapolation of the apparent rich cluster
radio/X-ray luminosity relation. Even if these
sources are misidentified distant clusters, they
would lie above the apparent rich cluster
radio/X-ray luminosity relation in the
literature, suggesting that detections of radio
halos and relics thus far may be more biased than
previously thought.
300 kpc
142135 z0.01, P1.4 1022.4 W/Hz, Lx ? 1041.2
erg/s. Left
Unfiltered (point sources remaining) WENSS in red
with DSS galaxies in green. The middle green
galaxy is S0 galaxy NGC 5580 the upper right
spiral is NGC 5588, neither of which are known
AGN. Middle ROSAT greyscale w/ WENSS contours
showing no clear X-ray emission. Right DSS
field showing all galaxies and nearby groups at
z0.01
Introduction Observationally, there are
difficulties with the Lr-Lx correlation at low
luminosities. For high X-ray luminosities but
low radio luminosities, deep radio observations
are needed to observe even a factor of few in
flux lower than the correlation line. At lower
X-ray luminosities, selection biases in searching
for radio emission have restricted the radio halo
and relic samples purely to massive clusters.
Our unbiased search of the entire sky covered by
the Westerbork Northern Sky Survey (WENSS) at
327MHz begins to search for diffuse radio
emission in low-mass systems. The WENSS covers
the sky north of d 30 with an angular
resolution of 54 by 54 csc ?. The publicly
available images have a typical noise value of
3.6 mJy/beam. We did not limit our search,
instead using a multiresolution filtering
technique (Rudnick 2002) to decompose WENSS into
structures on different scales. The resulting
open'' maps contain the large scale structure
greater than 3 up to 1, while the filtered maps
contain the small scale structure. Open maps
were then searched by eye for diffuse structures,
resulting in a wide variety of sources (e.g. SNe,
HII regions, radio galaxies, known cluster
halos/relics) and five sources with no clear
identification. Here we discuss the three
diffuse radio sources which may be associated
with groups or poor clusters of galaxies.
091430 z0.02, P1.4 1023 W/Hz,
Lx ? 1041.5 erg/s. Top Left Filtered WENSS in
red with DSS galaxies in green. Top Right
ROSAT greyscale with WENSS contours. Weak X-ray
is present. Left DSS image with groups and
galaxies with known z0.02. HCG 37 shows
evidence of merging galaxies and AGN activity but
is 400kpc away from the diffuse radio emission.
Conclusions We find that poor clusters and
groups can have diffuse radio emission
luminosities comparable to those of some rich
clusters. With X-ray luminosities much lower
than clusters, the groups fall approximately two
orders of magnitude above the LX-PR relation for
rich clusters. The sources, on the order of a
few hundred kpc, are smaller than but have
similar morphology as halos and relics in rich
clusters. It is possible that these sources may
be generated through internal processes such AGN
activity, or during group formation during infall
along large scale filaments. As these low mass
systems are important to understand for
cosmological reasons, it is vital to continue
searching for radio emission in groups. Whether
or not these are a new class of diffuse radio
objects or involve similar processes as modeled
for rich cluster sources, our ideas of particle
acceleration and magnetic fields will change. We
are currently beginning to search for diffuse
radio structures specifically within groups of
galaxies.
AUTOMATIC FILTERING Multi-resolution filtering,
is a useful technique for a wide variety of image
analysis purposes. We use the simple filtering
method described in Rudnick (2002, PASP 114, 427)
to decompose WENSS images into structures on
different scales. The resulting open maps
contain the large scale structure, while the
filtered maps contain the small scale structure.
Open maps were then searched for diffuse
structures.
Radio power at 1.4GHz vs. X-ray bolometric
luminosity. Rich cluster relics are shown with
circles along with the best fit line to the
cluster halo data (Feretti 2005, X-Ray and
Radio Connections, astro-ph/0406090) extrapolated
down to lower luminosities. Squares are the
groups/poor clusters, with upper limits on the
X-ray data except for a detection of 091430. The
two distinct pieces of 080939 are plotted
separately. The dashed line shows the dependence
on the assumed redshift for the diffuse sources
even if these sources are misidentified cluster
relics, they would not fall on the
previously-observed correlation.
Above Slices showing model map deconvolved into
open (large scale) and filtered (small scale)
components. Right greyscale image of model
source with arrow showing the location of the
slice
ACKNOWLEDGEMENTS Data from WENSS (Rengelink et
al. 1997), Digital Sky Survey (Abazajian et al.
2003, 2004, 2005) and ROSAT and XMM-Newton
archives. Partial support for this research
comes from NSF grant 03-07600 to the Univ. of
Minnesota.