arXiv subjects

E. Carretti

Publications and source records attributed to E. Carretti.

At least 91 records · Page 5Linked to original sources

arXiv2015

A Broadband Polarization Catalog of Extragalactic Radio Sources

An understanding of cosmic magnetism requires converting the polarization properties of extragalactic radio sources into the rest-frame in which the corresponding polarized emission or Faraday rotation is produced. Motivated by this requirement, we present a catalog of multiwavelength linear polarization and total intensity radio data for polarized sources from the NRAO VLA Sky Survey (NVSS). We cross-match these sources with a number of complementary measurements -- combining data from major radio polarization and total intensity surveys such as AT20G, B3-VLA, GB6, NORTH6CM, Texas, and WENSS, together with other polarization data published over the last 50 years. For 951 sources, we present spectral energy distributions (SEDs) in both fractional polarization and total intensity, each containing between 3 and 56 independent measurements from 400 MHz to 100 GHz. We physically model these SEDs, and where available provide the redshift of the optical counterpart. For a superset of 25,649 sources we provide the total intensity spectral index, $α$. Objects with steep versus flat $α$ generally have different polarization SEDs: steep-spectrum sources exhibit depolarization, while flat-spectrum sources maintain constant polarized fractions over large ranges in wavelength. This suggests the run of polarized fraction with wavelength is predominantly affected by the local source environment, rather than by unrelated foreground magnetoionic material. In addition, a significant fraction (21%) of sources exhibit "repolarization", which further suggests that polarized SEDs are affected by different emitting regions within the source, rather than by a particular depolarization law. This has implications for the physical interpretation of future broadband polarimetric surveys.

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arXiv2015

Using rotation measure grids to detect cosmological magnetic fields -- a Bayesian approach

Determining magnetic field properties in different environments of the cosmic large-scale structure as well as their evolution over redshift is a fundamental step toward uncovering the origin of cosmic magnetic fields. Radio observations permit the study of extragalactic magnetic fields via measurements of the Faraday depth of extragalactic radio sources. Our aim is to investigate how much different extragalactic environments contribute to the Faraday depth variance of these sources. We develop a Bayesian algorithm to distinguish statistically Faraday depth variance contributions intrinsic to the source from those due to the medium between the source and the observer. In our algorithm the Galactic foreground and the measurement noise are taken into account as the uncertainty correlations of the galactic model. Additionally, our algorithm allows for the investigation of possible redshift evolution of the extragalactic contribution. This work presents the derivation of the algorithm and tests performed on mock observations. With cosmic magnetism being one of the key science projects of the new generation of radio interferometers we have made predictions for the algorithm's performance on data from the next generation of radio interferometers. Applications to real data are left for future work.

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arXiv2015

Faraday Tomography of the North Polar Spur: Constraints on the distance to the Spur and on the Magnetic Field of the Galaxy

We present radio continuum and polarization images of the North Polar Spur (NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization angle versus wavelength squared over 2048 frequency channels from 1280 to 1750 MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM map with a published Faraday depth map of the entire Galaxy in this direction, we derive the Faraday depth introduced by the NPS and the Galactic interstellar medium (ISM) in front of and behind the NPS. The Faraday depth contributed by the NPS is close to zero, indicating that the NPS is an emitting only feature. The Faraday depth caused by the ISM in front of the NPS is consistent with zero at b>50 degree, implying that this part of the NPS is local at a distance of approximately several hundred parsecs. The Faraday depth contributed by the ISM behind the NPS gradually increases with Galactic latitude up to b=44 degree, and decreases at higher Galactic latitudes. This implies that either the part of the NPS at b<44 degree is distant or the NPS is local but there is a sign change of the large-scale magnetic field. If the NPS is local, there is then no evidence for a large-scale anti-symmetry pattern in the Faraday depth of the Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes b>50 degree can be explained by including a coherent vertical magnetic field.

