arXiv subjects

R. A. Laing

Publications and source records attributed to R. A. Laing.

At least 37 records · Page 2Linked to original sources

arXiv2010

Structure of the magnetoionic medium around the FR Class I radio galaxy 3C 449

The goal of this work is to constrain the strength and structure of the magnetic field associated with the environment of the radio source 3C 449, using observations of Faraday rotation, which we model with a structure function technique and by comparison with numerical simulations. We assume that the magnetic field is a Gaussian, isotropic random variable and that it is embedded in the hot intra-group plasma surrounding the radio source. For this purpose, we present detailed rotation measure images for the polarized radio source 3C 449, previously observed with the Very Large Array at seven frequencies between 1.365 and 8.385 GHz. We quantify the statistics of the magnetic-field fluctuations by deriving rotation measure structure functions, which we fit using models derived from theoretical power spectra. We quantify the errors due to sampling by making multiple two-dimensional realizations of the best-fitting power spectrum.We also use depolarization measurements to estimate the minimum scale of the field variations. We then make three-dimensional models with a gas density distribution derived from X-ray observations and a random magnetic field with this power spectrum. Under these assumptions we find that both rotation measure and depolarization data are consistent with a broken power-law magnetic-field power spectrum, with a break at about 11 kpc and slopes of 2.98 and 2.07 at smaller and larger scales respectively. The maximum and minimum scales of the fluctuations are around 65 and 0.2 kpc, respectively. The average magnetic field strength at the cluster centre is 3.5 +/-1.2 micro-G, decreasing linearly with the gas density within about 16 kpc of the nucleus.

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arXiv2008

Structures of the magnetoionic media around the FR I radio galaxies 3C 31 and Hydra A

We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365 to 8440MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the North and South sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron-emitting plasma. We establish that the residual degree of polarization in the short-wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We show that the observed RM variations over selected areas of 3C 31 are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power-law slope changes significantly on the scales sampled by our data. The power spectrum can only have the form expected for Kolmogorov turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also compare the global variations of RM across 3C 31 with the results of three-dimensional simulations of the magnetic-field fluctuations in the surrounding magnetoionic medium. We show that our data are consistent with a field distribution that favours the plane perpendicular to the jet axis - probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X-ray data. (Abridged)

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arXiv2008

Multifrequency VLA observations of the FR I radio galaxy 3C 31: morphology, spectrum and magnetic field

We present high-quality VLA images of the FR I radio galaxy 3C 31 in the frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40 arcsec. Our new images reveal complex, well resolved filamentary substructure in the radio jets and tails. We also use these images to explore the spectral structure of 3C 31 on large and small scales. We infer the apparent magnetic field structure by correcting for Faraday rotation. Some of the intensity substructure in the jets is clearly related to structure in their apparent magnetic field: there are arcs of emission where the degree of linear polarization increases, with the apparent magnetic field parallel to the ridges of the arcs. The spectral indices are significantly steeper (0.62) within 7 arcsec of the nucleus than between 7 and 50 arcsec (0.52 - 0.57). The spectra of the jet edges are also slightly flatter than the average for their surroundings. At larger distances, the jets are clearly delimited from surrounding larger-scale emission both by their flatter radio spectra and by sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of 3C 31's jets is very close to that found in other FR I galaxies where their jets first brighten in the radio and where X-ray synchrotron emission is most prominent. Farther from the nucleus, where the spectra flatten, X-ray emission is fainter relative to the radio. The brightest X-ray emission from FR I jets is therefore not associated with the flattest radio spectra, but with a particle-acceleration process whose characteristic energy index is 2.24. The spectral flattening with distance from the nucleus occurs where our relativistic jet models require deceleration, and the flatter-spectra at the jet edges may be associated with transverse velocity shear. (Slightly abridged)

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arXiv2008

An XMM-Newton study of the environments, particle content and impact of low-power radio galaxies

We present a detailed study of the environments of a sample of nine low-power radio galaxies, based on new and archival XMM-Newton observations. We report new detections of group-scale environments around three radio galaxies, 3C 296, NGC 1044 and 3C 76.1. As with previous studies, we find that FR-I radio galaxies inhabit group environments ranging over nearly two orders of magnitude in bolometric X-ray luminosity, but find no evidence for a tight relationship between large-scale X-ray environment and radio-source properties such as size, radio luminosity, and axial ratio. We confirm earlier work showing that equipartition internal pressures are typically lower than the external pressures acting on the radio lobes, so that additional non-radiating particles must be present or the lobes must be magnetically dominated. We present the first direct observational evidence that entrainment may provide this missing pressure, in the form of a relationship between radio-source structure and apparent pressure imbalance. Finally, our study provides further support for the presence of an apparent temperature excess in radio-loud groups compared to the group population as a whole. For five of eight temperature excesses, the energy required to inflate the radio lobes is comparable to the energy required to produce this excess by heating of the group gas; however, in three cases the current radio source appears too weak to produce the temperature excess. It remains unclear whether the temperature excess can be directly associated with the current phase of AGN activity, or whether it is instead either a signature of previous AGN activity or simply an indicator of the particular set of group properties most conducive to the growth of an FR-I radio galaxy.

