arXiv subjects

Federica Govoni

Publications and source records attributed to Federica Govoni.

32 records · Page 2Linked to original sources

arXiv2017

Narrow head-tail radio galaxies at very high resolution

Aims. Narrow-angle tailed (NAT) sources in clusters of galaxies can show on the large scale very narrow tails that are unresolved even at arcsecond resolution. These sources could therefore be classified as one-sided jets. The aim of this paper is to gain new insight into the structure of these sources, and establish whether they are genuine one-sided objects, or if they are two-sided sources. Methods. We observed a sample of apparently one-sided NAT sources at subarcsecond resolution to obtain detailed information on their structure in the nuclear regions of radio galaxies. Results. Most radio galaxies are found to show two-sided jets with sharp bends, and therefore the sources are similar to the more classical NATs, which are affected by strong projection effects.

astro-ph.GA↗

arXiv2016

A Strong Merger Shock in Abell 665

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 $\pm$ 0.6, propagating in front of a cold disrupted cloud. This makes Abell~665 the second cluster where a strong merger shock of $M \approx$ 3 has been detected, after the Bullet cluster. The shock velocity from jump conditions is consistent with (2.7 $\pm$ 0.7) $\times$ 10$^3$ km sec$^{-1}$. 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.

astro-ph.HE↗

arXiv2015

Using SKA Rotation Measures to Reveal the Mysteries of the Magnetised Universe

We know that magnetic fields are pervasive across all scales in the Universe and over all of cosmic time and yet our understanding of many of the properties of magnetic fields is still limited. We do not yet know when, where or how the first magnetic fields in the Universe were formed, nor do we fully understand their role in fundamental processes such as galaxy formation or cosmic ray acceleration or how they influence the evolution of astrophysical objects. The greatest challenge to addressing these issues has been a lack of deep, broad bandwidth polarimetric data over large areas of the sky. The Square Kilometre Array will radically improve this situation via an all-sky polarisation survey that delivers both high quality polarisation imaging in combination with observations of 7-14 million extragalactic rotation measures. Here we summarise how this survey will improve our understanding of a range of astrophysical phenomena on scales from individual Galactic objects to the cosmic web.

astro-ph.CO↗

arXiv2015

Statistical methods for the analysis of rotation measure grids in large scale structures in the SKA era

To better understand the origin and properties of cosmological magnetic fields, a detailed knowledge of magnetic fields in the large-scale structure of the Universe (galaxy clusters, filaments) is crucial. We propose a new statistical approach to study magnetic fields on large scales with the rotation measure grid data that will be obtained with the new generation of radio interferometers.

astro-ph.CO↗

arXiv2015

Mega-parsec scale magnetic fields in low density regions in the SKA era: filaments connecting galaxy clusters and groups

The presence of magnetic fields in galaxy clusters has been well established in recent years, and their importance for the understanding of the physical processes at work in the Intra Cluster Medium has been recognized. Halo and relic sources have been detected in several tens clusters. A strong correlation is present between the halo and relic radio power and the X-ray luminosity. Since cluster X-Ray luminosity and mass are related, the correlation between the radio power and X-ray luminosity could derive from a physical dependence of the radio power on the cluster mass, therefore the cluster mass could be a crucial parameter in the formation of these sources. The goal of this project is to investigate the existence of non-thermal structures beyond the Mpc scale, and associated with lower density regions with respect to clusters of galaxies: galaxy filaments connecting rich clusters. We present a piece of evidence of diffuse radio emission in intergalactic filaments. Moreover, we present and discuss the detection of radio emission in galaxy groups and in faint X-Ray clusters, to analyze non-thermal properties in low density regions with physical conditions similar to galaxy filaments. We discuss how SKA1 observations will allow the investigation of this topic and the study of the presence of diffuse radio sources in low density regions. This will be a fundamental step to understand the origin and properties of cosmological magnetic fields.