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arXiv2015

A radio-polarisation and rotation measure study of the Gum Nebula and its environment

The Gum Nebula is 36 degree wide shell-like emission nebula at a distance of only 450 pc. It has been hypothesised to be an old supernova remnant, fossil HII region, wind-blown bubble, or combination of multiple objects. Here we investigate the magneto-ionic properties of the nebula using data from recent surveys: radio-continuum data from the NRAO VLA and S-band Parkes All Sky Surveys, and H-alpha data from the Southern H-Alpha Sky Survey Atlas. We model the upper part of the nebula as a spherical shell of ionised gas expanding into the ambient medium. We perform a maximum-likelihood Markov chain Monte-Carlo fit to the NVSS rotation measure data, using the H-halpha data to constrain average electron density in the shell $n_e$. Assuming a latitudinal background gradient in RM we find $n_e=1.3^{+0.4}_{-0.4} {\rm cm}^{-3}$, angular radius $ϕ_{\rm outer}=22.7^{+0.1}_{-0.1} {\rm deg}$, shell thickness $dr=18.5^{+1.5}_{-1.4} {\rm pc}$, ambient magnetic field strength $B_0=3.9^{+4.9}_{-2.2} μ{\rm G}$ and warm gas filling factor $f=0.3^{+0.3}_{-0.1}$. We constrain the local, small-scale (~260 pc) pitch-angle of the ordered Galactic magnetic field to $+7^{\circ}\lesssim\wp\lesssim+44^{\circ}$, which represents a significant deviation from the median field orientation on kiloparsec scales (~-7.2$^{\circ}$). The moderate compression factor $X=6.0\,^{+5.1}_{-2.5}$ at the edge of the H-alpha shell implies that the 'old supernova remnant' origin is unlikely. Our results support a model of the nebula as a HII region around a wind-blown bubble. Analysis of depolarisation in 2.3 GHz S-PASS data is consistent with this hypothesis and our best-fitting values agree well with previous studies of interstellar bubbles.

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arXiv2015

New radio observations of anomalous microwave emission in the HII region RCW175

We have observed the HII region RCW175 with the 64m Parkes telescope at 8.4GHz and 13.5GHz in total intensity, and at 21.5GHz in both total intensity and polarization. High angular resolution, high sensitivity, and polarization capability enable us to perform a detailed study of the different constituents of the HII region. For the first time, we resolve three distinct regions at microwave frequencies, two of which are part of the same annular diffuse structure. Our observations enable us to confirm the presence of anomalous microwave emission (AME) from RCW175. Fitting the integrated flux density across the entire region with the currently available spinning dust models, using physically motivated assumptions, indicates the presence of at least two spinning dust components: a warm component with a relatively large hydrogen number density n_H=26.3/cm^3 and a cold component with a hydrogen number density of n_H=150/cm^3. The present study is an example highlighting the potential of using high angular-resolution microwave data to break model parameter degeneracies. Thanks to our spectral coverage and angular resolution, we have been able to derive one of the first AME maps, at 13.5GHz, showing clear evidence that the bulk of the AME arises in particular from one of the source components, with some additional contribution from the diffuse structure. A cross-correlation analysis with thermal dust emission has shown a high degree of correlation with one of the regions within RCW175. In the center of RCW175, we find an average polarized emission at 21.5GHz of 2.2\pm0.2(rand.)\pm0.3(sys.)% of the total emission, where we have included both systematic and statistical uncertainties at 68% CL. This polarized emission could be due to sub-dominant synchrotron emission from the region and is thus consistent with very faint or non-polarized emission associated with AME.

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arXiv2015

Cluster magnetic fields through the study of polarized radio halos in the SKA era

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 relativistic 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.

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arXiv2015

Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA

We investigate the possibility for the SKA to detect and study the magnetic fields in galaxy clusters and in the less dense environments surrounding them using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic field models for galaxy clusters of different masses and in different stages of their evolution, and derive mock rotation measure observations of background radiogalaxies. According to our results, already in phase I, we will be able to infer the magnetic field properties in galaxy clusters as a function of the cluster mass, down to $10^{13}$ solar-masses. Moreover, using cosmological simulations to model the gas density, we have computed the expected rotation measure through shock-fronts that occur in the intra-cluster medium during cluster mergers. The enhancement in the rotation measure due to the density jump will permit to constraint the magnetic field strength and structure after the shock passage. SKA observations of polarised sources located behind galaxy clusters will answer several questions about the magnetic field strength and structure in galaxy clusters, and its evolution with cosmic time.

