arXiv subjects

T. Venturi

Publications and source records attributed to T. Venturi.

At least 145 records · Page 8Linked to original sources

arXiv2002

Radio emission from the A3571 cluster complex: the final stage of a cluster merger?

In this paper we report and discuss the results of a radio survey in the A3571 cluster complex, a structure located in the Shapley Concentration core, and formed by the three clusters A3571, A3572 and A3575. The survey was carried out simultaneously at 22 cm and 13 cm with the Australia Telescope Compact Array, and led to the detection of 124 radio sources at 22 cm. The radio source counts in this region are in agreement with the background counts. Among the 36 radio sources with optical counterpart, six have measured redshift which places them at the distance of the A3571 cluster complex, and nine radio sources have optical counterparts most likely members of this cluster complex. All radio galaxies emit at low power level, i.e. logP$_{22cm}$ (W Hz$^{-1}$) $\le$ 22.6. A number of them are likely to be starburst galaxies. The radio luminosity function of early type galaxies is in agreement with that derived by Ledlow & Owen (1996) if we restrict our analysis to the A3571 cluster. On the basis of the multiwavelength properties of the A3571 cluster complex, we propose that it is a very advanced merger, and explain the radio properties derived from our study in the light of this hypothesis.

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arXiv2001

Radio monitoring of a sample of X- and gamma-ray loud blazars

In this paper we present the results of a 4-year (1996 - 1999) radio flux density monitoring program for a sample of X- and $γ$-ray loud blazars. Our program started in January 1996 and was carried out on monthly basis at the frequencies of 5 GHz and 8.4 GHz with the 32-m antennas located in Medicina (Bologna, Italy) and Noto (Siracusa, Italy). 22 GHz data collected in Medicina from January 1996 to June 1997 will also be presented. The sample of selected sources comprises most radio loud blazars with $δ\ge -10^{\circ}$ characterised by emission in the X- and $γ$-ray regimes, and target sources for the BeppoSAX X-ray mission. All sources in the sample, except J1653+397 (MKN 501), are variable during the four years of our monitoring program. We classified the type of variability in each source by means of a structure function analysis. We also computed th$α_{5}^{8.4}$ for all epochs with nearly simultaneous observations ate spectral index and found that $α_{5}^{8.4}$ starts flattening at the very beginning o f a radio flare, or flux density increase.

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arXiv2001

A new sample of large angular size radio galaxies. II. The optical data

We constructed and presented in the first paper of this series a new sample of 84 large angular size radio galaxies by selecting from the NRAO VLA Sky Survey objects with angular size > 4', declination above +60 degrees and total flux density at 1.4 GHz > 100 mJy. In this paper we present optical spectra and images of those galaxies associated with the radio emission for which no redshift was known prior to our observations. Optical counterparts have been identified for all (but one) members of the sample. After our observations, a reliable spectroscopic redshift is available for 67 objects (80%) from the sample. This paper, second of a series of three, contributes to increase the number of well defined samples of radio galaxies with ample spectroscopic information.

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arXiv2001

Two-sided radio emission in ON231 (W Comae)

Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show remarkable new features in the source structure compared to those previously published. The images were obtained from observations made with the European VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical outburst occurred in Spring 1998. The up-to-date B band historic light curve of ON231 is also presented together with the R band luminosity evolution in the period 1994--1999. We identify the source core in the radio images with the brightest component having the flattest spectrum. A consequence of this assumption is the existence of a two--sided emission in ON231 not detected in previous VLBI images. A further new feature is a large bend in the jet at about 10 mas from the core. The emission extends for about 20 mas after the bend, which might be due to strong interaction with the environment surrounding the nucleus. We suggest some possible interpretations to relate the changes in the source structure with the optical and radio flux density variation in the frame of the unification model.

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arXiv2001

VLBA Polarimetry of Three Powerful Radio Galaxy Cores

We present sensitive, high dynamic range, Very Long Baseline Array (VLBA) polarimetric observations of the cores of three powerful radio galaxies: 3C166, 3C236 and 3C390.3. Significant polarization is detected in one source (3C166) allowing us to map out the Faraday Rotation Measure (RM) distribution and projected magnetic field direction. The inner jet of 3C166 is found to have a rest frame RM of -2300 rad/m**2, similar to those found in quasars cores. No polarized flux was detected from the other two sources, but in both counterjets are seen. The counterjet in 3C236 was previously known, but the detection in 3C390.3 is a new discovery. We suggest that the low fractional polarization in radio galaxy cores is the result of Faraday depolarization by ionized gas associated with the accretion disk. The lower polarization of radio galaxy cores compared to quasars is then naturally explained by unified models as a result of the viewing angle.