astro-ph.GA↗

arXiv2014

Non-thermal emission from galaxy clusters: feasibility study with SKA1

Galaxy clusters are known to host a variety of extended radio sources: tailed radio galaxies whose shape is modelled by the interaction with the intra-cluster medium (ICM); radio bubbles filling cavities in the ICM distribution and rising buoyantly through the thermal gas; diffuse giant radio sources ("halos" and "relics") revealing the presence of relativistic electrons and magnetic fields in the intra-cluster volume. It is currently the subject of an active debate how the non-thermal components that we observe at radio wavelengths affect the physical properties of the ICM and depend on the dynamical state of galaxy clusters. In this work we start our SKA1 feasibility study of the "radio cluster zoo" through simulations of a typical radio-loud cluster, hosting several bright tailed radio galaxies and a diffuse radio halo. Realistic simulations of SKA1 observations are obtained through the MeqTrees software. A new deconvolution algorithm, based on sparse representations and optimised for the detection of faint diffuse astronomical sources, is tested and compared to the classical CLEAN method.

astro-ph.CO↗

arXiv2014

White Paper: Radio Emission and Polarization Properties of Galaxy Clusters with VLASS

We outline the science case for extended radio emission and polarization in galaxy clusters which would be a scientifically important area of research for an upcoming Jansky Very Large Array Sky Survey. The survey would provide a major contribution in three key areas of the physics of clusters: 1) the active galactic nucleus population and the impact of feedback on the evolution of the intra-cluster medium, 2) the origin and evolution of diffuse cluster radio sources to probe the physics of mergers with implications for cosmology, and 3) the origin and role of magnetic fields in the ICM and in large scale structures. Considering all three areas, a survey must have sufficient spatial resolution to study the tailed galaxies which trace the cluster weather as well as the radio lobes driving energy into the cluster from the central AGN. The survey must also have sensitivity to low surface brightness emission and large angular scales to probe radio halos and relics as well as the WHIM residing in the large scale structure filaments. Finally, we note that full polarization information would be a highly valuable tool to probe a number of cluster-related issues. Due to the general steep spectral index of the emission we consider the survey is best suited to this science when conducted in P, L, or S bands. We conclude that the choices of S Band + D Configuration, L Band + C Configuration, and P Band + B Configuration offer optimal resolutions for constraining galactic interactions and feedback in cluster environments, while still probing large scale structure and the bulk cluster environment itself. While the push to probe higher redshifts and lower mass limits strongly favors a narrow and deep (or even targeted) survey strategy, we note that a wide survey covering roughly 1/4-2/3 of the sky will have significant scientific return, discovery potential, and archival value.

astro-ph.CO↗

arXiv2012

Comparisons of Cosmological MHD Galaxy Cluster Simulations to Radio Observations

Radio observations of galaxy clusters show that there are $μ$G magnetic fields permeating the intra-cluster medium (ICM), but it is hard to accurately constrain the strength and structure of the magnetic fields without the help of advanced computer simulations. We present qualitative comparisons of synthetic VLA observations of simulated galaxy clusters to radio observations of Faraday Rotation Measure (RM) and radio halos. The cluster formation is modeled using adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the assumption that the initial magnetic fields are injected into the ICM by active galactic nuclei (AGNs) at high redshift. In addition to simulated clusters in Xu et al. (2010, 2011), we present a new simulation with magnetic field injections from multiple AGNs. We find that the cluster with multiple injection sources is magnetized to a similar level as in previous simulations with a single AGN. The RM profiles from simulated clusters, both $|RM|$ and the dispersion of RM ($σ_{RM}$), are consistent at a first-order with the radial distribution from observations. The correlations between the $σ_{RM}$ and X-ray surface brightness from simulations are in a broad agreement with the observations, although there is an indication that the simulated clusters could be slightly over-dense and less magnetized with respect to those in the observed sample. In addition, the simulated radio halos agree with the observed correlations between the radio power versus the cluster X-ray luminosity and between the radio power versus the radio halo size. These studies show that the cluster wide magnetic fields that originate from AGNs and are then amplified by the ICM turbulence (Xu et al. 2010) match observations of magnetic fields in galaxy clusters.

