Astrometric Survey Research Articles

Gravitationally lensed radio sources in the Jodrell Bank-VLA Astrometric Survey

We report the final results of the search for gravitationally lensed flat-spectrum radio sources in the 2384-source Jodrell Bank–VLA Astrometric Survey (JVAS). We describe the process of selecting candidate lensed sources from 8.4-GHz VLA snapshot maps. 66 sources were selected as lens candidates and were followed up with multifrequency VLA and/or high-resolution MERLIN 5-GHz observations. To illustrate the candidate elimination process, we discuss examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. There are six gravitational lens systems in the JVAS, five of which are new discoveries. We summarize the properties of all the systems.

CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey

The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been completed, bringing the total number of sources imaged to over 15,000 in the CLASS and Jodrell-VLA Astrometric Survey combined survey. In the VLA observations carried out in 1998 March and April, two new candidate lensed systems were discovered: CLASS B1152+199 and B1359+154. B1152+199 is a 16 double, with a background quasar at z = 1.019 lensed by a foreground galaxy at z = 0.439. The relatively flat radio spectra of the lensed images (α = -0.32), combined with a previous ROSAT detection of the source, make B1152+199 a strong candidate for time delay studies at both radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z = 3.235, with a maximum image separation of 17. As yet, the redshift of the lensing object in this system is undetermined. The steep spectral index of the source (α = -0.9) suggests that B1359+154 will not exhibit strong variability and is therefore unlikely to be useful for determining H0 from measured time delays.

Time delay for the gravitational lens system B0218+357

Measurement of the time delay between multiple images of a gravitational lens system is potentially an accurate method of determining the Hubble constant over cosmological distances. One of the most promising candidates for an application of this technique is the system B0218+357 which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of two images of a compact radio source, separated by 335 milliarcsec, and an Einstein ring which can provide a strong constraint on the mass distribution in the lens. We present here the results of a three-month VLA monitoring campaign at two frequencies. The data are of high quality and both images show clear variations in total flux density, percentage polarization and polarization position angle at both frequencies. The time delay between the variations in the two images has been calculated using a chi-squared minimization to be 10.5+/-0.4 days at 95 per cent confidence, with the error being derived from Monte-Carlo simulations of the light curves. Although mass modelling of the system is at a preliminary stage, taking the lensing galaxy to be a singular isothermal ellipsoid and using the new value for the time delay gives a value for the Hubble constant of 69+13/-19 km/s/Mpc, again at 95 per cent confidence.

Gigahertz Peaked Spectrum sources from the Jodrell Bank–VLA Astrometric Survey

Observations with MERLIN at 408 MHz have been used to establish the low-frequency part of the spectra of more than a hundred compact radio sources taken from the part of the Jodrell Bank–VLA Astrometric Survey limited by . These sources were selected from JVAS and other catalogues to have convex spectra between 1.4 and 8.4 GHz, characteristic of Gigahertz Peaked Spectrum (GPS) sources. We have confirmed convex shapes of the spectra of 76 objects (one half of our initial candidates) thereby yielding the largest genuine sample of GPS sources compiled so far. Seven of 17 identified quasars in the sample have large () redshifts.

The Gravitational Lens System B1030+074. Discovery and Follow-up

We report the discovery of a new double image gravitational lens system B1030+074 which was found during the Jodrell Bank - VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA as well as HST WFPC2 and NICMOS observations. The lensed images are separated by 1.56 arcseconds and their flux density ratio at centimetric wavelengths is approximately 14:1 although the ratio is slightly frequency dependent and the images appear to be time variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be either part of the galaxy or a companion object. We have modeled B1030+074 using a Singular Isothermal Ellipsoid that yielded a time delay of 156/h50 days. This lens is likely to be suitable for the measurement of the Hubble constant.

The new gravitational lens system B1030 + 074

We report the discovery of a new double-image gravitational lens system, B1030 + 074, which was found during the Jodrell Bank–VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA, and optical observations using WFPC2 on the HST. The lensed images are separated by 1.56 arcsec and their flux density ratio at centimetric wavelengths is approximately 14:1, although the ratio is slightly frequency-dependent and the images appear to be time-variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be a spiral arm or an interacting galaxy. We have modelled B1030 + 074 using a singular isothermal ellipsoid which yields a time delay of 156/h50 d. This lens is likely to be suitable for the measurement of the Hubble constant.

