Galaxy clusters are unique laboratories to investigate turbulent fluid motions and large scale magnetic fields. Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relat
ivistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission from radio halos is extremely important to constrain the properties of intracluster magnetic fields and the physics of the acceleration and transport of the relativistic particles. However, detecting this polarized signal is a very hard task with the current radio facilities.We use cosmological magneto-hydrodynamical simulations to predict the expected polarized surface brightness of radio halos at 1.4 GHz. We compare these expectations with the sensitivity and the resolution reachable with the SKA1. This allows us to evaluate the potential for studying intracluster magnetic fields in the surveys planned for SKA1.
Synchrotron radio halos at the center of merging galaxy clusters provide the most spectacular and direct evidence of the presence of relativistic particles and magnetic fields associated with the intracluster medium. The study of polarized emission f
rom radio halos has been shown to be extremely important to constrain the properties of intracluster magnetic fields. However, detecting this polarized signal is a very hard task with the current radio facilities.We investigate whether future radio observatories, such as the Square Kilometer Array (SKA) and its precursors and pathfinders, will be able to detect the polarized emission of radio halos in galaxy clusters.On the basis of cosmological magnetohydrodynamical simulations with initial magnetic fields injected by active galactic nuclei, we predict the expected radio halo polarized signal at 1.4 GHz. We compare these expectations with the limits of current radio facilities and explore the potential of the forthcoming radio interferometers to investigate intracluster magnetic fields through the detection of polarized emission from radio halos.The resolution and sensitivity values that are expected to be obtained in future sky surveys performed at 1.4 GHz using the SKA precursors and pathfinders (like APERTIF and ASKAP) are very promising for the detection of the polarized emission of the most powerful (L1.4GHz>10e25 Watt/Hz) radio halos. Furthermore, the JVLA have the potential to already detect polarized emission from strong radio halos, at a relatively low resolution.However, the possibility of detecting the polarized signal in fainter radio halos (L1.4GHz~10e24 Watt/Hz) at high resolution requires a sensitivity reachable only with SKA.
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, consid
ering 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.
M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy gen
erated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.
We present the study of five `dying nearby radio galaxies belonging to the WENSS minisurvey and to the B2 bright catalogs: WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. These sources have been selected on the basis of their ex
tremely steep broad-band radio spectra. 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 by two distinct dying galaxies, which appear blend 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 detected also in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources of our sample are located (at least in projection) at the center of an X-ray emitting cluster. Our results suggest that the duration of the dying phase for a radio source in cluster can be significantly higher with respect to that of a radio galaxy in the field. The simplest interpretation is that the low-frequency emission from the fading radio lobes last longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. Radio sources in dense environment, like e.g. 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 samples of objects.
Radio galaxies with a projected linear size > 1 Mpc are classified as giant radio sources. According to the current interpretation these are old sources which have evolved in a low-density ambient medium. Since radiative losses are negligible at low
frequency, extending spectral ageing studies in this frequency range will allow to determine the zero-age electron spectrum injected and then to improve the estimate of the synchrotron age of the source. We present Very Large Array images at 74 MHz and 327 MHz of two giant radio sources: 3C35 and 3C223. We performed a spectral study using 74, 327, 608 and 1400 GHz images. The spectral shape is estimated in different positions along the source. The radio spectrum follows a power-law in the hot-spots, while in the inner region of the lobe the shape of the spectrum shows a curvature at high frequencies. This steepening is in agreement with synchrotron aging of the emitting relativistic electrons. In order to estimate the synchrotron age of the sources, the spectra have been fitted with a synchrotron model of emission. Using the models, we find that 3C35 is an old source of about 143 Myr, while 3C223 is a younger source of 72 Myr.
The goal of this work is to constrain the strength and structure of the magnetic field associated with the environment of the radio source 3C 449, using observations of Faraday rotation, which we model with a structure function technique and by compa
rison with numerical simulations. We assume that the magnetic field is a Gaussian, isotropic random variable and that it is embedded in the hot intra-group plasma surrounding the radio source. For this purpose, we present detailed rotation measure images for the polarized radio source 3C 449, previously observed with the Very Large Array at seven frequencies between 1.365 and 8.385 GHz. We quantify the statistics of the magnetic-field fluctuations by deriving rotation measure structure functions, which we fit using models derived from theoretical power spectra. We quantify the errors due to sampling by making multiple two-dimensional realizations of the best-fitting power spectrum.We also use depolarization measurements to estimate the minimum scale of the field variations. We then make three-dimensional models with a gas density distribution derived from X-ray observations and a random magnetic field with this power spectrum. Under these assumptions we find that both rotation measure and depolarization data are consistent with a broken power-law magnetic-field power spectrum, with a break at about 11 kpc and slopes of 2.98 and 2.07 at smaller and larger scales respectively. The maximum and minimum scales of the fluctuations are around 65 and 0.2 kpc, respectively. The average magnetic field strength at the cluster centre is 3.5 +/-1.2 micro-G, decreasing linearly with the gas density within about 16 kpc of the nucleus.
