Low Frequency Radio Studies of Halos and Relics

Tracy E. Clarke1, A. Cohen2, W. Peters1, S. W. Randall3, P. E. J. Nulsen3, M. S. Owers4, C. L. Sarazin5, W. R. Forman3, S. S. Murray3 1Naval Research Lab., 2Johns Hopkins University Applied Physics Lab., 3Harvard‐Smithsonian Center for Astrophysics, 4Centre for Astrophysics and Supercompung, Swinburne University, 5Department of Astronomy, University of Virginia Abstract: Radio observaons toward clusters reveal the presence of diffuse, steep spectrum emission (halos and relics) associated only with clusters displaying evidence of merger acvity. The observed connecon between cluster mergers and diffuse radio emission points toward the potenal use of these enigmac radio sources as signposts to idenfy merging systems in large samples of clusters. One excing possibility is that the radio properes may provide a proxy for correcng the cluster mass esmates for the effects of the merger boosts. Due to their steep radio spectrum, diffuse radio halos and relics are best studied at low frequencies where single observaons are able to cover large areas of the sky efficiently. We present new low frequency radio observaons of the relics in Abell 133 and Abell 2443 and discuss the features seen in these images and spectral index maps. Detailed spectral studies of these systems are a key component to idenfying and understanding the origin(s) of the emission as well as probing the details of cluster formaon and evoluon. We also present an overview of a new study which uses the VLA Low Frequency Sky Survey (VLSS) to probe the shape of the radio spectrum for known clusters with diffuse emission.

Abell 133 Abell 2443 (Randall et al., submied) (Cohen and Clarke, submied) • Luminous, nearby (z = 0.0566) cluster with opcal and X‐ray merger evidence • Rich, intermediate redshi (z = 0.108) cluster with opcal subclumps nearby • Chandra and XMM data show complex X‐ray morphology including an X‐ray plume • Wen et al. (2007) idenfied 289 cluster members along a NW to SE distribuon (Fujita et al. 2002, 2004) • 74 MHz VLSS (Cohen et al. 2007) revealed diffuse steep‐spectrum radio emission • Radio emission reveals steep spectrum relic (Figure 1A, Slee et al. 2001) • Deep 1425, 325, and 74 MHz images (Figure 1) show complex structure with A B C several head‐tail galaxies and an extended region of emission visible only at the lower frequencies

A B C D

D E F

Figure 1: Mul‐frequency radio images of Abell 2443. A: VLA C‐configuraon image at 1425 MHz showing compact radio sources. B: VLA B‐configuraon 325 MHz image showing extended emission in the relic region. C: VLA A‐Configuraon 74 MHz image highlighng the diffuse extended emission. Figure 1: Mul‐frequency VLA radio images of Abell D: Spectral index map between 325 and 74 MHz. 133. A: 1400 MHz θ~5”, B: 1400 MHz, θ~15”, C: 330 Figure 2: A 330 MHz image with labels. MHz, θ~10”, D: 330 MHz, θ~30”, E: 74 MHz, θ~30”, Spectral index maps B – D are labeled with resoluon and frequencies. • Matched resoluon mul‐frequency data reveal emission is diffuse, extremely F: Chandra 0.6‐5.0 keV smoothed to 4” steep spectrum, irregular, and not associated with any cluster member • New 1400, 330, and 74 MHz data reveal large scale radio structure not previously • Comparison with ROSAT X‐ray (Figure 2 le) shows radio emission is peripheral reported (Figure 1 B, C, D, and E) • Spectral index of diffuse emission (α=‐2.8 between 325 and 1400 MHz, α=‐1.7 • Radio structure is likely a giant radio galaxy associated with a z=.293 background between 74 and 330 MHz) places it in the rare category of ultra‐steep spectrum system, the northern lobe of which may overlie the radio relic in A133 sources. • Our new deep Chandra data reveal an outer surface brightness edge which is consistent with a sloshing cold front in the core (Figure 3) ‐> recent merger • X‐ray spectral fits suggest that the wings are formed by gas displacement from AGN lobes rather than a weak shock passage through the cool core ICM sloshing Galaxy/ICM interface

Figure 3: Le: 0.3 ‐2.0 keV Chandra image showing the core (blue cross), inner and outer X‐ray Figure 2: Le; Smoothed ROSAT X‐ray image in greyscale with VLA 325 MHz contours overlaid. Right; edges. Right: Emission measure profile along the semi‐minor axis with best‐fit 3D density model. Flux density as a funcon of frequency for the diffuse emission (relic: red) and several radio galaxies The two vercal dashed lines show the best‐fit break radii of the density disconnuies. (A, B, and C)

Low Frequency Spectrum of Halos and Relics

• Undertaking a re‐reducon of the 74 MHz VLSS data (Cohen et al. 2007) • Inial tests show an ancipated 40% increase in source counts, a decrease in the rms of 15%, and sensivity to extended emission on scales larger than 17’ • Pilot fields were selected on all known radio halo and relic clusters to study the low frequency spectral index • Preliminary results reveal that at least some of the sources show spectral flaening to 74 MHz • Results have implicaons for cluster detecon using the new generaon of low frequency instruments such as the Long Wavelength Array (LWA) and the LOw Frequency Array (LOFAR) as well as the upcoming low frequency upgrade on the Expanded Very Large Array (see Kassim et al. poster)

Basic research in radio astronomy at the Naval Research Laboratory is funded by 6.1 Base funding.