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Search Results: 1 - 10 of 219559 matches for " C. Fanti "
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The B3-VLA CSS sample. VIII: New optical identifications from the Sloan Digital Sky Survey. The ultraviolet-optical spectral energy distribution of the young radio sources
C. Fanti,R. Fanti,A. Zanichelli,D. Dallacasa,C. Stanghellini
Physics , 2011, DOI: 10.1051/0004-6361/201015379
Abstract: Compact steep-spectrum radio sources and giga-hertz peaked spectrum radio sources (CSS/GPS) are generally considered to be mostly young radio sources. In recent years we studied at many wavelengths a sample of these objects selected from the B3-VLA catalog: the B3-VLA CSS sample. Only ~ 60 % of the sources were optically identified. We aim to increase the number of optical identifications and study the properties of the host galaxies of young radio sources. We cross-correlated the CSS B3-VLA sample with the Sloan Digital Sky Survey (SDSS), DR7, and complemented the SDSS photometry with available GALEX (DR 4/5 and 6) and near-IR data from UKIRT and 2MASS. We obtained new identifications and photometric redshifts for eight faint galaxies and for one quasar and two quasar candidates. Overall we have 27 galaxies with SDSS photometry in five bands, for which we derived the ultraviolet-optical spectral energy distribution (UV-O-SED). We extended our investigation to additional CSS/GPS selected from the literature. Most of the galaxies show an excess of ultra-violet (UV) radiation compared with the UV-O-SED of local radio-quiet ellipticals. We found a strong dependence of the UV excess on redshift and analyzed it assuming that it is generated either from the nucleus (hidden quasar) or from a young stellar population (YSP). We also compare the UV-O-SEDs of our CSS/GPS sources with those of a selection of large size (LSO) powerful radio sources from the literature. If the major process of the UV excess is caused by a YSP, our conclusion is that it is the result of the merger process that also triggered the onset of the radio source with some time delay. We do not see evidence for a major contribution from a YSP triggered by the radio sources itself.
A sample of small size compact steep-spectrum radio sources. VLBI images and VLA polarization at 5 GHz
D. Dallacasa,M. Orienti,C. Fanti,R. Fanti,C. Stanghellini
Physics , 2013, DOI: 10.1093/mnras/stt710
Abstract: Global VLBI observations at 5 GHz have been performed to study the source morphology in 10 compact steep-spectrum (CSS) sources selected from the Peacock & Wall catalogue with the aim of finding asymmetric structures produced by the interaction with the ambient medium. The combination of these data and earlier 1.7-GHz observations allows the study of the spectral index distribution across the source structure and the unambiguous determination of the nature of each component. In seven sources we detected the core component with a flat or inverted spectrum. In six sources the radio emission has a two-sided morphology and comes mainly from steep-spectrum extended structures, like lobes, jets, and hotspots. Only one source, 0319+121, has a one-sided core-jet structure. In three out of the six sources with a two-sided structure the flux density arising from the lobes is asymmetric, and the brightest lobe is the one closest to the core, suggesting that the jets are expanding in an inhomogeneous ambient medium which may influence the source growth. The interaction between the jet and the environment may slow down the source expansion and enhance the luminosity due to severe radiative losses, likely producing an excess of CSS radio sources in flux density limited samples. The lobes of the other three asymmetric sources have a brighter-when-farther behaviour, in agreement with what is expected by projection and relativistic effects. Simultaneous VLA observations carried out to investigate the polarization properties of the targets detected significant polarized emission (~5.5%) only from the quasar 0319+121.
A complete sample of GHz-Peaked-Spectrum radio sources and its radio properties
C. Stanghellini,C. P. O'Dea,D. Dallacasa,S. A. Baum,R. Fanti,C. Fanti
Physics , 1998, DOI: 10.1051/aas:1998270
Abstract: 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%.
VLBA observations of GHz-Peaked-Spectrum radio sources at 15 GHz
C. Stanghellini,D. Dallacasa,C. P. O'Dea,S. A. Baum,R. Fanti,C. Fanti
Physics , 2001, DOI: 10.1051/0004-6361:20011101
Abstract: We present VLBA observations at 15 GHz of ten GHz Peaked Spectrum (GPS) radio sources. The cores are often difficult or impossible to locate. When likely cores are found, they account for a small fraction of the flux density in GPS galaxies - around or below 2%, while in GPS quasars they can account for more than 20% of the total flux density. We detect low polarization in the GPS sources -- i.e., typically less than a few percent and often less than one percent. This establishes that low polarization in the parsec scale structure is an important defining characteristic of the GPS sources. The dichotomy in the radio morphology versus optical identification, i.e., galaxies are symmetric and quasars are not, is basically confirmed from these new data, which also indicate that the radio emission from GPS quasars is dominated by a jet, with often a weak or hidden core, suggesting they are at moderate angles to the line of sight, and so are only moderately beamed.
The B3-Vla CSS sample. III: Evn & Merlin images at 18 cm
D. Dallacasa,C. Fanti,S. Giacintucci,C. Stanghellini,R. Fanti,L. Gregorini,M. Vigotti
Physics , 2002, DOI: 10.1051/0004-6361:20020578
Abstract: EVN and MERLIN observations at 18 cm are presented for 18 Compact Steep--spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources were marginally resolved in previous VLA A-configuration observations at 4.9 and 8.4 GHz or had peculiar morphologies, two of them looking like core-jets. The MERLIN images basically confirm the VLA structures at 8.4 GHz while the EVN and/or the combined images reveal several additional details.