We present new radio continuum observations, taken at 1.4 GHz with the Very Large Array, and new observations in the CO line, taken with the Plateau de Bure interferometer, of two regions of the spiral galaxy NGC 3627. Comparing these observations wi
th archival Spitzer and H_{alpha} data we study the correlation of the radio continuum (RC), infrared-8 um and CO emissions at a spatial resolution of 100 pc. We compare the point-by-point variations of the RC, CO, and 8um brightnesses in two distinct regions of 2 kpc times 2 kpc in size of NGC 3627. We examined scale much lower than the electron diffusion scale, where a break down of the correlations would be expected. However no evidence for such correlation break down has been found. The RC emission follows well the distribution of CO and the widths of several structures, measured along slices across them, are comparable. Furthermore, we found that down to a spatial scale of 100 pc the radio continuum emission is correlated with dust emissions at 4.5, 5.8, and 8 um, that traces different dust temperatures. We present a new perspective with which visualizing and studying the RC-CO-24 um and RC-CO-8 um correlations, by using a three-dimensional representation. We fit the observed data with a three dimensional line, obtaining a rms of 0.25 dex. The observed correlation enhances the complexity of the electrons diffusion, losses and injection mechanisms, and of their connection to star formation processes, described by molecular and dust emissions. We plan to further investigate this connection using spatially resolved spectral index studies at low radio frequencies where the thermal emission component is seemingly negligible.
The goal of this work is to put constraints on the strength and structure of the magnetic field in the cluster of galaxies A2382. We investigate the relationship between magnetic field and Faraday rotation effects in the cluster, using numerical simu
lations as a reference for the observed polarization properties. For this purpose we present Very Large Array observations at 20 cm and 6 cm of two polarized radio sources embedded in A2382, and we obtained detailed rotation measure images for both of them. We simulated random three-dimensional magnetic field models with different power spectra and thus produced synthetic rotation measure images. By comparing our simulations with the observed polarization properties of the radio sources, we can determine the strength and the power spectrum of intra-cluster magnetic field fluctuations that best reproduce the observations. The data are consistent with a power law magnetic field power spectrum with the Kolmogorov index $n=11/3$, while the outer scale of the magnetic field fluctuations is of the order of 35 kpc. The average magnetic field strength at the cluster center is about 3 $mu$G and decreases in the external region as the square root of the electron gas density. The average magnetic field strength in the central 1 Mpc$^{3}$ is about 1 $mu$G.
Up till now very few dying sources were known, presumably because the dying phase is short at centimeter wavelengths. We therefore have tried to improve the statistics on sources that have ceased to be active, or are intermittently active. The latter
sources would partly consist of a fossil radio plasma left over from an earlier phase of activity, plus a recently restarted core and radio jets. Improving the statistics of dying sources will give us a better handle on the evolution of radio sources, in particular the frequency and time scales of radio activity. We have used the WENSS and NVSS surveys, in order to find sources with steep spectral indices, associated with nearby elliptical galaxies. In the cross correlation we presently used only unresolved sources, with flux densities at 1.4 GHz larger than 10 mJy. The eleven candidates thus obtained were observed with the VLA in various configurations, in order to confirm the steepness of the spectra, and to check whether active structures like flat-spectrum cores and jets are present, perhaps at low levels. We estimated the duration of the active and relic phases by modelling the integrated radio spectra using the standard models of spectral evolution. We have found six dying sources and three restarted sources, while the remaining two candidates remain unresolved also with the new VLA data and may be Compact Steep Spectrum sources, with an unusually steep spectrum. The typical age of the active phase, as derived by spectral fits, is in the range 10^7 - 10^8 years. For our sample of dying sources, the age of the relic phase is on average shorter by an order of magnitude than the active phase.