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arXiv2014

The Spectral Variability of the GHz-Peaked Spectrum Radio Source PKS 1718-649 and a Comparison of Absorption Models

Using the new wideband capabilities of the Australia Telescope Compact Array (ATCA), we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over three epochs spanning approximately 21 months, reveal variability both above the spectral peak at ~3 GHz and below the peak. The combination of the low and high frequency variability cannot be easily explained using a single absorption mechanism, such as free-free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free-free optical depth on our line-of-sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of Active Galactic Nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free-free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free-free absorption model characterised by a power-law distribution of free-free absorbing clouds. This agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free-free and synchrotron self-absorption) that are based on simple homogenous structures.

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arXiv2014

First Look Murchison Widefield Array observations of Abell 3667

The Murchison Widefield Array (MWA) is a new low frequency interferometric radio telescope, operating in the remote Murchison Radio Observatory in Western Australia. In this paper we present the first MWA observations of the well known radio relics in Abell 3667 (A3667) between 120 and 226 MHz. We clearly detect the radio relics in A3667 and present flux estimates and spectral indices for these features. The average spectral index of the north-west (NW) and south-east (SE) relics is -0.9 +/- 0.1 between 120 and 1400 MHz. We are able to resolve spatial variation in the spectral index of the NW relic from -1.7 to -0.4, which is consistent with results found at higher frequencies.

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arXiv2014

The first Murchison Widefield Array low frequency radio observations of cluster scale non-thermal emission: the case of Abell 3667

We present the first Murchison Widefield Array observations of the well-known cluster of galaxies Abell 3667 (A3667) between 105 and 241 MHz. A3667 is one of the best known examples of a galaxy cluster hosting a double radio relic and has been reported to contain a faint radio halo and bridge. The origins of radio halos, relics and bridges is still unclear, however galaxy cluster mergers seems to be an important factor. We clearly detect the North-West (NW) and South-East (SE) radio relics in A3667 and find an integrated flux density at 149 MHz of 28.1 +/- 1.7 and 2.4 +/- 0.1 Jy, respectively, with an average spectral index, between 120 and 1400 MHz, of -0.9 +/- 0.1 for both relics. We find evidence of a spatial variation in the spectral index across the NW relic steepening towards the centre of the cluster, which indicates an ageing electron population. These properties are consistent with higher frequency observations. We detect emission that could be associated with a radio halo and bridge. How- ever, due to the presence of poorly sampled large-scale Galactic emission and blended point sources we are unable to verify the exact nature of these features.

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arXiv2014

Galactic interstellar turbulence in the southern sky seen through spatial gradients of the polarization vector

Radio synchrotron polarization maps of the Galaxy can be used to infer the properties of interstellar turbulence in the diffuse warm ionized medium (WIM). In this paper, we investigate the spatial gradient of linearly polarized synchrotron emission ($|\nabla\textbf{P}|/|\textbf{P}|$) as a tracer of turbulence, the relationship of the gradient to the sonic Mach number of the WIM, and changes in morphology of the gradient as a function of Galactic position in the southern sky. We use data from the S-band Polarization All Sky Survey (S-PASS) to image the spatial gradient of the linearly polarized synchrotron emission ($|\nabla \textbf{P}|/|\textbf{P}|$) of the entire southern sky at $2.3$~GHz. The spatial gradient of linear polarization reveals rapid changes of the density and magnetic fluctuations in the WIM due to magnetic turbulence as a function of Galactic position; we make comparisons of these data to ideal MHD numerical simulations. In order to constrain the sonic Mach number ($M_{s}$), we apply a high order moments analysis to the observations and to the simulated diffuse, isothermal ISM with ideal magneto-hydrodynamic turbulence. We find that polarization gradient maps reveal elongated structures, which we associate with turbulence in the ISM. Our analysis corroborates the view of a turbulent WIM in a transonic regime $M_{s}\lesssim 2$. Filamentary structures with typical widths down to the angular resolution are seen and the observed morphologies match closely with numerical simulations and in some cases H$α$ contours. The $|\nabla \textbf{P}|/|\textbf{P}|$ intensity is found to be approximately log-normal distributed. No systematic variations of the sonic Mach number are observed as a function of Galactic coordinates, which is consistent with turbulence in the WIM inferred by the analysis of H$α$ data.

