We have found a class of circular radio objects in the Evolutionary Map of the Universe Pilot Survey, using the Australian Square Kilometre Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs, about one arcmin diameter, that are unlike other objects previously reported in the literature. We explore several possible mechanisms that might cause these objects, but none seems to be a compelling explanation.

With their sizes larger than 0.7 Mpc, Giant Radio Galaxies (GRGs) are the largest individual objects in the Universe. To date, the reason why they reach such enormous extensions is still unclear. One of the proposed scenarios suggests that they are the result of multiple episodes of jet activity. Cross-correlating the INTEGRAL+Swift AGN population with radio catalogues (NVSS, FIRST, SUMSS), we found that 22% of the sources are GRG (a factor four higher than those selected from radio catalogues). Remarkably, all of the sources in the sample show signs of restarting radio activity. The X-ray properties are consistent with this scenario, the sources being in a high-accretion, high-luminosity state with respect to the previous activity responsible for the radio lobes.

We present a selection of our VLBI images of SN 1993J, along with our latest results on its expansion and its shell structure.

We present the preliminary results of 235 MHz, 327 MHz and 610 MHz observations of the galaxy cluster A3562 in the core of the Shapley Concentration (SC). The purpose of these observations, carried out with the Giant Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo located at the centre of A3562 and determine the shape of its radio spectrum at low frequencies, in order to understand the origin of this source. In the framework of the re–acceleration model, the preliminary analysis of the halo spectrum suggests that we are observing a young source (few $10^8$ yrs) at the beginning of the re–acceleration phase.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Cluster mergings are expected to have an influence on the radio emission of the galaxy population. We present the results of a deep radio survey in the A3558 complex in the central region of the Shapley Concentration, in order to further explore our hypothesis of a dependence of the radio luminosity function on the age of the merging.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present preliminary results of a multifrequency and multiresolution study carried out with the Australia Telescope Compact Array for nine of the ten extended radio galaxies located in the merging cluster complexes A3558 and A3528, at the centre of the Shapley Concentration. We found that 5 out of the 9 extended radio galaxies are active radio galaxies, i.e. they have a clear active radio nucleus coincident with the central region of the associated optical galaxy, radio jets and extended lobes; the remaining four lack an obvious radio nucleus, have a very diffuse and amorphous morphology and exhibit peculiar spectral properties. We call these radio sources as remnants and propose that they are (a) either radio galaxies where the nuclear activity has ceased; or (b) regions where pre-existing electrons have been reaccelerated as consequence of shocks due to cluster mergers.

Twenty-two consecutive VLBI images of supernova 1993J in the galaxy M81 taken over 7 years show, in unprecedented detail, the dynamic evolution of the expanding radio shell of an exploded star. High precision astrometry using phase-referencing shows that the supernova expands isotropically, and that its geometric center has a formal proper motion of 190±110 km s−1 w.r.t. the core of M81. Systematic changes in the images most likely reflect a pattern of inhomogeneities in the medium left over from the progenitor star, or possibly instabilities in the expanding shell. As the shockfront sweeps up the medium, it is progressively decelerated, and after 7 years it has slowed to less than 1/2 its original expansion velocity. SN1993J is likely now entering the early stages of the adiabatic phase common in much older supernova remnants.

We performed a multifrequency and multiepoch study of the quasar PKS 1510-089 with ground and space VLBI, in order to study its nuclear properties and test current models for the production of the emission at X- and γ-ray energies. A preliminary analysis of our images suggests that the lower limit to the Lorentz factor γ at the distance of ∼ 12 pc from the central engine is γmin = 5. We used Ho = 100 km s−1 Mpc−1.

Twenty consecutive VLBI images of supernova 1993J in M81 from the time of explosion to the present show the dynamic evolution of the expanding radio shell of an exploded star. No clear sign of a pulsar nebula, expected to have a spectral luminosity 10 to 1,000 times larger than that of the Crab, has yet been seen. The upper limit on the brightness at 8.4 GHz in the center of the shell in one of the latest images is 0.15 mJy per beam of 0.4 mas2, corresponding to a spectral luminosity of that of the Crab. Any nebula that may have formed in the center is probably still obscured by the surrounding thermal matter with no substantial filamentation having yet occurred in the latter.

We present multi-frequency VLBA polarimetric observations of a small sample of quasars revealing their Faraday rotation measure (RM) structure at ~ 1 mas resolution. Two of the sources (OQ 172 and 3C 216) were chosen on the basis of high RMs seen by the VLA and single dish telescopes. A few more typical quasars (3C 273, 3C 279, 3C 345 and 3C 380) were also included in the study. In OQ 172, 3C 273, 3C 279 and 3C 380 we find high RMs (>3000 rad m−2) associated with the nuclear region. Only 1-10 mas from the nucleus the jet |RM|s in all 4 of these sources fall to less than 100 rad m−2. This RM structure may be produced by strong magnetic fields in the the narrow line region of the quasar.

From VLBI observations of 11 FR I radio galaxies we find that: 1. parsec scale jets are relativistic; 2. 3C 264 shows a relativistic jet decelerating moving from the core to the extended lobes; 3. 3C 338 is a source with asymmetric parsec scale structure and morphological changes, implying proper motion on both sides of the source; 4. 1144+35 is an extended low power radio galaxy with an apparent superluminal motion in its parsec scale radio structure.

We report on phase-referenced 5 and 8.4 GHz VLBI images of supernova SN 1993J in M81, tracking the evolution of the source from 50 to 1000 days after the explosion.

We shortly discuss here the parsec scale information presently available on the extended low power radio galaxy (FR I, Fanaroff and Riley, 1974) NGC6251 (Jones and Wehrle, 1994) and on 11 radio galaxies from the sample presented in Giovannini et al. (1990).

The Fanaroff-Riley type I radio galaxies (Fanaroff & Riley, 1974) presented in this paper belong to the complete sample of low-intermediate luminosity radio galaxies published in Giovannini, Feretti & Comoretto (1990). This sample includes radio galaxies with different morphologies on the arcsecond scale, such as compact sources, core-halos, FRIs and FRIIs.

We observed the cluster of galaxies A3556 (< v > = 14300 km/sec), belonging to the supercluster of galaxies known as the Shapley Concentration (Bardelli et al., 1994, MNRAS 267, 255, and references therein), with the Australia Telescope Compact Array. Our observations took place in the continuum band at 20 cm with the configurations 1.5D and 6C, and at 20 cm and 13 cm in the configuration 6A, for a total of 12×3 hours, with a resolution ranging from ≃ 6″ to ≃ 40″. We observed a sky region of ≃ 2° × 1° around the cluster center taking advance of the mosaicing technique developed ATNF.

The Coma cluster is a rich cluster of galaxies nested in an even larger supercluster of galaxies. The plane of the supercluster appears to be defined by the Coma cluster itself and another galaxy cluster, Abell 1367, that lies about 40 Mpc (H0 = 75 Mpc km−1s−1 (≡h75)) farther west (Tifft and Gregory 1976).

Compact Steep Spectrum (CSS) radio sources are defined as having straight steep spectra typical of extended double-lobed radio sources and structures unresolved by conventional interferometers (Peacock and Wall 1982; van Breugel et al., 1984; Fanti et al., 1985). Their spectra remain straight up to quite high frequencies, indicating that no dominant core is present. Sometimes the spectrum bends at frequencies lower than 1 GHz. The angular sizes imply projected linear sizes generally < 10 kpc.