Context. X-shaped radio galaxies (XRGs) exhibit a pair of bright primary lobes and a pair of weak secondary lobes (“wings”), which are oriented with an angle that gives the structure a cross-like ...shape. Though several theoretical models have been proposed to explain their origin, there is currently not a general consensus on a formation scenario. Aims. We analysed new multifrequency Karl G. Jansky Very Large Array (JVLA) radio data at 1.5, 5.5, 6, and 9 GHz of the candidate XRG in Abell 3670 (A3670) in order to characterise and classify it for the first time and to investigate its origin. Methods. We produced flux, spectral index, and radiative age maps of A3670 by means of the new radio data. We investigated the connection between the radio galaxy and its host, a brightest cluster galaxy (BCG) with two optical nuclei classified as a dumbbell galaxy. Finally we discussed the literature models and compared them to the observed properties of A3670. Results. We classify A3670 as a Fanaroff–Riley I-type XRG and measured a 1.4 GHz radio power of P1.4 = 1.7 × 1025 W Hz−1. By estimating the radiative age of the various source components, we find that the wings are Δt ≃ 20 Myr older than the lobes. We verified that the lobes and wings are aligned with the major and minor axes of the optical galaxy, respectively, and we estimated a black hole mass of MBH ∼ 109 M⊙, which is in agreement with the typical properties of the XRGs. Conclusions. Among the discussed scenarios, the jet-shell interaction model may best reproduce the observed properties of A3670. The gas of a stellar shell is responsible for the deflection of the jets, thus forming the wings. The presence of stellar shells in A3670 is plausible, but it needs further optical observations to be confirmed.
Aims. We present and release photometric redshifts for a uniquely large and deep sample of 522286 objects with $i'_{\rm AB}\le 25$ in the Canada-France Hawaii Telescope Legacy Survey (CFHTLS) “Deep ...Survey” fields D1, D2, D3, and D4, which cover a total effective area of 3.2 $\deg^2$. Methods. We use 3241 spectroscopic redshifts with $0 \leq z \leq 5$ from the VIMOS VLT Deep Survey (VVDS) as a calibration and training set to derive these photometric redshifts. Using the “Le Phare” photometric redshift code, we developed a robust calibration method based on an iterative zero-point refinement combined with a template optimisation procedure and the application of a Bayesian approach. This method removes systematic trends in the photometric redshifts and significantly reduces the fraction of catastrophic errors (by a factor of 2), a significant improvement over traditional methods. We use our unique spectroscopic sample to present a detailed assessment of the robustness of the photometric redshift sample. Results. For a sample selected at $i'_{\rm AB}\le 24$, we reach a redshift accuracy of $\sigma_{\Delta z/(1+z)}=0.029$ with $\eta=3.8\%$ of catastrophic errors (η is defined strictly as those objects with $|\Delta z|/(1+z) > 0.15$). The reliability of our photometric redshifts decreases for faint objects: we find $\sigma_{\Delta z/(1+z)}=0.025, 0.034$ and $\eta=1.9\%, 5.5\%$ for samples selected at $i'_{\rm AB}=17.5$–22.5 and 22.5–24 respectively. We find that the photometric redshifts of starburst galaxies are less reliable: although these galaxies represent only 22% of the spectroscopic sample, they are responsible for 50% of the catastrophic errors. An analysis as a function of redshift demonstrates that our photometric redshifts work best in the redshift range $0.2\le z \le 1.5$. We find an excellent agreement between the photometric and the VVDS spectroscopic redshift distributions at $i'_{\rm AB}\le 24$. Finally, we compare the redshift distributions of i' selected galaxies on the four CFHTLS deep fields, showing that cosmic variance is still present on fields of 0.7–0.9 deg2. These photometric redshifts are made publicly available at http://terapix.iap.fr (complete ascii catalogues) and http://cencos.oamp.fr/cencos/CFHTLS/ (searchable database interface).
We report the detection of diffuse radio emission which might be connected to a large-scale filament of the cosmic web covering a 8° × 8° area in the sky, likely associated with a z ≈ 0.1 overdensity ...traced by nine massive galaxy clusters. In this work, we present radio observations of this region taken with the Sardinia Radio Telescope. Two of the clusters in the field host a powerful radio halo sustained by violent ongoing mergers and provide direct proof of intracluster magnetic fields. In order to investigate the presence of large-scale diffuse radio synchrotron emission in and beyond the galaxy clusters in this complex system, we combined the data taken at 1.4 GHz with the Sardinia Radio Telescope with higher resolution data taken with the NRAO VLA Sky Survey. We found 28 candidate new sources with a size larger and X-ray emission fainter than known diffuse large-scale synchrotron cluster sources for a given radio power. This new population is potentially the tip of the iceberg of a class of diffuse large-scale synchrotron sources associated with the filaments of the cosmic web. In addition, we found in the field a candidate new giant radio galaxy.
