1. DIFFUSE RADIO SOURCES in GROUPS and POOR CLUSTERS
Kisha Delain and Lawrence Rudnick, University of Minnesota
z=0.04, P1.4 ~ W/Hz, Lx  and Lx  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 z=0.04, stars at z=0.075, circles at z=0.01 and crosses at z=0.02.
ABSTRACT
We have discovered new diffuse radio sources likely associated with groups of galaxies at low redshift ( ) 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.
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z=0.01, P1.4~ W/Hz, Lx  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 z=0.01
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.
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z=0.02, P1.4~ 1023 W/Hz, Lx  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 z= HCG 37 shows evidence of merging galaxies and AGN activity but is 400kpc away from the diffuse radio emission.
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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/ ) extrapolated down to lower luminosities. Squares are the groups/poor clusters, with upper limits on the X-ray data except for a detection of The two distinct pieces of 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 to the Univ. of Minnesota.