Search

Some merging galaxy clusters host diffuse extended radio emission, so-called radio halos and relics, unrelated to individual galaxies. The origin of these halos and relics is still debated, although there is compelling evidence now that they are related to cluster merger events. Here we present detailed Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT) radio observations between 147 MHz and 4.9 GHz of a new radio-selected galaxy cluster 1RXS J0603.3+4214, for which we find a redshift of 0.225. The cluster is detected as an extended X-ray source in the ROSAT All Sky Survey with an X-ray luminosity of LX, 0.1-2.4 keV ∼ 1 × 1045 erg s-1. The cluster hosts a large bright 1.9 Mpc radio relic, an elongated ∼2 Mpc radio halo, and two fainter smaller radio relics. The large radio relic has a peculiar linear morphology. For this relic we observe a clear spectral index gradient from the front of the relic towards the back, in the direction towards the cluster center. Parts of this relic are highly polarized with a polarization fraction of up to 60%. We performed rotation measure (RM) synthesis between 1.2 and 1.7 GHz. The results suggest that for the west part of the large relic some of the Faraday rotation is caused by the intracluster medium and not only due to galactic foregrounds. We also carried out a detailed spectral analysis of this radio relic and created radio color-color diagrams. We find (i) an injection spectral index of -0.6 to -0.7; (ii) steepening spectral index and increasing spectral curvature in the post-shock region; and (iii) an overall power-law spectrum between 74 MHz and 4.9 GHz with α = -1.10 ± 0.02. Mixing of emission in the beam from regions with different spectral ages is probably the dominant factor that determines the shape of the radio spectra. Changes in the magnetic field, total electron content, or adiabatic gains/losses do not play a major role. A model in which particles are (re)accelerated in a first order Fermi process at the front of the relic provides the best match to the observed spectra. We speculate that in the postshock region particles are re-accelerated by merger induced turbulence to form the radio halo as the relic and halo are connected. The orientation of the bright relic and halo indicate a north-south merger event, but the peculiar linear shape and the presence of another relic, perpendicular to the bright relic, suggest a more complex merger event. Deep X-ray observations will be needed to determine the merger scenario. [ABSTRACT FROM AUTHOR]

Gigahertz-peaked spectrum (GPS) radio sources are thought to be young objects which later evolve into Fanaro–Riley type I (FR I) and FR II radio galaxies. We have used the Australia Telescope 20 GHz (AT20G) survey catalogue to select a uniform sample of GPS sources with spectral peaks above 5 GHz, which should represent the youngest members of this class. In this paper, we present e-VLBI observations of 10 such objects which are associated with nearby galaxies and so represent a new population of local, low-power GPS sources. Our e-VLBI observations were carried out at 4.8 GHz with the Australia Telescope Long Baseline Array (LBA) using a real-time software correlator. All 10 sources were detected, and were unresolved on scales of ∼100 mas, implying that they are typically less than 100 pc in linear size. [ABSTRACT FROM AUTHOR]

We present deep multifrequency observations using the Giant Metrewave Radio Telescope (GMRT) at 153, 244, 610 and 1260 MHz of a field centred on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as haloes and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey (WENSS), NRAO (National Radio Astronomy Observatories) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters Survey (FIRST). Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index, ], which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with . We examine their optical fields and discuss the nature of some of these sources. [ABSTRACT FROM AUTHOR]

We present the results of the first successful observations of the Pie Town link with the Very Large Array at 74 MHz on Hercules A. The improvement in resolution from 25 to 10 arcsec resolves the helical- and ring-like features seen at higher frequencies. We also present new high dynamic range images of this powerful radio galaxy at 325 MHz. Our low-frequency observations confirm the multiple outburst interpretation of the spectral index differences at high frequencies. A comparison between our radio andROSATX-ray data does not reveal any association between the X-ray emission from the cluster and the radio lobes. There are no extra regions of radio emission at 74 MHz. [ABSTRACT FROM AUTHOR]