Aims. 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. Methods. For this purpose we present Very Large Array observations at 20 cm and 6 cm for 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 reproduce the observations best. Results. 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 close to 35 kpc. The average magnetic field strength at the cluster centre is about 3 μG and decreases in the external region as the square root of the electron gas density. The average magnetic field strength in the central 1 Mpc3 is about 1 μG.
ABSTRACT
We present deep total intensity and polarization observations of the Coma cluster at 1.4 and 6.6 GHz performed with the Sardinia Radio Telescope. By combining the single-dish 1.4 GHz data ...with archival Very Large Array observations, we obtain new images of the central radio halo and of the peripheral radio relic where we properly recover the brightness from the large-scale structures. At 6.6 GHz, we detect both the relic and the central part of the halo in total intensity and polarization. These are the highest frequency images available to date for these radio sources in this galaxy cluster. In the halo, we find a localized spot of polarized signal, with fractional polarization of about 45 per cent. The polarized emission possibly extends along the north-east side of the diffuse emission. The relic is highly polarized, up to 55 per cent, as usually found for these sources. We confirm the halo spectrum is curved, in agreement with previous single-dish results. The spectral index is α = 1.48 ± 0.07 at a reference frequency of 1 GHz and varies from α ≃ 1.1, at 0.1 GHz, up to α ≃ 1.8, at 10 GHz. We compare the Coma radio halo surface brightness profile at 1.4 GHz (central brightness and e-folding radius) with the same properties of the other haloes, and we find that it has one of the lowest emissivities observed so far. Reanalysing the relic’s spectrum in the light of the new data, we obtain a refined radio Mach number of M = 2.9 ± 0.1.
Abstract
We observed the galaxy cluster CIZA J2242.8+5301 with the Sardinia Radio Telescope to provide new constraints on its spectral properties at high frequency. We conducted observations in three ...frequency bands centred at 1.4, 6.6 and 19 GHz, resulting in beam resolutions of 14, 2.9 and 1 arcmin, respectively. These single-dish data were also combined with archival interferometric observations at 1.4 and 1.7 GHz. From the combined images, we measured a flux density of S
1.4 GHz = (158.3 ± 9.6) mJy for the central radio halo and S
1.4 GHz = (126 ± 8) and (11.7 ± 0.7) mJy for the northern and the southern relics, respectively. After the spectral modelling of the discrete sources, we measured at 6.6 GHz S
6.6 GHz = (17.1 ± 1.2) and (0.6 ± 0.3) mJy for the northern and southern relics, respectively. Assuming simple diffusive shock acceleration, we interpret measurements of the northern relic with a continuous injection model represented by a broken power law. This yields an injection spectral index αinj = 0.7 ± 0.1 and a Mach number M = 3.3 ± 0.9, consistent with recent X-ray estimates. Unlike other studies of the same object, no significant steepening of the relic radio emission is seen in data up to 8.35 GHz. By fitting the southern relic spectrum with a simple power law (S
ν ∝ ν−α), we obtained a spectral index α ≈ 1.9 corresponding to a Mach number (M ≈ 1.8) in agreement with X-ray estimates. Finally, we evaluated the rotation measure of the northern relic at 6.6 GHz. These results provide new insights on the magnetic structure of the relic, but further observations are needed to clarify the nature of the observed Faraday rotation.
We present detailed imaging of Faraday rotation and depolarization for the radio galaxies 0206+35, 3C 270, 3C 353 and M 84, based on Very Large Array observations at multiple frequencies in the range ...1365 to 8440 MHz. All of the sources show highly anisotropic banded rotation measure (RM) structures with contours of constant RM perpendicular to the major axes of their radio lobes. All except M84 also have regions in which the RM fluctuations have lower amplitude and appear isotropic. We give a comprehensive description of the banded RM phenomenon and present an initial attempt to interpret it as a consequence of interactions between the sources and their surroundings. We show that the material responsible for the Faraday rotation is in front of the radio emission and that the bands are likely to be caused by magnetized plasma which has been compressed by the expanding radio lobes. We present a simple model for the compression of a uniformly magnetized external medium and show that RM bands of approximately the right amplitude can be produced, but only for special initial conditions. A two-dimensional magnetic structure in which the field lines are a family of ellipses draped around the leading edge of the lobe can produce RM bands in the correct orientation for any source orientation. We also report the first detections of rims of high depolarization at the edges of the inner radio lobes of M 84 and 3C 270. These are spatially coincident with shells of enhanced X-ray surface brightness, in which both the field strength and the thermal gas density are likely to be increased by compression. The fields must be tangled on small scales.
