R. A. Perley

Multifrequency radio observations of Cygnus A - Spectral aging in powerful radio galaxies

A detailed analysis of the radio spectrum across the lobes of Cygnus A is presented in order to critically test the synchroton spectral aging theory. The results are in good agreement with the jet model for powerful radio galaxies, involving particle acceleration at the hot spots and outflow into the radio lobes, with subsequent energy loss due to synchrotron radiation. The hot spot spectra are well represented by a spectral aging model involving continuous injection of relativistic particles. Both hot spots have spectral break frequencies around 10 GHz. An injection index of 0.5 is found for both hot spots, consistent with diffusive shock acceleration at a strong nonrelativistic shock in a Newtonian fluid. The LF hot spot emission spectrum falls below the injected power law. This effect is isolated to the hot spots, and is best explained by a low-energy cutoff in the particle distribution.

The Expanded Very Large Array: A New Telescope for New Science

Since its commissioning in 1980, the Very Large Array (VLA) has consistently demonstrated its scientific productivity. However, its fundamental capabilities have changed little since 1980, particularly in the key areas of sensitivity, frequency coverage, and velocity resolution. These limitations have been addressed by a major upgrade of the array, which began in 2001 and will be completed at the end of 2012. When completed, the Expanded VLA?the EVLA?will provide complete frequency coverage from 1 to 50?GHz, a continuum sensitivity of typically 1 ?Jy beam?1 (in 9 hr with full bandwidth), and a modern correlator with vastly greater capabilities and flexibility than the VLAs. In this Letter, we describe the goals of the EVLA project, its current status, and the anticipated expansion of capabilities over the next few years. User access to the array through the Open Shared Risk Observing and Resident Shared Risk Observing programs is described. The following papers in this special issue, derived from observations in its early science period, demonstrate the astonishing breadth of this most flexible and powerful general-purpose telescope.

The properties of the X-ray holes in the intracluster medium of the Perseus cluster

Abstract : High-resolution X-ray and low-frequency radio imaging now allow us to examine in detail the interaction and physical properties of the radio source 3C 84 and the surrounding thermal gas. The radiative and dynamical properties of the inner X-ray holes, which coincide with the radio lobes, indicate that the ratio of the energy factor k to the filling factor f is in the range 180 < k/f < 500. We define k to be the ratio of the total particle energy to that of the electrons radiating above a fiducial frequency of 10 MHz. The relativistic plasma and magnetic field are not in equipartition, since the field must be a factor of 4 or more lower than required for pressure balance. Unexpected steep-spectrum spurs in the low-frequency radio maps point to outer X-ray holes, which are plausibly buoyant old radio lobes. The evidence that the inner lobes are currently expanding subsonically, yet have not detached due to buoyancy, and the requirement that the synchrotron cooling time must exceed the age of the hole enable us to constrain the jet power of the nucleus to between 10(exp 44) and 10(exp 45) erg s (exp-1), depending on the filling factor of the relativistic plasma.

VLA images of 23 extragalactic radio sources

The paper presents radio images at 20 cm of 22 radio galaxies and one quasar made with NRAO Very Large Array. The sources are drawn from the z less than 0.5 subset of the complete radio sample defined by Laing et al. (1983). The images have resolutions of 1.1{minus}5.8 arcsec, chosen to give about 50 beamwidths across each source, and sufficient sampling of the aperture plane to represent accurately the largest-scale structure. With the current images, nearly all members of the Laing et al. sample with z less than 0.5 have high-sensitivity images from the VLA or Westerbork. 46 refs.

Faraday rotation of Cygnus A - magnetic fields in cluster gas

Multifrequency, multiconfiguration observations with the VLA have allowed, for the first time, detailed mapping of the rotation measure (RM) of the prominent extragalactic radio source Cygnus A. It is found that this source lies behind a deep Faraday screen, with the rotation measure varying from -4000 to +3000 rad/sq m for the eastern lobe, and from -2000 to +1500 rad/sq m for the western lobe. Gradients in both lobes commonly exceed 300 rad/sq m/arcsec. The overall pattern in RM is the same in each lobe, increasing from east to west, leading to the conclusion that the magnetic field is ordered on scales of about 20 to 30 kpc. There is no evidence for internal depolarization. The authors examine, and reject, the hypothesis that this Faraday screen has a Galactic origin and propose that its origin is in the intracluster gas in which Cygnus A is embedded. 66 references.

VLA observations of 3C 295 - A young radio galaxy

VLA observations of the luminous radio galaxy 3C 295 at 2, 3.6, 6, and 20 cm are presented. The combination of its extreme luminosity with small physical size results in a very high energy density and equipartition magnetic field strengths of about 500 microG. The spectrum is unusually steep for a radio galaxy, and shows curvature at all frequencies. In addition, the largest Faraday rotation measures (RMs) yet measured in an extragalactic source, exceeding 20,000 radians/sq m in places, has been found. These RMs are caused by cluster gas and magnetic fields of strength about 30 microG, and are not produced within the source or at its boundary. This is a young radio galaxy, less than 2 million years old. 33 refs.