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arXiv2013

SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl - First Science from the Pilot Region

SPLASH (the Southern Parkes Large-Area Survey in Hydroxyl) is a sensitive, unbiased and fully-sampled survey of the Southern Galactic Plane and Galactic Centre in all four ground-state transitions of the hydroxyl (OH) radical. The survey provides a deep census of 1612-, 1665-, 1667- and 1720-MHz OH absorption and emission from the Galactic ISM, and is also an unbiased search for maser sources in these transitions. We present here first results from the SPLASH pilot region, which covers Galactic longitudes 334 to 344 degrees and latitudes of -2 to +2 degrees. Diffuse OH is widely detected in all four transitions, with optical depths that are always small (averaged over the Parkes beam), and with departures from LTE common even in the 1665- and 1667-MHz main lines. To a 3$σ$ sensitivity of 30 mK, we find no evidence of OH envelopes extending beyond the CO-bright regions of molecular cloud complexes, and conclude that the similarity of the OH excitation temperature and the level of the continuum background is at least partly responsible for this. We detect masers and maser candidates in all four transitions, approximately 50 per cent of which are new detections. This implies that SPLASH will produce a substantial increase in the known population of ground-state OH masers in the Southern Galactic Plane.

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arXiv2013

Absolutely calibrated radio polarimetry of the inner Galaxy at 2.3 GHz and 4.8 GHz

We present high sensitivity and absolutely calibrated images of diffuse radio polarisation at a resolution of about 10 arcmin covering the range 10 degr <l<34 degr and |b|<5 degr at 2.3 GHz from the S-band Parkes All Sky Survey and at 4.8 GHz from the Sino-German 6 cm polarisation survey of the Galactic plane. Strong depolarisation near the Galactic plane is seen at 2.3 GHz, which correlates with strong Halpha emission. We ascribe the depolarisation to spatial Faraday rotation measure fluctuations of about 65 rad/m2 on scales smaller than 6-9 pc. We argue that most (about 90%) of the polarised emission seen at 4.8 GHz originates from a distance of 3-4 kpc in the Scutum arm and that the random magnetic field dominates the regular field there. A branch extending from the North Polar Spur towards lower latitudes can be identified from the polarisation image at 4.8 GHz but only partly from the polarised image at 2.3 GHz, implying the branch is at a distance larger than 2-3 kpc. We show that comparison of structure functions of complex polarised intensity with those of polarised intensity can indicate whether the observed polarised structures are intrinsic or caused by Faraday screens. The probability distribution function of gradients from the polarisation images at 2.3 GHz indicates the turbulence in the warm ionised medium is transonic.

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arXiv2013

A 20 GHz Bright Sample for Delta > 72 deg - II. Multi-frequency Follow-up

We present follow-up observations at 5, 8 and 30 GHz of the K-band Northern Wide Survey (KNoWS) 20 GHz Bright Sample, performed with the 32-m Medicina Radio Telescope and the 32-m Torun Radio Telescope. The KNoWS sources were selected in the Northern Polar Cap (Delta > 72 deg) and have a flux density limit S(20GHz) = 115 mJy. We include NVSS 1.4 GHz measurements to derive the source radio spectra between 1.4 and 30 GHz. Based on optical identifications, 68 per cent of the sources are QSOs, and 27 per cent are radio galaxies. A redshift measurement is available for 58 per cent of the sources. The radio spectral properties of the different source populations are found to be in agreement with those of other high-frequency selected samples.