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arXiv2001

Radio Properties of the Shapley Concentration. IV. The A3528 Cluster Complex

We report and discuss the results of a 22 cm radio survey carried out with the Australia Telescope Compact Array (ATCA) covering the A3528 complex, a chain formed by the merging ACO clusters A3528-A3530-A3532, located in the central region of the Shapley Concentration. Simultaneous 13 cm observations are also presented. Our final catalogue includes a total of 106 radio sources above the flux density limit of 0.8 mJy. By cross correlation with optical and spectroscopic catalogues we found 32 optical counterparts, nine of them belonging tothe A3528 complex. We explored the effects of cluster mergers on the radio emission properties of radio galaxies by means of the radio luminosity function (RLF) and radio source counts. We found that the radio source counts are consistent with the background counts, as already found for the A3558 complex (Paper III). The RLF for this complex is consistent, both in shape and normalisation, with the general cluster luminosity function for early-type galaxies derived by Ledlow & Owen (LO96). This result is different from what we obtained for the A3558 merging complex, whose RLF is significantly lower than LO96. We propose that the different stage of the merger is responsible for the different RLFs in the two cluster complexes in the core of the Shapley Concentration. The early stage of merger for the A3528 complex, proposed by many authors, may have not yet affected the radio properties of cluster galaxies, while in the more much advanced merger in the A3558 region we actually see the effects of this process on the radio emission.

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arXiv2001

A new sample of large angular size radio galaxies. I. The radio data

We present a new sample of 84 large angular size radio galaxies selected from the NRAO VLA Sky Survey. Radio sources with declination above +60 degrees, total flux density greater than 100 mJy at 1.4 GHz and angular size larger than 4 arcmin have been selected and observed with the VLA at 1.4 and 4.9 GHz. The radio observations attempt to confirm the large angular size sources and to isolate the core emission for optical identification. In this paper, the first of a series of three, we present radio maps of 79 sources from the sample and discuss the effects of the selection criteria in the final sample. 37 radio galaxies belong to the class of giants, of which 22 are reported in this paper for the first time.

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arXiv2001

VLBI Observations of a Complete Sample of Radio Galaxies. 10 Years Later

A complete sample of 27 radio galaxies was selected from the B2 and 3CR catalogs, in order to study their properties on the milliarcsecond scale. In the Appendix of this paper we present new radio images for 12 of them. Thanks to the present data, all the sources in this sample have been imaged at mas resolution. We discuss the general results. In particular we stress the evidence for high velocity jets in low power radio galaxies, we compare high and low power sources, and discuss the source properties in the light of the unified scheme models. We derive that the properties of parsec scale jets are similar in sources with different total radio power and kpc scale morphology. From the core - total radio power correlation, we estimate that relativistic jets with Lorentz factor $γ$ in the range 3 - 10 are present in high and low power radio sources. We discuss also the possible existence of a two velocity structure in the jets (fast spine and lower velocity external shear layer).

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arXiv2000

The violent environment of the Shapley Concentration: a multiwavelength view

Rich superclusters are the ideal environment for the detection of cluster mergings, because the high peculiar velocities induced by the enhanced local density of the large-scale structure favour the cluster-cluster collisions, in the same way as seen in the simulations. The Shapley Concentration supercluster represents a unique laboratory where it is possible to follow cluster mergings and to test related astrophysical consequences, as the formation of shocks, radio halos, relics and wide angle tail radiosources, and the presence of galaxies with enhanced star formation. We present the results of an extensive multiwavelength survey of the central part of the Shapley Concentration, with the use of optical spectra, radio and X-ray data.

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arXiv2000

EVN Observations of GRS1915+105

We obtained EVN observations of the X-ray transient source GRS1915+105 in its radio-loud state. In 2 images obtained before a major flare, the source is resolved, showing two opposite asymmetric jets. This indicates that steady jets exist also before a major flare, with an estimated velocity of 0.2-0.6c.

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arXiv2000

Two classes of radio flares in the blazar PKS 0420$-$014

The two 5GHz VLBI (Very Long Baseline Interferometry) observations (1996 June and 1997 November) presented in this paper, combined with several formerb VLBI observations at 8.4GHz and 5GHz, suggest that the radio flares of the blazar PKS 0420$-$014 can be divided into two classes according to their geometric origins in 5 or 8.4GHz VLBI maps and the properties of light curves. One class of flares, which we call {\it core flares}, originate from the core. Core flares have large lags between the light curves at different frequencies, and will probably lead to the ejection of new jet components. The other class of flares, which we call {\it jet flares}, come from jet components. Jet flares vary simultaneously at different wavelengths, and may due to the Doppler boosting effect of rotating knots moving along a helical jet. The radio flare in 1991, accompanied by a simultaneous gamma-ray flare, was identified as a core flare.