VLA observations of the radio galaxy Hydra A (3C 218)

An extensive series of observations of the radio galaxy Hydra A using the VLA is reported. The radio emission consists of two components: (1) a pair of well-collimated, curved jets with S-symmetry, each of which makes a turn through about 40 deg between the core and the lobe, and (2) diffuse radio lobes, also rotation-symmetric, although straighter, which extend north out to 5 arcmin (420 kpc) and south out to 3 arcmin (250 kpc). Rotation measures (RMs) ranging from -1000 to +3300 rad/sq m are found in the northern lobe with gradients in the RM of up to 1000 rad/sq m/arcsec. In the southern lobe the RM gradients are probably in excess of + or - 3750 rad/sq m, the maximum value that can be unambiguously determined here. 48 refs.

HST optical spectral index map of the jet of 3C 273

We present HST images at 622 nm and 300 nm of the jet in 3C 273 and determine the run of the optical spectral index at 0: 00 2 along the jet. The smoothness of spectral index changes shows that the physical conditions are varying smoothly across the jet. There is no correlation between the optical flux and spectral index, as would be expected for relativistic electrons suering strong cooling due to synchrotron emission. We nd no evidence for localized acceleration or loss sites. This suggests that the spectral shape is not changing much throughout the jet. We show that relativistic beaming and/or sub-equipartition magnetic elds cannot remove the discrepancy between light-travel time along the jet and the lifetime of electrons emitting optical synchrotron radiation. We consider this further evidence in favour of a distributed electron acceleration process.

The Evolution of NGC 7027 at Radio Frequencies: A New Determination of the Distance and Core Mass

We present the results of a 25 yr program to monitor the radio flux evolution of the planetary nebula NGC 7027. We find significant evolution of the spectral flux densities. The flux density at 1465 MHz, where the nebula is optically thick, is increasing at a rate of 0.251% ± 0.015% yr −1, caused by the expansion of the ionized nebula. At frequencies where the emission is optically thin, the spectral flux density is changing at a rate of –0.145% ± 0.005% yr −1, caused by a decrease in the number of ionizing photons coming from the central star. A distance of 980 ± 100 pc is derived. By fitting interpolated models of post-AGB evolution to the observed changes, we find that over the 25 yr monitoring period, the stellar temperature has increased by 3900 ± 900 K and the stellar bolometric luminosity has decreased by 1.75% ± 0.5% . We derive a distance-independent stellar mass of 0.655 ± 0.010 M☉ adopting the Blocker stellar evolution models, or about 0.04 M☉ higher when using models of Vassiliadis & Wood. A Cloudy photoionization model is used to fit all epochs at all frequencies simultaneously. The differences between the radio flux density predictions and the observed values show some time-independent residuals of typically 1%. A possible explanation is inaccuracies in the radio flux scale of Baars and coworkers. We propose an adjustment to the flux density scale of the primary radio flux calibrator 3C 286, based on the Cloudy model of NGC 7027. We also calculate precise flux densities for NGC 7027 for all standard continuum bands used at the VLA, as well as for some new 30 GHz experiments.

Active Galactic Nucleus and Starburst Radio Emission from Optically Selected Quasi-stellar Objects

We used the 1.4 GHz NVSS to study radio sources in two color-selected QSO samples: a volumelimited sample of 1313 QSOs defined by Mi < −23 in the redshift range 0.2 < z < 0.45 and a magnitude-limited sample of 2471 QSOs with mr ≤ 18.5 and 1.8 < z < 2.5. About 10% were detected above the 2.4 mJy NVSS catalog limit and are powered primarily by AGNs. The space density of the low-redshift QSOs evolves as ρ ∝ (1 + z). In both redshift ranges the flux-density distributions and luminosity functions of QSOs stronger than 2.4 mJy are power laws, with no features to suggest more than one kind of radio source. Extrapolating the power laws to lower luminosities predicts the remaining QSOs should be extremely radio quiet, but they are not. Most were detected statistically on the NVSS images with median peak flux densities Sp(mJy beam ) ≈ 0.3 and 0.05 in the lowand high-redshift samples, corresponding to spectral luminosities log[L1.4GHz(W Hz )] ≈ 22.7 and 24.1, respectively. We suggest that the faint radio sources are powered by star formation at rates Ṁ ∼ 20M⊙ yr −1 in the moderate luminosity (median 〈Mi〉 ≈ −23.4) low-redshift QSOs and Ṁ ∼ 500M⊙ yr −1 in the very luminous (〈Mi〉 ≈ −27.5) high-redshift QSOs. Such luminous starbursts [〈log(LIR/L⊙)〉 ∼ 11.2 and 12.6, respectively] are consistent with “quasar mode” accretion in which cold gas flows fuel both AGN and starburst. Subject headings: galaxies: active—galaxies: starburst—quasars: general—radio continuum: galaxies