C. R. Lawrence

Optical Spectra of a Complete Sample of Radio Sources. I. The Spectra

Spectra covering the observed wavelength range 3800 to 9400 A have been obtained for a complete, flux-limited sample of 64 sources that has been studied extensively at radio wavelengths at high angular resolution. The sample is now completely identified—four new finding charts are given, and one source was identified spectroscopically. The spectra were obtained with the 200 inch Hale telescope, primarily with the Double Spectrograph, with spectral resolution of about 9 A at short wavelengths and 18 A at long wavelengths. Redshifts are now known for 62 of the 64 sources; tentative redshifts are given for the remaining two. The redshift range of the sample is 0.0171–2.55. The goal of these observations is a quantitative comparison between the radio structures in these objects, especially the compact nuclei, and the optical spectra. Many of the objects in the sample are faint, and many others have almost featureless spectra. Considerable care was required in the reduction and calibration of the spectra to preserve valuable information on these difficult objects. The spectra are fitted with various components: analytic functions for emission and absorption lines, nonthermal continua, Balmer continua, and thermal continua, and template spectra for starlight and Fe II blends. The signal-to-noise ratio of the spectra, averaged over wavelength, ranges from 2 to 164, with a median of 23. In this paper we describe the observations and data reduction, present the spectra, and give the parameters of components fitted to the spectra.

Spitzer Observations of 3C Quasars and Radio Galaxies: Mid-Infrared Properties of Powerful Radio Sources

We have measured mid-infrared radiation from an orientation-unbiased sample of 3CRR galaxies and quasars at redshifts 0.4 ≤ z ≤ 1.2 with the IRS and MIPS instruments on Spitzer. Powerful emission (L24 μm > 1022.4 W Hz-1 sr-1) was detected from all but one of the sources. We fit the Spitzer data and other measurements from the literature with synchrotron and dust components. The IRS data provide powerful constraints on the fits. At 15 μm, quasars are typically 4 times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of nonthermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars. Fitted optical depths are anticorrelated with core dominance, from which we infer an equatorial distribution of dust around the central engine. The median optical depth at 9.7 μm for objects with core dominance factor R > 10-2 is ≈0.4; for objects with R ≤ 10-2, it is ≈1.1. We have thus addressed a long-standing question in the unification of FR II quasars and galaxies: quasars are more luminous in the mid-infrared than galaxies because of a combination of Doppler-boosted synchrotron emission in quasars and extinction in galaxies, both orientation-dependent effects.

A gravitational lens candidate with an unusually red optical counterpart

The properties of the strong radio source MG0414 + 0534 are described. It is found to display many of the properties expected in a gravitational lens system. At radio wavelengths and 0.5-arcsec resolution, MG0414 + 0534 is made up of four compact components whose unusual configuration and relative flux densities are similar to those found in confirmed four-image gravitational lens systems. At optical wavelengths three objects are detected, consistent with there being optical objects at the positions of the radio components, given the lower optical resolution. The radio and optical centroid positions agree within the astrometric errors, and the relative ordering of the fluxes is the same. The colors and radiooptical spectral indices are similar, but there are differences larger than the photometric errors and the measured variability (about 30 percent). Extinction by dust might simultaneously explain the unusually red color and the absence of light from a lens.

Discovery of a New Gravitational Lens System

A new gravitational lens system, the triple radio source MG2016+112, has been discovered. Five emission lines at a redshift of 3.2733�0.0014 have been identified in the spectra of two stellar objects of magnitude 22.5 coincident with radio components 3.4 arc seconds apart. The lines are the narrowest ever observed in objects at such a large redshift. The redshift of a 23rd-magnitude extended optical object coincident with the third radio component has not been determined spectroscopically, but its known optical properties are consistent with those of a giant elliptical galaxy with a redshift of about 0.8.

The MIT-Green Bank (MG) 5 GHz survey

The catalog yielded by the MIT-Green Bank 5-GHz survey contains 5974 sources with S/N ratio greater than 5; a supplemental catalog contains 3836 possible detections with S/N of less than 5. The reliability of the main list is measured to be about 96 percent, with a completeness of about 95 percent. Flux density errors are a function of flux density. A comparison is presently made of survey sources from the Douglas et al. (1980) 365-MHz Texas survey; spectral indices are computed for coincident sources, and the distribution of spectral indices is discussed.

5 GHz radio structure and optical identifications of sources from the MG survey. II. Maps and finding charts

Nearly 1000 sources from the 5 GHz MIT-Green Bank survey have been observed with the VLA. Radio maps are given for 460 resolved sources, finding charts for 390 optical identifications, and spectral indices between 1.4 and 5 GHz for 632 sources. 20 references.

A gravitationally-lensed ring in MG 1549+3047

We report the discovery of a new gravitational lens in the radio source MG 1549+3047. The source has a compact core and two lobes, with a faint optical counterpart to the core. The brighter lobe is shaped like a ring, and centered on a foreground galaxy with known redshift. The radio structure of the ring can be reproduced by distorting a typical radio lobe through a simple elliptical gravitational lensing potential, which models the mass distribution in the foreground galaxy. From a preliminary lens model, we show that the mass distribution follows the observed luminosity, and we predict a central velocity dispersion of about 230 km s −1 . This corresponds to a blue mass-to-light ratio of about 18h solar units

Extended and compact X-ray emission in powerful radio galaxies

We report ROSAT X-ray observations of two powerful radio galaxies. 3C 280 provides evidence for a mixture of unresolved and extended emission, with the latter produced by hot plasma of insufficient pressure to confine the radio lobes and insufficient density for a cooling flow to have begun. 3C 220.3 gives only an X-ray upper limit, but one consistent with our interpretation of the X-ray emission from powerful radio-loud active galactic nuclei (AGNs) in terms of obscured and unobscured components.

Deep optical and radio observations of the gravitational lens system 2016 + 112

New optical and radio observations of the gravitational lens system 2016 + 112 are presented. Deep optical images reveal the presence of an extremely red galaxy located in the midst of the three radio positions. There is no evidence for a cluster of galaxies in the region; the lensing appears to be dominated by the new galaxy and the galaxy associated with the third radio source. The A image has remained constant in brightness since its discovery nearly a year ago, but B has faded by about 0.3 mag. A new spectrum of B shows that the equivalent widths of its emission lines have increased by roughly a factor of 2. A wide-field radio map shows that the postulated third image must either be more than 100 times fainter than A and B, or else lie close to one of the detected radio sources. The discovery of the new lensing mass makes the geometry of this lens system somewhat easier to understand, but no model based only on idealized mass distributions at the locations of the two known galaxies can explain all the observed properties. 18 references.

The third image, the redshift of the lens, and new components of the gravitational lens 2016 + 112

Three new observations of the gravitational lens system 2016 + 112 are reported. A new, more sensitive spectrograph has produced a redshift for the red galaxy D, and direct imaging through a narrowband filter centered on the Lyman alpha line of the lensed object has detected a previously unknown point source and two radio-quiet extended emission line regions, all apparently at the redshift of the two brighter images A and B. Some preliminary results from a new VLA map of the system are reported, and the implications of the results for models of gravitational lenses are discussed.