Evidence for cospatial optical and radio polarized emission in active galactic nuclei

We investigate the relationship between the optical and radio emission of active galactic nuclei (AGN) by analysing optical and 15+22+43 GHz Very Long Baseline Array (VLBA) polarization observations simultaneous to within a day for 11 BL Lacertae (BL Lac) objects and the blazar 3C279. We have determined and corrected for the Faraday rotation measures in the very long baseline interferometry (VLBI) cores, enabling us to compare the intrinsic (zero-wavelength) VLBI-core polarization angles and the optical polarization angles χ_opt. A clear alignment between these two angles emerges in the transition toward higher radio frequencies, and a prominent peak at 0° is visible in the distribution of |χ_opt−χ_43 GHz|. This correlation implies that the magnetic-field orientations in the regions giving rise to the optical and radio polarization are the same, and can be easily understood if the radio and optical polarization are roughly cospatial. It is difficult to rule out the possibility that they arise in different regions in a straight jet with a uniform magnetic-field structure, but this seems less likely, since the VLBI jets of AGN are often bent on parsec-scales. This may suggest that much or all of the strong optical polarization in these sources arises in the inner radio jets, possibly associated with the formation and emergence of compact new VLBI components.     

1144+35: A giant radio galaxy with superluminal motion

We report on centimeter VLA and VLBI observations of the giant, low power radio galaxy 1144+35. On the parsec scale, we see a complex jet component moving away from the center of activity at 2.7h₅₀⁻¹ c. We detect a faint parsec-scale counter-jet and derive a jet velocity of 0.95c and an angle to the line of sight of 25°, consistent with an intrinsically symmetric ejection. These findings lend credence to the claim that even the jets of low-power radio galaxies start out relativistically.     

Polarization observations of the radio cores of AGN — I. A sample of quasars

We present total-intensity and linear-polarization observations at a single epoch for a sample of 11 quasars and one BL Lac object. The data were taken with the VLA A array at λλ 20, 18, 6 and 2 cm. We examine the variation of the degree of polarization, p , and polarization position angle, PA, with wavelength, and attempt to determine the rotation measure, RM, of the cores in these sources. The degree of polarization does not exhibit any systematic variation with wavelength, the median values ranging from 2.3 to 3.5 per cent at the different wavelengths. The variation of PA with λ² is not linear over the entire wavelength range. However, for most sources the λλ 20-, 18- and 6-cm PAs do follow such a linear relationship, yielding a median |RM| of about 15 rad m⁻². In contrast, the λλ 6- and 2-cm observations give a median |RM| of about 129 rad m⁻². The long-wavelength emission is likely to originate from a spatially different part of the milliarcsec-scale jet from the λ 2-cm emission, which could turn over at a higher frequency and is likely to be more compact and located closer to the quasar nucleus. We have attempted to obtain linear fits over the entire wavelength range allowing for n  π ambiguities in the PAs, but the fits are not statistically significant. The low values of RM for these core-dominated sources suggest that either the radio emission from the jet intercepts few of the emission-line clouds and their confining medium, or the clouds have a small filling factor and are possibly magnetically confined.     

FR II radio galaxies with z < 0.3 — I. Properties of jets, cores and hotspots

In previous papers we have discussed high-resolution observations of a large sample of powerful radio galaxies with z  < 0.3. Jets are detected in up to 80 per cent of the sample, and radio cores in nearly all the objects; in addition, we are able to resolve the hotspots in most sources. In this paper we present measurements of the radio properties of these components.   The prominences of the jets detected do not appear to be a function of radio luminosity, providing the clearest evidence yet that the reported low detection rate of jets in radio galaxies has been an artefact of low-sensitivity observations. We find a positive correlation between the total source length and core prominence in the narrow-line radio galaxies. We have found evidence for a relationship between hotspot size and total source size, but few other significant relationships between hotspot properties and those of the jets or lobes. We compare our measurements with those of Bridle et al., based on observations of a sample of quasars, and argue that the results are consistent with a modification of the unified model in which the broad-line radio galaxies are the low-luminosity counterparts of quasars, although the situation is complicated by contamination with low-excitation radio galaxies which appear to have radio properties different from the high-excitation objects. We discuss the classes of empirical model that can be fitted to the data set.     

