arXiv subjects

W. D. Cotton

Publications and source records attributed to W. D. Cotton.

At least 109 records · Page 6Linked to original sources

arXiv2003

Parsec Scale Properties of Markarian 501

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 GHz and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb brightened structure in the jet is clearly visible in our data and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous multi-wavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

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arXiv2003

The Low Frequency Radio Counterpart of the XMM Large Scale Structure Survey

The XMM Large Scale Structure Survey (XMM-LSS) is a major project to map the large scale structure of the universe out to cosmological distances. An 8 degree by 8 degree region will be surveyed by XMM with planned optical follow-up to produce a three-dimensional map of many hundreds of clusters out to a redshift of z = 1. To explore the relation of the large scale structure to the location and properties of extragalactic radio sources, the XMM-LSS project also includes a low frequency radio survey of this region. This combination will provide unprecedented insight into how the radio source formation and evolution are affected by the local environment. Here, we present preliminary results from our 325 MHz and 74 MHz surveys in this region. At 325 MHz, we have a flux limit of 4 mJy/beam, a resolution of 6.3", and a total of 256 source detections over 5.6 square degrees. At 74 MHz, we have a flux limit of 275 mJy/beam, a resolution of 30", and a total of 211 source detections over 110 square degrees. We describe these results and explore what they tell us about the population of extra-galactic low frequency radio sources. The 74 MHz survey represents the first presentation of a deep, sub-arcminute resolution survey at such a low frequency. This was made possible by recent advances in both hardware and data reduction algorithms which we describe in detail.

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arXiv2002

Parsec Scale Properties of Radio Sources

We present a new complete sample of radio sources selected from the B2 and 3CR catalogues, with no bias respect to the jet velocity and orientation. Using preliminary data we investigate the parsec scale properties of radio sources with different radio power and kpc scale morphology. We stress the evidence for high velocity pc scale jets in all sources and conclude that the properties of parsec scale jets are similar regardless of the source total power and large scale morphology. Moreover we show two epoch images of two nearby radio galaxies: a low power compact source and a CSO. The comparison of the two epoch images suggests that in both sources the size is increasing with time. We derive a possible advance speed and estimate their dynamic age.

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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

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

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

The giant radio galaxy 8C0821+695 and its environment

We present new VLA and Effelsberg observations of the radio galaxy 8C0821+695. We have obtained detailed images in total intensity and polarization of this 2 Mpc sized giant. The magnetic field has a configuration predominantly parallel to the source main axis. We observe Faraday rotation at low frequencies, most probably produced by an ionized medium external to the radio source. The spectral index distribution is that typical of FR II radio galaxies, with spectral indices gradually steepening from the source extremes towards the core. Modeling the spectrum in the lobes using standard synchrotron loss models yields the spectral age of the source and the mean velocity of the jet-head with respect to the lobe material. The existence of a possible backflow in the lobe is considered to relate spectral with dynamical determinations of the age and the velocity with respect to the external medium. Through a very simple model, we obtain a physical characterization of the jets and the external medium in which the radio galaxy expands. The results in 8C0821+695 are consistent with a relativistic jet nourishing the lobes which expand in a hot, low density halo. We infer a deceleration of the source expansion velocity which we explain through a progressive increase in the hot-spot size.

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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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arXiv1999

Restarting activity in the giant radio galaxy J1835+620

We present radio and optical observations of the peculiar radio galaxy J1835+620, a member of a new sample of large angular size radio galaxies selected from the NRAO VLA Sky Survey. Its optical counterpart is in a group of at least three galaxies and shows strong, narrow emission lines which allow us to measure its redshift as z=0.518. The derived cosmological distance places J1835+620 among the giant radio galaxies. The outstanding aspect of J1835+620 at radio wavelengths is the existence of two symmetric bright components within a typical Fanaroff-Riley type II structure. We interpret the source as the result of two distinct phases of core activity. Radio maps and radio polarization properties are consistent with a dense new ejection evolving through an older underlying jet. We suggest that interaction with nearby galaxies could be the reason for restarting the activity in J1835+620.

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arXiv1999

B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion

We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. These observations are sensitive to structures on scales from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum lobes on the megaparsec scale are consistent with an age of $\sim$ 10$^8$ years. On the parsec scale, a complex jet component is seen to move away from the center of activity with an apparent velocity 2.7 h$^{-1}_{50}$ c. It shows a central spine -- shear layer morphology. A faint parsec scale counterjet is detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight of 25$^\circ$ are derived, consistent with an intrinsically symmetric ejection. The central spine in the parsec scale jet is expected to move at a higher velocity and a Lorentz factor $γ$ $\sim$ 15 has been estimated near the core.The age of this inner VLBI structure is $\sim$ 300 years. Assuming a constant angle to the line-of-sight, the jet velocity is found to decrease from 0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc on the plane of the sky). These findings lend credence to the claim that (1) even the jets of low power radio galaxies start out relativistic; and (2) these jets are decelerated to subrelativistic velocities by the time they reach kiloparsec scales.

