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Register a new userShock heating by FR I radio sources in galaxy clusters
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Authors
M. Brueggen
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Feedback by active galactic nuclei (AGN) is frequently invoked to explain the cut-off of the galaxy luminosity function at the bright end and the absence of cooling flows in galaxy clusters. Meanwhile, there are recent observations of shock fronts around radio-loud AGN. Using realistic 3D simulations of jets in a galaxy cluster, we address the question what fraction of the energy of active galactic nuclei is dissipated in shocks. We find that weak shocks that encompass the AGN have Mach numbers of 1.1-1.2 and dissipate at least 2% of the mechanical luminosity of the AGN. In a realistic cluster medium, even a continuous jet can lead to multiple shock structures, which may lead to an overestimate of the AGN duty cycles inferred from the spatial distribution of waves.
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The dying radio sources represent a very interesting and largely unexplored stage of the active galactic nucleus (AGN) evolution. They are considered to be very rare, and almost all of the few known ones were found in galaxy clusters. However, considering the small number detected so far, it has not been possible to draw any firm conclusions about their X-ray environment. We present X-ray observations performed with the Chandra satellite of the three galaxy clusters Abell 2276, ZwCl 1829.3+6912, and RX J1852.1+5711, which harbor at their center a dying radio source with an ultra-steep spectrum that we recently discovered. We analyzed the physical properties of the X-ray emitting gas surrounding these elusive radio sources. We determined the global X-ray properties of the clusters, derived the azimuthally averaged profiles of metal abundance, gas temperature, density, and pressure. Furthermore, we estimated the total mass profiles. The large-scale X-ray emission is regular and spherical, suggesting a relaxed state for these systems. Indeed, we found that the three clusters are also characterized by significant enhancements in the metal abundance and declining temperature profiles toward the central region. For all these reasons, we classified RX J1852.1+5711, Abell 2276, and ZwCl 1829.3+6912 as cool-core galaxy clusters.
Observations support the view that feedback, in the form of radio outbursts from active nuclei in central galaxies, prevents catastrophic cooling of gas and rapid star formation in many groups and clusters of galaxies. Variations in jet power drive a succession of weak shocks that can heat regions close to the active galactic nuclei (AGN). On larger scales, shocks fade into sound waves. The Braginskii viscosity determines a well-defined sound damping rate in the weakly magnetized intracluster medium (ICM) that can provide sufficient heating on larger scales. It is argued that weak shocks and sound dissipation are the main means by which radio AGN heat the ICM, in which case, the power spectrum of AGN outbursts plays a central role in AGN feedback.
We present a model for the compression and heating of the ICM by powerful radio galaxies and quasars. Based on a self-similar model of the dynamical evolution of FRII-type objects we numerically integrate the hydrodynamic equations governing the flow of the shocked ICM in between the bow shock and the radio lobes of these sources. The resulting gas properties are presented and discussed. The X-ray emission of the shocked gas is calculated and is found to be in agreement with observations. The enhancement of the X-ray emission of cluster gas due to the presence of powerful radio galaxies may play an important role in the direct detection of cluster gas at high redshifts.
Extra-galactic radio sources are a significant contaminant in cosmic microwave background and Sunyaev-Zeldovich effect experiments. Deep interferometric observations with the BIMA and OVRO arrays are used to characterize the spatial, spectral, and flux distributions of radio sources toward massive galaxy clusters at 28.5 GHz. We compute counts of mJy source fluxes from 89 fields centered on known massive galaxy clusters and 8 non-cluster fields. We find that source counts in the inner regions of the cluster fields (within 0.5 arcmin of the cluster center) are a factor of 8.9 (+4.3,-2.8) times higher than counts in the outer regions of the cluster fields (radius greater than 0.5 arcmin). Counts in the outer regions of the cluster fields are in turn a factor of 3.3 (+4.1,-1.8) greater than those in the non-cluster fields. Counts in the non-cluster fields are consistent with extrapolations from the results of other surveys. We compute spectral indices of mJy sources in cluster fields between 1.4 and 28.5 GHz and find a mean spectral index of alpha = 0.66 with an rms dispersion of 0.36, where flux is proportional to frequency raised to negative alpha. The distribution is skewed, with a median spectral index of 0.72 and 25th and 75th percentiles of 0.51 and 0.92, respectively. This is steeper than the spectral indices of stronger field sources measured by other surveys.
The aim of this work is to analyse the radio properties of the massive and dynamical disturbed clusters Abell 1451 and Zwcl 0634.1+4750, especially focusing on the possible presence of diffuse emission. We present new GMRT 320 MHz and JVLA 1.5 GHz observations of these two clusters. We found that both Abell 1451 and Zwcl 0634.1+4750 host a radio halo with a typical spectrum ($alphasim1-1.3$). Similarly to a few other cases reported in the recent literature, these radio halos are significantly fainter in radio luminosity with respect to the current radio power-mass correlations and they are smaller than classical giant radio halos. These underluminous sources might contribute to shed light on the complex mechanisms of formation and evolution of radio halos. Furthermore, we detected a candidate radio relic at large distance from the cluster center in Abell 1451 and a peculiar head tail radio galaxy in Zwcl 0634.1+4750, which might be interacting with a shock front.
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