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arXiv2007

Jet-environment interactions in FRI radio galaxies

There is now unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows. On the assumption that they are axisymmetric and intrinsically symmetrical (a good approximation close to the nucleus), we can make models of their geometry, velocity, emissivity and field structure whose parameters can be determined by fitting to deep VLA observations. Mass entrainment - either from stellar mass loss within the jet volume or via a boundary layer at the jet surface - is the most likely cause for deceleration. This idea is quantitatively consistent with the velocity field and geometry inferred from kinematic modelling and the external gas density and pressure profiles derived from X-ray observations. The jets must initially be very light, perhaps with an electron-positron composition.

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arXiv2007

Jet spectra in FR I radio galaxies: implications for particle acceleration

We describe very accurate imaging of radio spectral index for the inner jets in three FR I radio galaxies. Where the jets first brighten, there is a remarkably small dispersion around a spectral index of 0.62. This is also the region where bright X-ray emission is detected. Further from the nucleus, the spectral index flattens slightly to 0.50 - 0.55 and X-ray emission, although still detectable, is fainter relative to the radio. The brightest X-ray emission from the jets is therefore not associated with the flattest radio spectra, but rather with some particle-acceleration process whose characteristic energy index is 2.24. The change in spectral index occurs roughly where our relativistic jet models require rapid deceleration. Flatter-spectrum edges can be seen where the jets are isolated from significant surrounding diffuse emission and we suggest that these are associated with shear.

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arXiv2007

Magnetic-field fluctuations around the radio galaxy 3C 31

We present an analysis of the magnetic-field fluctuations in the magnetoionic medium in front of the radio galaxy 3C 31 derived from rotation-measure (RM) fits to high-resolution polarization images. We first show that the Faraday rotation must be due primarily to a foreground medium. We determine the RM structure functions for different parts of the source and infer that the simplest form for the power spectrum is a power law with a high-frequency cutoff. We also present three-dimensional simulations of RM produced by a tangled magnetic field in the hot plasma surrounding 3C 31, and show that the observed RM distribution is consistent with a spherical plasma distribution in which the radio source has produced a cavity.

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arXiv2007

Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region

A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy, resolved by MERLIN+VLA at 0".2-2".0 resolution. 55 have Chandra X-ray counterparts including 18 with a hard X-ray photon index and high luminosity characteristic of a type-II (obscured) AGN. >70% of the radio sources have been classified as starbursts or AGN using radio morphologies, spectral indices and comparisons with optical appearance and MIR emission. Starbursts outnumber radio AGN 3:1. This study extends the VO methods previously used to identify X-ray-selected obscured type-II AGN to investigate whether very luminous radio and X-ray emission originates from different phenomena in the same galaxy. The high-redshift starbursts have typical sizes of 5--10 kpc and star formation rates of ~1000 Msun/yr. There is no correlation between radio and X-ray luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness. Most extended radio starbursts at z>1.3 host X-ray selected obscured AGN. Radio emission from most of these ultra-luminous objects is dominated by star formation but it contributes less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts.

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arXiv2007

The inner jet of radio galaxy NGC 315 as observed with Chandra and the VLA

We present Chandra X-ray results for the jet, nucleus, and gaseous atmosphere of NGC 315, a nearby radio galaxy whose jet kinematics are known through deep radio mapping. Diffuse X-ray synchrotron emission is detected from the jet out to 30 arcsec from the nucleus, through regions both of fast bulk flow and deceleration. The X-ray to radio flux ratio drops considerably where the flow decelerates, but the X-ray and radio emissions show similar transverse extents throughout, requiring distributed particle acceleration to maintain the supply of X-ray-emitting electrons. A remarkable knotty filament within the jet is seen in both the radio and X-ray, contributing roughly 10 per cent of the diffuse emission along its extent at both wavelengths. No completely satisfactory explanation for the filament is found, though its oscillatory appearance, roughly aligned magnetic field, and requirements for particle acceleration, suggest that it is a magnetic strand within a shear layer between fast inner and slower outer flow.

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arXiv2006

A Molecular Gas Study of Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radio galaxies, selected from the B2 catalogue. We investigate whether the CO properties of our sample are correlated with the properties of the host galaxy, and in particular with the dust component. We find strong evidences for a physical link between the dust disks probed by HST in the galaxy cores and the molecular gas probed by the CO spectral lines, which in two cases display a double-horn shape, consistent with ordered rotation. On the other hand, from a preliminary comparison with other samples of radio sources we find no significant differences in molecular gas properties between FRI and FRII radio sources. In order to confirm the suggestion that the CO is dynamically associated with the core dust disks, the most suitable sources of our sample will be proposed for interferometric imaging at PdBI.

