arXiv subjects

D. Dallacasa

Publications and source records attributed to D. Dallacasa.

167 records · Page 10Linked to original sources

arXiv1999

Extended radio emission in BL Lac objects - I: the images

We have observed 28 sources selected from the 1Jy sample of BL Lac objects (Stickel et al. 1991) with the Very Large Array (VLA) in A, B and D configurations at 1.36, 1.66 and 4.85 GHz, and/or with the Westerbork Synthesis Radio Telescope (WSRT) at 1.40 GHz. In this paper we present high sensitivity images at arcsecond resolution of the 18 objects showing extended structure in our images, and of another source from the FIRST (Faint Images of the Radio Sky at Twenty-cm) survey (Becker et al. 1995). In general our high sensitivity images reveal an amount of extended emission larger than previously reported. In some objects the luminosity of the extended structure is comparable with that of FR~II radio sources. A future paper will be devoted to the interpretation of these results.

astro-ph↗

arXiv1999

The radio galaxies and the magnetic field in Abell 119

We present new, multiwavelength Very Large Array observations of the 3 radio galaxies in the cluster Abell 119: 0053-015, 0053-016 and 3C 29. The first two radio galaxies, which lie close to the cluster center, show a narrow-angle-tailed structure, with many twists in the tails. The third radio source is located at the cluster periphery, and shows an undistorted FR I morphology. All three radio sources are strongly polarized at the highest frequencies, and all three show both more depolarization and higher Faraday Rotation Measures with increasing proximity to the cluster center. We interpret this polarization behaviour as induced by the magneto-ionized intracluster medium, whose magnetic field is estimated to be in the range 5-10 microG.

astro-ph↗

arXiv1998

A radio survey of merging clusters in the Shapley Concentration

We present here the first results of a 22cm survey of the Shapley Concentration core. The observations were carried out with the Australia Telescope Compact Array. Our radio observations completely and uniformely cover the A3558 complex, allowing a thorough multifrequency study, by comparison of our results with the available optical spectroscopic and X-ray data of the whole chain. We will present here some statistical results of our survey and compare them with the information on the dynamics of the chain and on the properties of the intracluster gas. Attention will also be devoted to the extended radio galaxies found in our survey.

astro-ph↗

arXiv1998

A complete sample of GHz-Peaked-Spectrum radio sources and its radio properties

We define a complete sample of thirty-three GHz-Peaked-Spectrum (GPS) radio sources based on their spectral properties. We present measurements of the radio spectra and polarization of the complete sample and a list of additional GPS sources which fail one or more criteria to be included in the complete sample. The majority of the data have been obtained from quasi-simultaneous multi-frequency observations at the Very Large Array (VLA) during 3 observing sessions. Low frequency data from the Westerbork Synthesis Radio Telescope (WSRT) and from the literature have been combined with the VLA data in order to better define the spectral shape. The objects presented here show a rather wide range of spectral indices at high and low frequencies, including a few cases where the spectral index below the turnover is close to the theoretical value of 2.5 typical of self-absorbed incoherent synchrotron emission. Faint and diffuse extended emission is found in about 10% of the sources. In the majority of the GPS sources, the fractional polarization is found to be very low, consistent with the residual instrumental polarization of 0.3%.

astro-ph↗

arXiv1995

The magnetic field in the Coma cluster

The polarization data of the radio galaxy NGC4869, belonging to the Coma cluster and located in its central region, allow us to obtain information on the structure of magnetic field associated with the cluster itself. A magnetic field of $\sim$ 8.5 $μ$G, tangled on scales of the order of less than 1 kpc, is required to explain the observed fluctuations of the rotation measure. This magnetic field is more than one order of magnitude stronger than the equipartition value obtained for Coma C. This implies that the halo source Coma C may not be at the equipartition. Moreover, the need of efficient reacceleration mechanisms for the electrons radiating in Coma C is stronger. The energy supply to the Coma C radiating electrons is probably provided by the cluster merger process.

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