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F. Govoni

Publications and source records attributed to F. Govoni.

At least 73 records · Page 4Linked to original sources
arXiv2010

GMRT observations of the Ophiuchus galaxy cluster

VLA observations at 1477 MHz revealed the presence of a radio mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies. In this work we present a study of the radio emission from this cluster of galaxies at lower radio frequencies. We observed the Ophiuchus cluster at 153, 240, and 614 MHz with the GMRT. The mini-halo is clearly detected at 153 and 240 MHz while it is not detected at 610 MHz. The most prominent feature at low frequencies is a patch of diffuse steep spectrum emission located at about 5' south-east from the cluster center. By combining these images with that at 1477 MHz, we derived the spectral index of the mini-halo. Globally, the mini-halo has a low-frequency spectral index of alpha_240^153 ~1.4 +/- 0.3 and an high-frequency spectral index of alpha_1477^240 ~ 1.60 +/- 0.05. Moreover, we measure a systematic increase of the high-frequency spectral index with radius: the azimuthal radial average of alpha_1477^240 increases from about 1.3, at the cluster center, up to about 2.0 in the mini-halo outskirts. The observed radio spectral index is in agreement with that obtained by modeling the non-thermal hard X-ray emission in this cluster of galaxies. We assume that the X-ray component arises from inverse Compton scattering between the photons of the cosmic microwave background and a population of non-thermal electrons which are isotropically distributed and whose energy spectrum is a power law with index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of gamma_min < 700 up to a maximum Lorentz factor of gamma_max =3.8 x 10^4 with an index p=3.8 +/- 0.4. The volume-averaged strength for a completely disordered intra-cluster magnetic field is B_V ~0.3 +/- 0.1 micro-G.

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arXiv2010

The intracluster magnetic field power spectrum in Abell 665

The goal of this work is to investigate the power spectrum of the magnetic field associated with the giant radio halo in the galaxy cluster A665. For this, we present new deep Very Large Array total intensity and polarization observations at 1.4 GHz. We simulated Gaussian random three-dimensional turbulent magnetic field models to reproduce the observed radio halo emission. By comparing observed and synthetic radio halo images we constrained the strength and structure of the intracluster magnetic field. We assumed that the magnetic field power spectrum is a power law with a Kolmogorov index and we imposed a local equipartition of energy density between relativistic particles and field. Under these assumptions, we find that the radio halo emission in A665 is consistent with a central magnetic field strength of about 1.3 micro-G. To explain the azimuthally averaged radio brightness profile, the magnetic field energy density should decrease following the thermal gas density, leading to an averaged magnetic field strength over the central 1 Mpc^3 of about 0.75 micro-G. From the observed brightness fluctuations of the radio halo, we infer that the outer scale of the magnetic field power spectrum is ~450 kpc, and the corresponding magnetic field auto-correlation length is ~100 kpc.

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arXiv2010

Structure of the magnetoionic medium around the FR Class I radio galaxy 3C 449

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 comparison 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.

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arXiv2010

The Coma cluster magnetic field from Faraday rotation measures

The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday Rotation Measure (RM) images with numerical simulations of the magnetic field. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday Rotation Measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B_0 and radial power-law slope eta, where eta indicates how the field scales with respect to the gas density profile. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ~ 2 kpc and maximum scale ~ 34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B_0=3.9 microG; eta=0.4) and (B_0=5.4 microG; eta=0.7) within 1sigma. The best agreement between observations and simulations is achieved for B_0=4.7 microG; eta=0.5. Values of B_0>7 microG and <3 microG as well as eta < 0.2 and eta > 1.0 are incompatible with RM data at 99 % confidence level.

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arXiv2010

The diffuse radio filament in the merging system ZwCl 2341.1+0000

In some clusters of galaxies, a diffuse non-thermal emission is present, not obviously associated with any individual galaxy. These sources have been identified as relics, mini-halos, and halos according to their properties and position with respect to the cluster center. Moreover in a few cases have been reported the existence of a diffuse radio emission not identified with a cluster, but with a large scale filamentary region. The aim of this work is to observe and discuss the diffuse radio emission present in the complex merging structure of galaxies ZwCl 2341.1+0000. We have obtained VLA observations at 1.4 GHz to derive a deep radio image of the diffuse emission. Low resolution VLA images show a diffuse radio emission associated to the complex merging region with a largest size = 2.2 Mpc. In addition to the previously reported peripheral radio emission, classified as a double relic, diffuse emission is detected along the optical filament of galaxies. The giant radio source discussed here shows that magnetic fields and relativistic particles are present also in filamentary structures. Possible alternate scenarios are: a giant radio halo in between two symmetric relics, or the merging of two clusters both hosting a central radio halo.