A new search for distant radio galaxies in the Southern hemisphere – I. Sample definition and radio properties

This paper introduces a new program to find high-redshift radio galaxies in the Southern hemisphere through ultrasteep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices α⁸⁴³₄₀₈≤−1.0 ( S _ν∝ν^α) and flux densities S ₄₀₈≥ 200 mJy in a region of 0.35 sr, chosen by cross-correlating the revised 408 MHz Molonglo Reference Catalogue, the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region −40° < δ < −30°. We present Australia Telescope Compact Array (ATCA) high-resolution 1384 and 2368 MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408–2368 MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368 MHz. We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample.     

The radio galaxy Centaurus B

Centaurus B (PKS B1343−601) is one of the brightest and closest radio galaxies, with flux density ∼250 Jy at 408 MHz and redshift 0.01215, but it has not been studied much because of its position (i) close to the Galactic plane (it is also known as G309.6+1.7 and Kes 19) and (ii) in the southern sky. It has recently been suggested as the centre of a highly obscured cluster behind the Galactic plane. We present radio observations made with the Australia Telescope Compact Array and Molonglo Observatory Synthesis Telescope to study the jets and lobes. The total intensity and polarization radio images of the FR I jets are used to determine the jet brightness and width variations, magnetic field structure and fractional polarization. The equipartition pressure calculated along the jets declines rapidly over the first 1 arcmin from the galaxy reaching a constant pressure of 10⁻¹³  h ^−4/7 Pa in the lobes blown in the intracluster medium.     

Deep low-frequency observations with the Giant Metrewave Radio Telescope: a search for relic radio emission

We present deep multifrequency observations using the Giant Metrewave Radio Telescope (GMRT) at 153, 244, 610 and 1260 MHz of a field centred on J0916+6348, to search for evidence of fossil radio lobes which could be due to an earlier cycle of episodic activity of the parent galaxy, as well as haloes and relics in clusters of galaxies. We do not find any unambiguous evidence of episodic activity in a list of 374 sources, suggesting that such activity is rare even in relatively deep low-frequency observations. We examine the spectra of all the sources by combining our observations with those from the Westerbork Northern Sky Survey (WENSS), NRAO (National Radio Astronomy Observatories) VLA (Very Large Array) Sky Survey (NVSS) and the Faint Images of the Radio Sky at Twenty-Centimeters Survey (FIRST). Considering only those which have measurements at a minimum of three different frequencies, we find that almost all sources are consistent with a straight spectrum with a median spectral index,  α∼ 0.8 [S(ν) ∝ν^−α  ], which appears steeper than theoretical expectations of the injection spectral index. We identify 14 very steep-spectrum sources with  α≥ 1.3  . We examine their optical fields and discuss the nature of some of these sources.     

Relationship between the position angles of elongated structures and polarization vector of the compact radio source 3C84 AT 2, 4, and 6-cm wavelengths

The position angles (PA) of the elongated structure of the compact source 3C84 are compared with those of the E polarization vector of its 2, 4, and 6-cm emission. It is shown that during 1972–1984, the E vector had two preferred directions with PA = -10° and PA = +17°, which correspond to the directions of the two principal elongated structures of the radio source, which contain moderately relativistic subparsec-scale jets. The parallel directions of the polarization vector and radio jets for the source 3C84 agree with the model of magnetic field and relativistic electron cloud distributions in the moderately relativistic jets of the BL Lac objects examined by Grabusda et al. (1994). Translated fromAstrofizika, Vol. 40, No. 1, pp. 19–28, February, 1997     

The radio jets in B2 0755+379

We present and discuss observations of the radio galaxy 0755+379 made with the VLA at 1.4 and 5.0 GHz and with MERLIN at 1.7 GHz. These data allow us to image the radio jets over two orders of magnitude in linear size and to investigate the hypothesis that jets in low-luminosity radio galaxies start with velocities close to c and then slow down to subrelativistic speeds. We apply a model for an adiabatically expanding relativistic jet to the observed surface brightness and derive velocity profiles along the jet for various assumed starting conditions. We show that these profiles are consistent with the observed jet/counter-jet brightness ratios provided that the angle to the line of sight θ ≃27°. The inferred velocity at a distance of 0.5 kpc from the nucleus is ≃0.9 c . Finally, we show that the predicted velocity at 10 kpc from the nucleus is consistent with that obtained independently from energy-balance arguments.     