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arXiv2006

A Search for Molecular Gas in Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radi galaxies, selected from the B2 catalogue. Most of such objects (16/23) have HST imaging. Our aim is to establish the distribution of molecular gas masses in low luminosity radio galaxies, in comparison with other radio source samples, confirm the suggestion that the CO is in ordered rotation, determine its relation to the dust disks observed in these objects with HST and establish targets for future interferometric imaging.

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arXiv2006

A relativistic model of the radio jets in 3C 296

We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We also derive the spectral-index and rotation-measure distributions. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58 deg to the line of sight. On-axis, they decelerate from a (poorly-constrained) initial velocity of 0.8c to approximately 0.4c around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power-law sections and the emission is centre-brightened. Our observations rule out a globally-ordered, helical magnetic-field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements ("arcs") in the main and counter-jets as an effect of relativistic aberration. (Abridged)

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arXiv2006

Faraday rotation variations along radio jets: the magnetic field in galaxy and group halos

Our modelling of FR I radio jets as decelerating, relativistic flows allows us to derive their orientations accurately. We present images of Faraday rotation for two of these these objects (3C 31 and NGC 315) and show that the fluctuations of rotation measure (RM) are larger in the fainter (receding) jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas density is much lower in NGC 315 and the RM fluctuations are only just detectable.

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arXiv2006

Multifrequency observations of the jets in the radio galaxy NGC 315

We present images of the jets in the nearby radio galaxy NGC 315 made with the VLA at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec FWHM. Within 15 arcsec of the nucleus, the spectral index of the jets is 0.61. Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from 0.55 on-axis to 0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner +/-400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1 - 2 rad/m^2. These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of approximately 225 arcsec and constraining its central density and magnetic-field strength. We also image the apparent magnetic-field structure over the first +/-200 arcsec from the nucleus.

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arXiv2006

Magnetic fields in jets: ordered or disordered?

The question of the degree of order in the magnetic fields of relativistic jets is important to any understanding of their production. Both vector-ordered (e.g. helical) and disordered, but anisotropic fields can produce the high observed degrees of polarization. We outline our models of jets in FR I radio galaxies as decelerating relativistic flows. We then present theoretical calculations of the synchrotron emission from different field configurations and compare them with observed emission from FR I jets. We show that large-scale helical fields (with significant poloidal and toroidal components) are inconsistent with observations. The combination of an ordered toroidal and disordered poloidal component is consistent with our data, as is an entirely disordered field. Jets must also contain small, but significant amounts of radial field.

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arXiv2005

A relativistic model of the radio jets in NGC 315

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38 degrees for the jets. Where they first brighten, their on-axis velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is roughly 0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at 3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved their rest-frame emission is centre-brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations.

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arXiv2004

Kinematics and dynamics of relativistic jets on large and small scales

Modelling of deep VLA images of the jets in FR I radio galaxies has allowed us to derive their three-dimensional distributions of velocity, emissivity and magnetic-field structure on kiloparsec scales. By combining our models of jet kinematics with measurements of the external pressure and density derived from Chandra observations, we can also determine the jet dynamics via a conservation-law approach. The result is a detailed and quantitative picture of jet deceleration. We discuss the potential application of these techniques on VLBI scales. Our fundamental assumption is that the jets are intrinsically symmetrical, axisymmetric, relativistic flows. Although this is likely to be a good approximation on average, the effects of non-stationarity in the flow (e.g. shocks/knots) may limit its applicability on pc scales. We also stress the need for both the main and counter-jet to be detected with good transverse resolution in linear polarization as well as total intensity. Two foreground effects (free-free absorption and Faraday rotation) must also be corrected. We comment on the implications of observed VLBI polarization and Faraday rotation for the field structure (and, by implication, collimation) of pc-scale jets. Where our VLA observations start to resolve the jets, about 1 kpc from the nucleus, they are travelling at about 0.8c. We briefly discuss the deceleration of jets from a much faster initial speed.

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arXiv2004

A Chandra observation of the X-ray environment and jet of 3C296

We have observed the twin-jet radio galaxy 3C296 with Chandra. X-ray emission is detected from the nucleus, from the inner parts of the radio jet, and from a small-scale thermal environment around the jet deceleration region. As we have found in previous observations of other twin-jet radio galaxies, the X-ray jet and a steep pressure gradient in the external thermal environment are associated with the region where strong bulk deceleration of the jet material is suggested by radio observations. Our observations provide additional evidence that the inner jets of twin-jet objects are always associated with a relatively cool, dense central X-ray emitting component with a short cooling time.

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