Diffuse steep-spectrum sources from the 74 MHz VLSS survey

Abstract

Context. Galaxy clusters grow by a sequence of mergers with other clusters and galaxy groups. During these mergers, shocks and/or turbulence are created within the intracluster medium (ICM). In this process, particles could be accelerated to highly relativistic energies. The synchrotron radiation from these particles is observed in the form of radio relics and halos that are generally characterized by a steep radio spectral index. Shocks can also revive fossil radio plasma from a previous episode of AGN activity, creating a socalled radio "phoenix". Here we present multi-frequency radio observations of diffuse steep-spectrum radio sources selected from the 74 MHz VLSS survey. Previous Giant Metrewave Radio Telescope (GMRT) observations showed that some of these sources had filamentary and elongated morphologies, which are expected for radio relics. Aims. We attempt to understand the nature of diffuse steep-spectrum radio sources and characterize their spectral index and polarization properties. Methods. We carried out radio continuum observations at 325 MHz with the GMRT. Observations with the Very Large Array (VLA) and Westerbork Synthesis Radio Telescope (WSRT) were taken at 1.4 GHz in full polarization mode. Optical images around the radio sources were taken with the William Herschel and Isaac Newton Telescope (WHT, INT). Results. Most of the sources in our sample consist of old radio plasma from AGNs located in small galaxy clusters. The sources can be classified as AGN relics or radio phoenices. The spectral indices across most of the radio sources display large variations. Conclusions. We conclude that diffuse steep-spectrum radio sources are not only found in massive X-ray luminous galaxy clusters but also in smaller systems. Future low-frequency surveys will uncover large numbers of steep-spectrum radio relics related to previous episodes of AGN activity.

AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS J1431.8+1331

“…We have observed the steep-spectrum radio source VLSS J1431.8+1331 using LOFAR, adding to an already existing data set taken with the GMRT and the VLA by van Weeren et al (2011). Using the resulting broad band frequency coverage and high resolution imaging, we were able to study the spectral properties of different regions across the source.…”

“…Figure 2 shows the LOFAR image of the target spanning the HBA band. We can discern two different source regions (described in detail by van Weeren et al 2011), the larger and brighter one (A) to the north-east, and a smaller region (B) to the south-west connected by a faint "bridge" of radio emission. Both regions seem to be slightly curved to the north-east.…”

“…This is more noticeable for Region B in the higher resolution image, outlined in contours in the right-hand panel. A hint of a faint radio core is visible in the high resolution VLA image of van Weeren et al (2011), while the brightest part of the target in the LOFAR image is the south-eastern part of Region A.…”

“…Several steep spectrum sources were studied by van Weeren et al (2009van Weeren et al ( , 2011, identified by matching the VLSS and NVSS 2 catalogues. One of these, VLSS J1431.8+1331, was identified as being potentially an AGN radio relic.…”

“…We also fitted JP and KGJP models (Appendix A) through all of the shifted data points to show that indeed the shifted spectra are self-similar and taken together produce a spectrum that is consistent with radiative ageing and IC scattering being the dominant energy loss mechanism of the electrons. The shifts made to the individual spectra to line them up can give us information about the physical parameters of the source van Weeren et al 2012). Specifically, shifts in the log(ν) axis relate to γ 2 B, while shifts along the log(S ) axis depend on N tot B, where N tot is the total number of energetic particles in the source volume for a given beam size.…”