Biases in the polarization position angles in the NRAO-VLA sky survey point source catalogue

We have examined the statistics of the polarization position angles determined for point sources in the NRAO-VLA Sky Survey (NVSS), and find that there is a statistically significant bias towards angles which are multiples of 45°. The formal probability that the polarization angles are drawn from a uniform distribution is exponentially small. When the sample of those NVSS sources with polarizations detected with a signal-to-noise ratio ≥3 is split either around the median polarized flux density or the median fractional polarization, the effect appears to be stronger for the more highly polarized sources. Regions containing strong sources and regions at low-Galactic latitudes are not responsible for the non-uniform distribution of position angles. We identify clean bias as the probable cause of the dominant effect, coupled with small multiplicative and additive offsets on each of the Stokes parameters. Our findings have implications for the extraction of science, such as information concerning galactic magnetic fields, from large-scale polarization surveys.     

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Königl's inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor Γ, viewing angle θ and electron number density n _e from radio very long-baseline interferometry and X-ray data for a sample of active galactic nuclei (AGNs) assuming that the X-rays are from the jet rather than the intracluster gas. The bulk kinetic power of jets is then calculated using the derived jet parameters. We find a strong correlation between the total luminosity of broad emission lines and the bulk kinetic power of the jets. This result supports the scenario that the accretion process is tightly linked with the radio jets, though how the disc and jet are coupled is not revealed by present correlation analysis. Moreover, we find a significant correlation between the bulk kinetic power and radio extended luminosity. This implies that the emission from the radio lobes is closely related with the energy flux transported through jets from the central part of AGNs.     

射电宇宙各向同性吗？

本文报告了小尺度河外射电源喷流和内禀偏振位置角分布的统计结果。我们发现，毫角秒喷流分布和内禀偏振位置角分布呈各向异性。这一现象的内禀真实性相当值得怀疑，因为它与我们的宇宙学三大基本原则之一相悖，但它在统计上的显著性是相当高的，至少我们可以认为在一些巡天工作中存在某种不清楚的传播效应，选择效应或缺陷     

Kinematical diagrams for conical relativistic jets

We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (υ _app = cβ _app) and the effective Doppler boosting factor (δ _eff) of the relativistically moving radio knot. A novel aspect of these analytical computations of β-δ diagrams is that they are made for parsec-scale jets having a conical shape, with modest opening angles (ω up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor, Γ), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis is surrounded by a slower moving sheath, possibly arising from a velocity shear.     

The magnetic field and geometry of the oblique shock in the jet of 3C 346

We investigate the brightest regions of the kpc-scale jet in the powerful radio galaxy 3C 346, using new optical Hubble Space Telescope ( HST ) ACS/F606W polarimetry together with Chandra X-ray data and 14.9 and 22.5 GHz Very Large Array (VLA) radio polarimetry. The jet shows a close correspondence between optical and radio morphology, while the X-ray emission shows a  0.80 ± 0.17 kpc  offset from the optical and radio peak positions. Optical and radio polarimetry show the same apparent magnetic field position angle and fractional polarization at the brightest knot, where the jet undergoes a large kink of almost 70° in the optical and radio images. The apparent field direction here is well aligned with the new jet direction, as predicted by earlier work that suggested the kink was the result of an oblique shock. We have explored models of the polarization from oblique shocks to understand the geometry of the 3C 346 jet, and find that the upstream flow is likely to be highly relativistic  (β_u= 0.91^+0.05_−0.07)  , where the plane of the shock front is inclined at an angle of  η= 51°± 11°  to the upstream flow which is at an angle  θ= 14⁺⁸₋₇  deg to our line of sight. The actual deflection angle of the jet in this case is only 22°.     

A multifrequency study of possible relic lobes in giant radio sources

We present low-frequency observations with the Giant Metrewave Radio Telescope of three giant radio sources (GRSs: J0139+3957, J0200+4049 and J0807+7400) with relaxed diffuse lobes which show no hotspots and no evidence of jets. The largest of these three, J0200+4049, exhibits a depression in the centre of the western lobe, while J0139+3957 and J0807+7400 have been suggested earlier by Klein et al. and Lara et al., respectively, to be relic radio sources. We estimate the ages of the lobes. We also present Very Large Array observations of the core of J0807+7400, and determine the core radio spectra for all three sources. Although the radio cores suggest that the sources are currently active, we explore the possibility that the lobes in these sources are due to an earlier cycle of activity.     