The B3-VLA CSS sample. II: VLBA images at 18 cm
D. Dallacasa,S. Tinti,C. Fanti,R. Fanti,L. Gregorini,C. Stanghellini,M. Vigotti
Physics , 2002, DOI: 10.1051/0004-6361:20020576
Abstract: VLBA observations at 18 cm are presented for 28 Compact Steep-spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources were unresolved in previous VLA observations at high frequencies or their brightness distribution was dominated by an unresolved steep spectrum component. More than half of them also showed a low frequency turnover in their radio spectrum. The VLBA images display in most cases a compact symmetric structure. Only in a minority of cases complex morphologies are present.
Synchrotron Spectra and Ages of Compact Steep Spectrum Radio Sources
M. Murgia,C. Fanti,R. Fanti,L. Gregorini,U. Klein,K. -H. Mack,M. Vigotti
Physics , 1999, DOI: 10.1016/S1387-6473(01)00200-7
Abstract: The high-frequency integrated spectra of Compact Steep Spectrum (CSS) sources show breaks with a moderate spectral steepening well fitted by continuous injection synchrotron spectra. In lobe-dominated CSS sources the radiative ages deduced by the synchrotron theory are in the range of up to 0.1 Myears, if equipartition magnetic fields are assumed. These radiative ages are well correlated with the source size indicating that the CSS sources are young. In order to maintain the frustration scenario, in which the sources' lifetimes are about 10 Myears, their equipartition magnetic field would be systematically decreased by a factor of more than 20. To complete the sample used in this work, we conducted observations at 230 GHz with the IRAM 30-m telescope of those sources which did not have such high-frequency observations up to now.
Multi-frequency Analysis of the two CSS quasars 3C 43 & 3C 298
C. Fanti,R. Fanti,D. Dallacasa,A. McDonald,R. T. Schilizzi,R. E. Spencer
Physics , 2002, DOI: 10.1051/0004-6361:20021410
Abstract: We present and discuss observations made with MERLIN and VLBI at 1.7 and 5 GHz of the two CSS quasars 3C 43 and 3C 298. They show quite different morphologies, the former being a very distorted triple radio source, the latter a small FRII type object. Relativistic effects and structural distortions are discussed. Source ages are evaluated to be of the order of ~10^5 years, therefore 3C 43 and 3C 298 can be considered fairly ''young'' radio sources. Some inference is also derived on the properties of the medium surrounding the radio emitting regions in these sub-galactic objects, whose density could be as low as 10^-3 cm^-3.
A tessitura plurivocal do trabalho: efeitos monológicos e dialógicos em tens o
Maria da Glória C?rrea Di Fanti
Alfa : Revista de Linguística , 2005,
Abstract: Temos o objetivo, neste artigo, de observar como a tens o entre efeitos monológicos e dialógicos se relaciona com gêneros do discurso/gêneros da atividade e revela singularidades de processos de produ o de sentido no desenvolvimento do trabalho. Para tanto, partindo do dialogismo como princípio epistemológico de base, refletimos sobre a interdependência entre atividade de linguagem e de trabalho, buscando situar a posi o assumida frente à análise do trabalho. A seguir, desenvolvemos nota es sobre movimentos alteritários interdependentes efeitos monológicos e dialógicos – como constitutivos da produ o discursiva e do desenvolvimento do trabalho. Na seqüência, trazemos para debate a quest o dos gêneros do discurso/da atividade como meios híbridos de desenvolver o trabalho, que s o estilizados em cada enuncia o. No desenrolar da reflex o, analisamos um fragmento de uma situa o laboral da esfera jurídico-trabalhista, procurando problematizar caminhos alteritários –rela o com o outro – percorridos para que o trabalho se desenvolva.
HST/NICMOS observations of the host galaxies of powerful radio sources: Does size matter?
W. H. de Vries,C. P. O'Dea,P. D. Barthel,C. Fanti,R. Fanti,M. D. Lehnert
Physics , 2000, DOI: 10.1086/316825
Abstract: We present near-infrared J and K band imaging of a sample of powerful radio source host galaxies with the Hubble Space Telescope NICMOS2 camera. These sources have been selected on their double lobed radio structure, and include a wide range of projected radio source sizes. The largest projected linear sizes range from the compact Gigahertz Peaked Spectrum (GPS, <1 kpc) and Compact Steep Spectrum (CSS, < 20 kpc) radio sources, up to the large-scale (> 20 kpc) classical doubles (FR II radio sources). We investigate the dependence of host galaxy properties (including near-IR surface brightness profiles) on radio source size, using both our own and published data. The absolute magnitudes and surface brightness profiles are consistent with the host galaxies being regular giant elliptical galaxies rather than Brightest Cluster Galaxies (BCGs). We find that the GPS, CSS, and FR II host galaxies are a uniform class of objects, consistent with a scenario in which a powerful radio source evolves along this size sequence.
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