Aims. We recently performed a study of a sample of relaxed, cooling core galaxy clusters with deep Very Large Array observations at 1.4 GHz. We find that in the central regions of A1835, A2029, and ...Ophiuchus the dominant radio galaxy is surrounded by a diffuse low-brightness radio emission that takes the form of a mini-halo. Here we present the results of the analysis of the extended diffuse radio emission in these mini-halos. Methods. In order to investigate the morphological properties of the diffuse radio emission in clusters of galaxies we propose to fit their azimuthally averaged brightness profile with an exponential, obtaining the central brightness and the e-folding radius from which the radio emissivity can be calculated. We investigate the radio properties of the mini-halos in A1835, A2029, and Ophiuchus in comparison with the radio properties of a representative sample of mini-halos and halos already known in the literature. Results. We find that radio halos can have quite different length-scales but their emissivity is remarkably similar from one halo to the other. In contrast, mini-halos span a wide range of radio emissivity. Some of them, like the Perseus mini-halo, are characterized by a radio emissivity which is more than 100 times greater than that of radio halos. On the other hand, the new mini-halos in cooling core clusters analyzed in this work, namely A2029, Ophiuchus, and A1835, have a radio emissivity which is much more typical of halos in merging clusters rather than similar to that of the other mini-halos previously known.
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We present new deep, high-resolution radio images of the diffuse minihalo in the cool core of the galaxy cluster RX J1720.1+2638. The images have been obtained with the Giant Metrewave Radio ...Telescope at 317, 617, and 1280 MHz and with the Very Large Array at 1.5, 4.9, and 8.4 GHz, with angular resolutions ranging from 1" to 10". This represents the best radio spectral and imaging data set for any minihalo. Most of the radio flux of the minihalo arises from a bright central component with a maximum radius of ~80 kpc. A fainter tail of emission extends out from the central component to form a spiral-shaped structure with a length of ~230 kpc, seen at frequencies 1.5 GHz and below. We find indication of a possible steepening of the total radio spectrum of the minihalo at high frequencies. Furthermore, a spectral index image shows that the spectrum of the diffuse emission steepens with increasing distance along the tail. A striking spatial correlation is observed between the minihalo emission and two cold fronts visible in the Chandra X-ray image of this cool core. These cold fronts confine the minihalo, as also seen in numerical simulations of minihalo formation by sloshing-induced turbulence. All these observations favor the hypothesis that the radio-emitting electrons in cluster cool cores are produced by turbulent re-acceleration.
Aims. We present new high sensitivity observations of the radio relic in A 521 carried out using the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. ...Methods. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327-610 MHz. In our present analysis we use our new GMRT observations in addition to proprietary and archival data. We search for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. Results. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is supported by our detection of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is probably a shock front.
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Dying radio galaxies in clusters Murgia, M.; Parma, P.; Mack, K.-H. ...
Astronomy & astrophysics,
02/2011, Volume:
526
Journal Article
Peer reviewed
Open access
Aims. We present a study of five “dying” nearby (z ≤ 0.2) radio galaxies belonging to both the WENSS minisurvey and the B2 bright catalogs WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 ...1610+29. Methods. These sources have been selected on the basis of their extremely steep broad-band radio spectra, which strongly indicates that either these objects belong to the rare class of dying radio galaxies or we are observing “fossil” radio plasma remaining from a previous instance of nuclear activity. We derive the relative duration of the dying phase from the fit of a synchrotron radiative model to the radio spectra of the sources. Results. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime, although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed of two distinct dying galaxies, which appear blended together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is also detected in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources in our sample are located (at least in projection) at the center of an X-ray emitting cluster. Conclusions. Our results suggest that the duration of the dying phase for a radio source in a cluster can be significantly higher than that of a radio galaxy in the field, although no firm conclusions can be drawn because of the small number statistics involved. The simplest interpretation of the tendency for dying galaxies to be found in clusters is that the low-frequency radio emission from the fading radio lobes lasts longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. We argue that radio sources in dense environments, such as the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger, statistically significant, samples of objects.
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A STRONG MERGER SHOCK IN ABELL 665 Dasadia, S.; Sun, M.; Sarazin, C. ...
Astrophysical journal. Letters,
03/2016, Volume:
820, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT Deep (103 ks) Chandra observations of Abell 665 have revealed rich structures in this merging galaxy cluster, including a strong shock and two cold fronts. The newly discovered shock has a ...Mach number of M = 3.0 0.6, propagating in front of a cold disrupted cloud. This makes Abell 665 the second cluster, after the Bullet cluster, where a strong merger shock of has been detected. The shock velocity from jump conditions is consistent with (2.7 0.7) × 103 km s−1. The new data also reveal a prominent southern cold front with potentially heated gas ahead of it. Abell 665 also hosts a giant radio halo. There is a hint of diffuse radio emission extending to the shock at the north, which needs to be examined with better radio data. This new strong shock provides a great opportunity to study the re-acceleration model with the X-ray and radio data combined.
We present Chandra gas temperature profiles at large radii for a sample of 13 nearby, relaxed galaxy clusters and groups, which includes A133, A262, A383, A478, A907, A1413, A1795, A1991, A2029, ...A2390, MKW 4, RX J1159+5531, and USGC S152. The sample covers a range of average temperatures from 1 to 10 keV. The clusters are selected from the archive or observed by us to have sufficient exposures and off-center area coverage to enable accurate background subtraction and reach the temperature accuracy of better than 20%-30% at least to r = (0.4-0.5)r sub(180) and for the three best clusters to (0.6-0.7)r sub(180). For all clusters, we find cool gas in the cores, outside of which the temperature reaches a peak at r 6 0.15r sub(180) and then declines to 60.5 of its peak value at r 0.5r sub(180). When the profiles are scaled by the cluster average temperature (excluding cool cores) and the estimated virial radius, they show large scatter at small radii but remarkable similarity at r > (0.1-0.2)r sub(180) for all but one cluster (A2390). Our results are in good agreement with previous measurements from ASCA by Markevitch et al. and from BeppoSAX by De Grandi & Molendi. Four clusters have recent XMM-Newton temperature profiles, two of which agree with our results, and we discuss reasons for disagreement for the other two. The overall shape of the temperature profiles at large radii is reproduced in recent cosmological simulations.
We combined deep Chandra, ROSAT HRI, and XMM-Newton observations of M87 to study the impact of active galactic nucleus (AGN) outbursts on its gaseous atmosphere. Many X-ray features appear to be a ...direct result of repetitive AGN outbursts. In particular, the X-ray cavities around the jet and counterjet are likely due to the expansion of radio plasma, while rings of enhanced emission at 14 and 17 kpc are probably shock fronts associated with outbursts that began 1-2 x 10 super(7) yr ago. The effects of these shocks are also seen in brightenings within the prominent X-ray arms. On larger scales, 650 kpc from the nucleus, depressions in the surface brightness may be remnants of earlier outbursts. As suggested for the Perseus Cluster by Fabian and his coauthors, our analysis of the energetics of the M87 outbursts argues that shocks may be the most significant channel for AGN energy input into the cooling-flow atmospheres of galaxies, groups, and clusters. For M87, the mean power driving the shock outburst, 2.4 x 10 super(43) ergs s super(-1) is 3 times greater than the radiative losses from the entire cooling flow. Thus, even in the absence of other energy inputs, outbursts every 3 x 10 super(7) yr are sufficient to quench the flow.
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