Abstract
We present high sensitivity (σP ≃ 0.6 mJy) polarimetric observations in seven bands, from 2.1 to 38 GHz, of a complete sample of 104 compact extragalactic radio sources brighter than 200 mJy ...at 20 GHz. Polarization measurements in six bands, in the range 5.5–38 GHz, for 53 of these objects were reported by Galluzzi et al. We have added new measurements in the same six bands for another 51 sources and measurements at 2.1 GHz for the full sample of 104 sources. Also, the previous measurements at 18, 24, 33, and 38 GHz were re-calibrated using the updated model for the flux density absolute calibrator, PKS1934−638, not available for the earlier analysis. The observations, carried out with the Australia Telescope Compact Array, achieved a 90 per cent detection rate (at 5σ) in polarization. 89 of our sources have a counterpart in the 72–231 MHz GLEAM (GaLactic and Extragalactic All-sky Murchison Widefield Array) survey, providing an unparalleled spectral coverage of 2.7 decades of frequency for these sources. While the total intensity data from 5.5 to 38 GHz could be interpreted in terms of single component emission, a joint analysis of more extended total intensity spectra presented here, and of the polarization spectra, reveals that over 90 per cent of our sources show clear indications of at least two emission components. We interpret this as an evidence of recurrent activity. Our high sensitivity polarimetry has allowed a 5σ detection of the weak circular polarization for ∼ 38 per cent of the data set, and a deeper estimate of 20 GHz polarization source counts than has been possible so far.
We present high-sensitivity polarimetric observations ( sigma P NOT approximately equal to 0.6 mJy) in six bands covering the 5.5-38 GHz range of a complete sample of 53 compact extragalactic radio ...sources brighter than 200 mJy at 20 GHz. The observations, carried out with the Australia Telescope Compact Array, achieved a 91 per cent detection rate (at 5 sigma ). Within this frequency range, the spectra of about 95 per cent of sources are well fitted by double power laws, both in total intensity and in polarization, but the spectral shapes are generally different in the two cases. Most sources were classified as either steep- or peaked-spectrum but less than 50 per cent have the same classification in total and in polarized intensity. No significant trends of the polarization degree with flux density or with frequency were found. The mean variability index in total intensity of steep-spectrum sources increases with frequency for a 4-5 yr lag, while no significant trend shows up for the other sources and for the 8 yr lag. In polarization, the variability index, which could be computed only for the 8 yr lag, is substantially higher than in total intensity and has no significant frequency dependence.
Abstract
We present Atacama Large Millimetre/submillimiter Array (ALMA) high sensitivity ($\sigma _\mathrm{ P} \simeq 0.4\,$ mJy) polarimetric observations at $97.5\,$ GHz (Band 3) of a complete ...sample of 32 extragalactic radio sources drawn from the faint Planck–ATCA Co-eval Observations (PACO) sample (b < −75°, compact sources brighter than $200\,$ mJy at $20\,$ GHz). We achieved a detection rate of $~97\, {\rm per\, cent}$ at $3\, \sigma$ (only 1 non-detection). We complement these observations with new Australia Telescope Compact Array (ATCA) data between 2.1 and $35\,$GHz obtained within a few months and with data published in earlier papers from our collaboration. Adding the co-eval GaLactic and Extragalactic All-sky Murchison widefield array (GLEAM) survey detections between $70\,$ and $230\,$ MHz for our sources, we present spectra over more than 3 decades in frequency in total intensity and over about 1.7 decades in polarization. The spectra of our sources are smooth over the whole frequency range, with no sign of dust emission from the host galaxy at mm wavelengths or of a sharp high frequency decline due, for example, to electron ageing. We do however find indications of multiple emitting components and present a classification based on the number of detected components. We analyse the polarization fraction behaviour and distributions up to $97\,$ GHz for different source classes. Source counts in polarization are presented at $95\,$ GHz.
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