astro-ph.CO↗

arXiv2012

Clusters of galaxies : observational properties of the diffuse radio emission

Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures...(abridged)... Most of the detailed knowledge of galaxy clusters has been obtained in recent years from the study of ICM through X-ray Astronomy. At the same time, radio observations have proved that the ICM is mixed with non-thermal components, i.e. highly relativistic particles and large-scale magnetic fields, detected through their synchrotron emission. The knowledge of the properties of these non-thermal ICM components has increased significantly, owing to sensitive radio images and to the development of theoretical models. Diffuse synchrotron radio emission in the central and peripheral cluster regions has been found in many clusters. Moreover large-scale magnetic fields appear to be present in all galaxy clusters, as derived from Rotation Measure (RM) studies. Non-thermal components are linked to the cluster X-ray properties, and to the cluster evolutionary stage, and are crucial for a comprehensive physical description of the intracluster medium. They play an important role in the cluster formation and evolution. We review here the observational properties of diffuse non-thermal sources detected in galaxy clusters: halos, relics and mini-halos. We discuss their classification and properties. We report published results up to date and obtain and discuss statistical properties. We present the properties of large-scale magnetic fields in clusters and in even larger structures: filaments connecting galaxy clusters. We summarize the current models of the origin of these cluster components, and outline the improvements that are expected in this area from future developments thanks to the new generation of radio telescopes.

astro-ph.CO↗

arXiv2006

Observations of magnetic fields in regular and irregular clusters

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. The most detailed evidence for the presence of cluster magnetic fields comes from radio observations. Magnetic fields can be investigated through the synchrotron emission of cluster-wide diffuse sources and from studies of the rotation measure of polarized radio galaxies. I will review efforts to measure magnetic field strengths and power spectra and the main issues that have led to our knowledge on magnetic fields in regular and irregular clusters of galaxies.

astro-ph↗

arXiv2004

A2255: the First Detection of Filamentary Polarized Emission in a Radio Halo

A deep radio observation of the A2255 cluster of galaxies has been carried out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the excellent (u,v) coverage and sensitivity achieved by our observation, the low brightness diffuse extended sources in the cluster (radio halo and relic) have been imaged with unprecedented resolution and dynamic range. We find that the radio halo has filamentary structures that are strongly polarized. The fractional linear polarization reaches levels of $\simeq$ 20$-$40% and the magnetic fields appear ordered on scales of $\sim$400 kpc. This is the first successful attempt to detect polarized emission from a radio halo and provides strong evidence that in this cluster the magnetic field is ordered on large scales.

astro-ph↗

arXiv2004

Magnetic Field in Clusters of Galaxies

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the $μ$G level, with values up to tens of $μ$G at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant through the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster radius with a rough dependence on the thermal gas density. Moreover, cluster magnetic fields are likely to fluctuate over a wide range of spatial scales with values from a few kpc up to hundreds kpc. Important information on the cluster field are obtained by comparing the observational results with the prediction from numerical simulations. The origin of cluster magnetic fields is still debated. They might originate in the early Universe, either before or after the recombination, or they could have been deposited in the intracluster medium by normal galaxies, starburst galaxies, or AGN. In either case, magnetic fields undergo significant amplification during the cluster merger processes.

astro-ph↗

arXiv2003

Magnetic Fields in Galaxy Clusters

Magnetic fields in galaxy clusters can be investigated using a variety of techniques. Recent studies including radio halos, Inverse Compton hard X-ray emissions and Faraday rotation measure, are briefly outlined. A numerical approach for investigating cluster magnetic fields strength and structure is presented. It consists of producing simulated rotation measure, radio halo images, and radio halo polarization, obtained from 3-dimensional multi-scale cluster magnetic field models, and comparing with observations.

astro-ph↗

arXiv2000

Radio Halos and Relics in Clusters of Galaxies and Detection Statistics

New sensitive VLA observations confirm the existence of halo and relic sources in 6 Abell clusters where a diffuse emission was found in the NVSS. We find evidence that the frequency of clusters with halos and relics is larger in clusters with high X-ray luminosity. The evidence that the occurrence of a halo source is larger at high redshifts is marginal.

astro-ph↗