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arXiv2009

A double radio halo in the close pair of galaxy clusters Abell 399 and Abell 401

Radio halos are faint radio sources usually located at the center of merging clusters of galaxies. These diffuse radio sources are rare, having so far been found only in about 30 clusters of galaxies, suggesting that particular conditions are needed to form and maintain them. It is interesting to investigate the presence of radio halos in close pairs of interacting clusters in order to possibly clarify their origin in relation to the evolutionary state of the merger. In this work, we study the case of the close pair of galaxy clusters A399 and A401. A401 is already known to contain a faint radio halo, while a hint of diffuse emission in A399 has been suggested based on the NVSS. To confirm this possibility, we analyzed deeper Very Large Array observations at 1.4 GHz of this cluster. We find that the central region of A399 is permeated by a diffuse low-surface brightness radio emission that we classify as a radio halo with a linear size of about 570 kpc and a central brightness of 0.3 micro-Jy/arcsec^2. Indeed, given their comparatively small projected distance of about 3 Mpc, the pair of galaxy clusters A401 and A399 can be considered as the first example of double radio halo system. The discovery of this double halo is extraordinary given the rarity of these radio sources in general and given that current X-ray data seem to suggest that the two clusters are still in a pre-merger state. Therefore, the origin of the double radio halo is likely to be attributed to the individual merging histories of each cluster separately, rather than to the result of a close encounter between the two systems.

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arXiv2009

Radio halos in nearby (z < 0.4) clusters of galaxies

The Intra-Cluster Medium is characterized by thermal emission, and by the presence of large scale magnetic fields. In some clusters of galaxies a diffuse non-thermal emission is also present, located at the cluster center and named radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume. In this paper we collect data on all known nearby cluster radio halos (z < 0.4), to discuss their statistical properties and to investigate their origin. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos. We present data on 31 radio halos, 1 new relic source, and 1 giant filament. We note the discovery of a small size diffuse radio emission in a cluster (A1213) with very low X-ray luminosity. Among statistical results we confirm the correlation between the average halo radio spectral index and the cluster temperature. We also discuss the high percentage of clusters where both a relic and a radio halo is present. The sample of radio halos discussed here represents the population of radio halos observable with present radio telescopes. The new telescope generation is necessary for a more detailed multifrequency study, and to investigate the possible existence of a population of radio halos with different properties.

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arXiv2009

Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745

Aims. To study at multiple frequencies the radio emission arising from the massive galaxy cluster MACS J0717.5+3745 (z=0.55). Known to be an extremely complex cluster merger, the system is uniquely suited for an investigation of the phenomena at work in the intra-cluster medium (ICM) during cluster collisions. Methods. We use multi-frequency and multi-resolution data obtained with the Very Large Array radio telescope, and X-ray features revealed by Chandra, to probe the non-thermal and thermal components of the ICM, their relations and interactions. Results. The cluster shows highly complex radio emission. A bright, giant radio halo is detected at frequencies as high as 4.8 GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a radio halo. This radio halo is also the most powerful ever observed, and the second case for which polarized radio emission has been detected, indicating that the magnetic field is ordered on large scales.

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arXiv2009

A search for diffuse radio emission in the relaxed, cool-core galaxy clusters A1068, A1413, A1650, A1835, A2029, and Ophiuchus

We analyze sensitive, high-dynamic-range, observations to search for extended, diffuse, radio emission in relaxed and cool-core galaxy clusters. We performed deep 1.4 GHz Very Large Array observations, of A1068, A1413, A1650, A1835, A2029, and complemented our dataset with archival observations of Ophiuchus. We find that, in the central regions of A1835, A2029, and Ophiuchus, the dominant radio galaxy is surrounded by diffuse low-brightness radio emission that takes the form of a mini-halo. We detect no diffuse emission in A1650, at a surface brightness level of the other mini-halos. We find low significance indications of diffuse emission in A1068 and A1413, although to be classified as mini-halos they would require further investigation, possibly with data of higher signal-to-noise ratio. In the Appendix, we report on the serendipitous detection of a giant radio galaxy with a total spatial extension of ~1.6 Mpc.