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arXiv2013

Thermal plasma in the giant lobes of the radio galaxy Centaurus A

We present a Faraday rotation measure (RM) study of the diffuse, polarized, radio emission from the giant lobes of the nearest radio galaxy, Centaurus A. After removal of the smooth Galactic foreground RM component, using an ensemble of background source RMs located outside the giant lobes, we are left with a residual RM signal associated with the giant lobes. We find the most likely origin of this residual RM is from thermal material mixed throughout the relativistic lobe plasma. The alternative possibility of a thin-skin/boundary layer of magnetoionic material swept up by the expansion of the lobes is highly unlikely since it requires, at least, an order of magnitude enhancement of the swept up gas over the expected intragroup density on these scales. Strong depolarisation observed from 2.3 to 0.96 GHz also supports the presence of a significant amount of thermal gas within the lobes; although depolarisation solely due to RM fluctuations in a foreground Faraday screen on scales smaller than the beam cannot be ruled out. Considering the internal Faraday rotation scenario, we find a thermal gas number density of ~10^{-4} cm^{-3} implying a total gas mass of ~10^{10} M_solar within the lobes. The thermal pressure associated with this gas (with temperature kT ~ 0.5 keV, obtained from recent X-ray results) is approximately equal to the non-thermal pressure, indicating that over the volume of the lobes, there is approximate equipartition between the thermal gas, radio-emitting electrons and magnetic field (and potentially any relativistic protons present).

astro-ph.CO↗

arXiv2013

Detection of a radio bridge in Abell 3667

We have detected a radio bridge of unpolarized synchrotron emission connecting the NW relic of the galaxy cluster Abell 3667 to its central regions. We used data at 2.3 GHz from the S-band Polarization All Sky Survey (S-PASS) and at 3.3 GHz from a follow up observation, both conducted with the Parkes Radio Telescope. This emission is further aligned with a diffuse X-ray tail, and represents the most compelling evidence for an association between intracluster medium turbulence and diffuse synchrotron emission. This is the first clear detection of a bridge associated both with an outlying cluster relic and X-ray diffuse emission. All the indicators point toward the synchrotron bridge being related to the post-shock turbulent wake trailing the shock front generated by a major merger in a massive cluster. Although predicted by simulations, this is the first time such emission is detected with high significance and clearly associated with the path of a confirmed shock. Although the origin of the relativistic electrons is still unknown, the turbulent re-acceleration model provides a natural explanation for the large-scale emission. The equipartition magnetic field intensity of the bridge is B_eq = 2.2 +/- 0.3 μG. We further detect diffuse emission coincident with the central regions of the cluster for the first time.

astro-ph.CO↗

arXiv2013

Giant Magnetized Outflows from the Centre of the Milky Way

The nucleus of the Milky Way is known to harbour regions of intense star formation activity as well as a super-massive black hole. Recent Fermi space telescope observations have revealed regions of γ-ray emission reaching far above and below the Galactic Centre, the so-called Fermi bubbles. It is uncertain whether these were generated by nuclear star formation or by quasar-like outbursts of the central black hole and no information on the structures' magnetic field has been reported. Here we report on the detection of two giant, linearly-polarized radio Lobes, containing three ridge-like sub-structures, emanating from the Galactic Centre. The Lobes each extend ~60 deg, bear a close correspondence to the Fermi bubbles, are located in the Galactic bulge, and are permeated by strong magnetic fields of up to 15 μG. Our data signal that the radio Lobes originate in a bi-conical, star-formation (rather than black hole) driven outflow from the Galaxy's central 200 pc that transports a massive magnetic energy of ~10^55 erg into the Galactic halo. The ridges wind around this outflow and, we suggest, constitute a `phonographic' record of nuclear star formation activity over at least 10 Myr.

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arXiv2012

Large Radio Telescopes for Anomalous Microwave Emission Observations

We discuss in this paper the problem of the Anomalous Microwave Emission (AME) in the light of ongoing or future observations to be performed with the largest fully steerable radio telescope in the world. High angular resolution observations of the AME will enable astronomers to drastically improve the knowledge of the AME mechanisms as well as the interplay between the different constituents of the interstellar medium in our galaxy. Extragalactic observations of the AME have started as well, and high resolution is even more important in this kind of observations. When cross-correlating with IR-dust emission, high angular resolution is also of fundamental importance in order to obtain unbiased results. The choice of the observational frequency is also of key importance in continuum observation. We calculate a merit function that accounts for the signal-to-noise ratio (SNR) in AME observation given the current state-of-the-art knowledge and technology. We also include in our merit functions the frequency dependence in the case of multifrequency observations. We briefly mention and compare the performance of four of the largest radiotelescopes in the world and hope the observational programs in each of them will be as intense as possible.

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