On the depolarization asymmetry seen in giant radio lobes

The depolarization asymmetry seen in double-lobed radio sources, referred to as the Laing-Garrington (L-G) effect where more rapid depolarization is seen in the lobe with no visible jet as the wavelength increases, can be explained either by internal differences between the two lobes, or by an external Faraday screen that lies in front of only the depolarized lobe. If the jet one-sidedness is due to relativistic beaming the depolarization asymmetry must be due to an intervening Faraday screen. If it is intrinsic the depolarization asymmetry must be related to internal differences in the lobes. For a random viewing angle distribution, which must be the case here where un-beamed lobe radiation dominates, jet one-sidedness is unrelated to viewing angle and therefore cannot be used either to estimate the viewing angle or to imply beaming. The outflow speed in the kpc jet is notoriously difficult to determine. However, although it has not yet been proven conclusively, we assume in this paper that the speed in the outer jet of several Fanaroff-Riley Class 1 (FRI) sources exhibiting the L-G effect is close to the 0.1c reported by several other investigators. For these sources we find that the jet one-sidedness cannot be explained by beaming and therefore must be intrinsic. In these FRI sources the L-G effect must be due to differences that originate inside the lobes themselves, with the outer regions of the relevant lobe acting as a Faraday screen. Although it is not known if the flow in the outer jets of FRII sources also slows to this speed it is suggested that the explanation of the L-G effect is likely to be the same in both types. This argument is strengthened by the recent evidence that FRII galaxies have very large viewing angles, which in turn implies that the L-G model cannot work regardless of the jet velocity. It may therefore be too soon to completely rule out internal depolarization in the lobes as the true explanation for the L-G effect.     

Interactions of radio galaxies and the intracluster medium in Abell 160 and Abell 2462

We present Chandra and Very Large Array observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and we test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores (∼30–40 kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C 465 and Hydra A. However, none of the published models of WAT formation offers a satisfactory explanation as to why this may be the case. Therefore, it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process.     

A MERLIN pilot study of faint variable VLA FIRST radio sources

We present the results of a pilot study in which we obtained Multi-Element Radio Linked Interferometer Network (MERLIN) L-band snapshot images of the 20 strongest radio sources from a sample of 123 radio objects that exhibit significant flux density variations at 1.4 GHz on a seven year base-line. The sample was created using FIRST VLA B-array data from 1995 and 2002 on a strip around zero declination near the south Galactic cap. The primary purpose is to assess the presence of radio jets and provide direct evidence between long-term variability and jet structures, as several models suggest. The radio structure of all the sources in this subsample appears very compact and our high resolution MERLIN observations reveal for the first time the presence of small milli-arcsecond (mas) jets and/or jet-like extensions in 17 of the 20 variable sources (literature data show core-jet structures or hints of extended structure in the other three objects which are point sources in our MERLIN observations). In the future we will use the radio results from the complete study, in combination with their optical Sloan Digital Sky Survey (SDSS) data, in order to investigate trends and possible differences or similarities between the quasars and galaxies that host these radio sources, and so examine existing unification schemes or open up new aspects in AGN unification.     

Giant radio sources

We present multifrequency Very Large Array (VLA) observations of two giant quasars, 0437−244 and 1025−229, from the Molonglo Complete Sample. These sources have well-defined FR II radio structure, possible one-sided jets, no significant depolarization between 1365 and 4935 MHz and low rotation measure (|RM|<20 rad m⁻²). The giant sources are defined to be those with overall projected size 1 Mpc. We have compiled a sample of about 50 known giant radio sources from the literature, and have compared some of their properties with a complete sample of 3CR radio sources of smaller sizes to investigate the evolution of giant sources, and test their consistency with the unified scheme for radio galaxies and quasars. We find an inverse correlation between the degree of core prominence and total radio luminosity, and show that the giant radio sources have similar core strengths to smaller sources of similar total luminosity. Hence their large sizes are unlikely to be caused by stronger nuclear activity. The degree of collinearity of the giant sources is also similar to that of the sample of smaller sources. The luminosity–size diagram shows that the giant sources are less luminous than our sample of smaller sized 3CR sources, consistent with evolutionary scenarios in which the giants have evolved from the smaller sources, losing energy as they expand to these large dimensions. For the smaller sources, radiative losses resulting from synchrotron radiation are more significant while for the giant sources the equipartition magnetic fields are smaller and inverse Compton loss owing to microwave background radiation is the dominant process. The radio properties of the giant radio galaxies and quasars are consistent with the unified scheme.