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arXiv1999

A Parsec Scale Accelerating Radio Jet in the Giant Radio Galaxy NGC315

Observations of the core of the giant radio galaxy NGC315 made with VLBI interferometers are discussed in the context of a relativistic jet. The sidedness asymmetry suggests Doppler favoritism from a relativistic jet. The presence of moving features in the jet as well as jet counter--jet brightness ratios hint at an accelerating, relativistic jet. An increasing jet velocity is also supported by a comparison of the jet's observed properties with the predictions of an adiabatic expansion model. On the parsec scale, the jet is unpolarized at a wavelength of 6 cm to a very high degree in clear distinction to the high polarization seen on the kiloparsec scale.

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arXiv1998

The Broad Line Radio Galaxy J2114+820

In the frame of the study of a new sample of large angular size radio galaxies selected from the NRAO VLA Sky Survey, we have made radio observations of J2114+820, a low power radio galaxy with an angular size of 6'. Its radio structure basically consists of a prominent core, a jet directed in north-west direction and two extended S-shaped lobes. We have also observed the optical counterpart of J2114+820, a bright elliptical galaxy with a strong unresolved central component. The optical spectrum shows broad emission lines. This fact, together with its low radio power and FR-I type morphology, renders J2114+820 a non-trivial object from the point of view of the current unification schemes of radio loud active galactic nuclei.

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arXiv1998

The Radio-Optical Jet in NGC 3862 from Parsec tu Sub-Kiloparsec Scales

We have observed the radio source NGC 3862 (3C264) simultaneously with the EVN and the MERLIN arrays, obtaining detailed images of its radio structure from parsec to sub-kiloparsec scales. 3C264 shows a one-sided jet, with evident variations in its morphological properties with distance. We have analyzed HST optical data of NGC 3862 finding a one-to-one correspondence between radio and optical features in the jet. The radio to optical spectral index is approximately constant along the jet. Synchrotron appears as the most plausible mechanism for the observed jet emission at radio and optical wavelengths. Local particle reacceleration or, alternatively, deviation from the condition of equipartition, are plausible mechanisms to explain the low synchrotron cooling and constant spectral index in the jet of 3C264.

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arXiv1998

VLBI Observations of Mkn 501 and Mkn 421

We present here two epochs of Space VLBI observations at 18 cm of the BL-Lac type object Mkn 501. Thanks to the high resolution of these new data we have found that the inner jet is centrally brightened at its beginning but becomes extended and limb brightened at ~ 8 mas from the core. Moreover a comparison between the two epochs shows the presence of a possible proper motion with apparent velocity = 6.7c. VLBI data at 6 and 18 cm of the BL-Lac type object Mkn 421 are also presented. Observational data have been used to constrain the jet velocity and orientation.

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arXiv1998

Space VLBI Observations of MKN501

We present here two epochs of Space VLBI Observations at 18 cm of the BL-Lac type object MKN 501. Thanks to the high resolution of these new data, we have found that the inner jet is centrally brightened at its beginning, but becomes extended and limb brightened at ~8 mas from the core. Moreover a comparison between the two epochs shows the presence of a proper motion with apparent velocity = 6.7c. Observational data have been used to constrain the jet velocity and orientation.

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arXiv1998

G359.87+0.18: A Young SNR Candidate Near the Galactic Center?

Sub-arcsecond radio continuum observations of the Galactic center region at $λ$6 and 2cm reveal a 0.5$^{\prime\prime}$ diameter source with a shell-like morphology. This source is linearly polarized at a level of 16% at $λ$6cm and has a steep nonthermal spectrum with spectral index 1.6 between $λ$6 and 2 cm. The distance to this source is not known but the large rotation measure value of 3000 rad m$^{-2}$ suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic.

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arXiv1997

VLBI Observations of a Complete Sample of Radio Galaxies: VIII - Proper Motion in 3C338

We present new VLA, MERLIN and VLBI images for the radio galaxy 3C 338, and the results of the monitoring of its arcsecond core flux density. Present high sensitivity observations allow us to investigate the radio structure of this source and to confirm the presence of two symmetric parsec scale jets. Morphological changes between different epochs are evident and a proper motion with $β\sim$ 0.4h$^{-1}$ has been derived allowing us to give a lower limit constraint for the Hubble constant. While the steep spectrum large scale structure of 3C 338 could be a relic emission, the small scale structure looks young, similar to the high power MSOs found at high redshift.

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