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arXiv2000

VLBI Observations of Five Compact Radio Sources

Five compact radio sources, include 0420-014, 1334-127, 1504-166, 2243-123, and 2345-167, were observed at 5GHz by European VLBI (Very Long Baseline Interferometry) Network (EVN) in June, 1996. The primary purpose of this observation was to confirm their superluminal proper motions. Here, the results of 1334-127, 1504-166, 2243-123 and 2345-167 are presented.

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arXiv2000

Parsec-scale properties of a complete sample of radio galaxies

We report the most important results on the parsec-scale properties of a complete sample of radio galaxies carried out at radio frequencies with the Global VLBI array and with the VLBA. Relativistic parsec-scale jets are common both in FRI and FRII radio galaxies, and their orientation to the line of sight is in agreement with the expectations from the unified schemes for radio loud AGNs. Proper motion has been detected in a few FRI galaxies in the sample. FRI and FRII radio galaxies exhibit very similar properties on the parsec-scale. Finally a few radio sources in the sample show evidence of velocity structure in the parsec-scale jet, i.e. a central spine with high Lorentz factor $γ$ and slower layers.

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arXiv2000

Radio cores in blazars

We present results and first epoch parsec-scale images from a monitoring program on five $γ$--ray loud blazars selected on the basis of their total flux density variability. We observed 0048$-$097, 0235+164, 0954+658, 1510$-$089 and 1749+096 with the Very Long Baseline Array simultaneously at 8.4 GHz and 22 GHz. Comparison between our high quality images and previous results from the literature suggest that 0954+658 and 1510$-$089 are superluminal radio sources, with a Lorentz factor $γ$ of the order of $\sim 2 - 3$.

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arXiv2000

Parsec-scale structures of radio galaxies in the 2-Jy sample

In this paper we present the results of VLBI observations of six radio galaxies belonging to the 2-Jy sample. The selected objects are 3C17, PKS 0620-52, PKS 0625-35, PKS 1318-43, PKS 1333-33 and 3C317. The first is a high power radio galaxy and the remainder are low power objects. Our observations were carried out with a set of different arrays and frequencies, and cover a range of resolutions from a few mas to a few tens of mas. Parsec-scale images are presented and discussed in the light of unification for radio loud galaxies. Estimates for the intrinsic plasma speed in these objects in the proximity of the parsec-scale core and tehir orientation to the line of sight are in agreement with the predictions from unified models and their optical and X-ray properties.

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arXiv2000

The compact core-jet region of the superluminal quasar 3C216

Space very long baseline interferometry (VLBI) observations of the quasar 3C216 with the VLBI Space Observatory Programme (VSOP) reveal that the parsec scale structure of the source can be well described by compact jet models developed for interpreting the core region of radio-loud active galactic nuclei (AGN). The measured brightness temperature of T_(b) = 7.9*10^{11} K is comparable to the inverse Compton limit, from which we determine a lower limit of delta ~ 3.17 for the Doppler-boosting factor. The apparent transvers velocity of the superluminal component is beta_(app) = (3.0+-0.2) h^{-1} asuming a constant velocity, but deceleration of the jet material cannot be excluded from our data. A combination of the above values indicates that the viewing angle of the core-jet to the line-of-sight is less than 18.4 degrees, and the jet Lorentz-factor exceeds 3.16. The observed small size of the source is probably caused by both interaction with the interstellar medium, and a projection effect.

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arXiv2000

Radio Properties of the Shapley Concentration. III. Merging Clusters in the A3558 Complex

We present the results of a 22 cm radio survey carried out with the A3558 complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and thetwo groups SC1327-312 and SC1323-313, located in the central region of the complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the two groups SC1327-312 and SC1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio halos and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than that for cluster type galaxies and for those not selected to be in clusters at radio powers logP(1.4) > 22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. The detection of a head-tail source in the centre of A3562, coupled with careful inspection of the 20 cm NRAO VLA Sky Survey (NVSS) and of 36 cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC1329-313, i.e. a candidate radio halo at the centre of A3562, and low brightness extended emission around a 14.96 magnitude Shapley galaxy.

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arXiv1999

Restarting activity in radio galaxies

We present observations of two radio galaxies, J1835+620 and 3C338, both with signs of having passed through different stages of core activity. The former presents two symmetric and bright components within a typical FR II structure, possibly resulting from two distinct phases of activity; the latter is a FR I radio galaxy with two separated regions with different age properties, possibly due to a switch-off and -on cycle in its core. In both sources, the optical counterpart lies in a group of galaxies with indications of mutual interaction, a scenario often invoked to explain triggering of core activity.

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