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arXiv2009

Comparative analysis of the diffuse radio emission in the galaxy clusters A1835, A2029, and Ophiuchus

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. 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. 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-halos, 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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arXiv2008

Double relics in Abell 2345 and Abell 1240: spectral index and polarization analysis

The aim of the present work is to study the radio properties of double relics in Abell 1240 and Abell 2345 in the framework of double relic formation models. We present new Very Large Array observations at 20 and 90 cm for these two clusters. We performed spectral index and polarization analysis and compared our results with expectations from theoretical models. The presence of double relics in these two cluster is confirmed by these new observations. Double relics in Abell 1240 show radio morphology, spectral index and polarization values in agreement with "outgoing merger shocks" models. One of the relics of Abell 2345 shows a peculiar morphology and spectral index profile, that are difficult to reconcile with present scenarios. We suggest a possible origin for this peculiar relic.

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arXiv2008

The dynamical state of A548 from XMM-Newton data: X-ray and radio connection

We present a detailed study of the X-ray properties of the galaxy cluster Abell 548b (z=0.04), using XMM-Newton data, and discuss the connection between the thermal properties and the presence of two extended relic radio sources located at the cluster periphery. We wish to analyze the dynamical state of the cluster and confirm the presence of a major merger. We will discuss the merger effects on the extended nonthermal emission. From the analysis of the temperature distribution and of the surface brightness profiles, we find evidence of a shock in the northern cluster region, just before the location of the two extended peripheral relics. From the optical analysis, we find that the cluster galaxies show a large $σ_V$, and two components are needed to fit their velocity distribution. Observational results were compared with a cluster simulation. The maps of gas temperature and density distribution from the simulation agree with the observational data in the case of a cluster merger nearly perpendicular to the plane of the sky and in the state after the maximum core collapse. We conclude that we are observing a galaxy cluster in a major merger phase, just after the maximum core collapse. The mass ratio is about 1:2, and the merger collision is nearly perpendicular to the plane of the sky. A shock is present in the northern cluster region, and it is very likely responsible for the electron reacceleration and the magnetic field amplification that will give cause the cluster relics. The relative position of the shock and the relics is strongly affected by projection effects.

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arXiv2008

Detection of diffuse radio emission at large distance from the center of the galaxy cluster A2255

Low-frequency radio observations of galaxy clusters are the key to detecting the diffuse extended emission associated with them. The presence and properties of such radio sources in galaxy clusters reveal the existence of magnetic fields on a large scale and allow theories to be tested concerning both the origin of relativistic particles in the ICM and their propagation. A deep radio bservation of the A2255 galaxy cluster was carried out at 85 cm with the WSRT. The good UV-coverage and sensitivity achieved by these observations allowed us to image the complex structure of the low-brightness, extended cluster sources (radio halo and relic). These sources show a larger extent than what has been imaged so far at this frequency, with two new structures located SW and NW of the cluster center and at projected distances of 2 Mpc from it. The physical properties of the newly detected structures, together with the active dynamical state of the cluster, support a connection with large-scale structure (LSS) formation shocks.

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arXiv2008

Observations of extended radio emission in clusters

We review observations of extended regions of radio emission in clusters; these include diffuse emission in `relics', and the large central regions commonly referred to as `halos'. The spectral observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.

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arXiv2007

The intracluster magnetic field power spectrum in Abell 2382

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 simulations 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 $μ$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 $μ$G.

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arXiv2007

In search of dying radio sources in the local universe

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.

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arXiv2007

New scaling relations in cluster radio halos and the re-acceleration model

In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P_R) vs size of the radio emitting region (R_H), and P_R vs total cluster mass (M_H) contained in the emitting region and cluster velocity dispersion (sigma_H) in this region. We search for these correlations by analyzing the most recent radio and X-ray data available in the literature for a well known sample of clusters with giant radio halos. In particular we find a good correlation between P_R and R_H and a very tight ``geometrical'' scaling between M_H and R_H. From these correlations P_R is also expected to scale with M_H and sigma_H and this is confirmed by our analysis. We show that all the observed trends can be well reconciled with expectations in the case of a slight variation of the mean magnetic field strength in the radio halo volume with M_H. A byproduct correlation between R_H and sigma_H is also found, and can be further tested by optical studies. In addition, we find that observationally R_H scales non-linearly with the virial radius of the host cluster, and this immediately means that the fraction of the cluster volume which is radio emitting increases with cluster mass and thus that the non-thermal component in clusters is not self-similar.

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