Flat-spectrum radio sources with kpc-scale structure

We have carried out the first systematic study of flat-spectrum radio sources which have significant structure on angular scales 90–300 milliarcsec (mas), corresponding to linear scales of ∼ 0.5–1.5 kpc at a redshift of 0.5. The principal aim of the study was to search for multiple gravitational imaging of compact radio components with image separations smaller than that of the smallest galaxy lens system known (335 mas) and corresponding to masses appropriate to compact/dwarf galaxies and to the bulges of spiral galaxies. A secondary aim was the morphological classification of a sample of flat-spectrum sources with kpc-scale structure. We particularly wanted to find out the frequency of occurrence of compact symmetric objects (CSOs: linear size < 1 kpc) and medium symmetric objects (MSOs: linear size < 15 kpc) which are believed to be the precursors of the large classical double radio sources. The parent sample consisted of 1665 flat-spectrum sources selected from two VLA surveys made with ∼ 200 mas resolution (the Jodrell Bank–VLA Astrometric Survey — JVAS — and the Cosmic Lens All-Sky Survey — CLASS), from which we used visibility data to select sources (55 in total) with significant structure in the size range 90–300 mas. Sources with multiple compact components having a flux density ratio < 8:1 could be high-magnification lens systems, CSO/MSOs or objects with strong jets. A step-by-step observational filtering process at successively higher angular resolution was employed to classify the 55-source subsample. Initial MERLIN observations at 5 GHz (50 mas resolution) enabled us to classify ∼ 75 per cent of the sources. The remaining sources were observed with the VLBA at 5 GHz (2 mas resolution) and in a few cases with MERLIN at 22 GHz (10 mas resolution). The resulting maps show that the majority of flat-spectrum sources with kpc-scale structure are asymmetric core–jets. The remaining sources include 23 new CSO/MSOs, a much smaller fraction of the parent sample (1.4 per cent) than is found in VLBI surveys of flat-spectrum sources (5–10 per cent). About half of the new CSO/MSOs constitute a hitherto unknown population: bright core CSO/MSOs. No definite high-magnification gravitational lenses were found. The implication is that the optical depth to lensing with image separations in the range 90–300 mas is several times less than on arcsec scales.

Light on dark lenses

We present an update of the status of the Cosmic Lens All-Sky Survey (CLASS). Twelve lenses have been found to date in CLASS together with the earlier Jodrell Bank-VLA Astrometric Survey (JVAS). Six other sources are under investigation as possible lenses. To date, all lens systems studied optically have been shown to have associated lensing galaxies to a limiting I magnitude of 23–24. These observations place severe constraints on published claims that dark ( M L > 100 ) galaxies form a significant fraction of the general population of galaxies.

Ground-Based Surveys For Low-Luminosity Stars

Most of what we know about the Luminosity and mass function of low-mass stars has been derived from analysis of ground-based photometric and astrometric Surveys. A major uncertainty in the Interpretation of such surveys lies in the appropriate correction for unresolved binaries and in the use of a reliable colour-magnitude relation. various new surveys are now uncovering large numbers of objects which should rapidly ameliorate these ambiguities.The recent discovery of brown dwarfs and planets has highlighted the importance of low-luminosity stars. once we understand their spectra and colours and have statistically significant samples, it will be important to use them to reliably constrain theories of star formation and chemical evolution as well as to use them as a probe of galactic structure and to reliably ascertain their contribution to baryonic dark matter. In this brief review ishall focus on ground-based field searches. Hambly (1997) reviews the very successful searches that have recently been carried out in nearby clusters.Until the 1970s, most work on the faint-star luminosity function was based on proper-motion searches primarily by dutch researchers, in particular Luyten (e.g. 1938, 1979). The Last 20 years have seen a switch from astrometric to photometric surveys employing plate-scanning machines or CCD cameras. Combined with substantial increases in computing power these technologies have led to the discovery of large numbers of very low-luminosity Stars.

A VLBA Calibrator Survey

AbstractThe Very Long Baseline Array (VLBA) Calibrator Survey consists of snapshot observations at 2.7 and 8.4 GHz of nearly 2000 flat-spectrum extragalactic sources selected from the Jodrell Bank–VLA Astrometric Survey (JVAS). When complete, this survey will assist in defining the extragalactic radio frame in the northern sky by providing positions for these sources which will be accurate on the milliarcsecond scale. This survey is accessible on the WWW at http://magnolia.nrao.edu/vlba_calib/index.html.

MEASURING THE ATMOSPHERIC INFLUENCE ON DIFFERENTIAL ASTROMETRY: A SIMPLE METHOD APPLIED TO WIDE FIELD CCD FRAMES

ABSTRACT Sets of short exposure, guided CCD frames are used to measure the noise added by the atmosphere to differential astrometric observations. Large nightly variations that are correlated with the seeing have been found in the data obtained over 2 years at the KPNO and CTIO 0.9-meter telescopes. The rms noise added by the atmosphere, after a linear transformation of the raw x, y data, is found to be 3 to 7 mas, normalized to 100 seconds exposure time and a field of view of 20 arcminutes near the zenith. An additional nearly constant noise (base-level) of 8.5 mas = 0.012 pixel is found for the KPNO and 6.0 mas = 0.015 pixel for the CTIO telescope. This implies that a ground-based, all sky, astrometric survey from guided CCD frames is more likely limited by the base-level noise than by the atmosphere and could reach an accuracy better than 10 mas under good seeing conditions.

Astrometry and Galactic Structure

The application of astrometric surveys to understanding the structure and kinematics of the Milky Way is discussed, with particular emphasis on some of the issues expected to be addressed in the next decade or so. It is shown that the results of complete proper motion surveys of field stars at the Galactic poles are in remarkable agreement concerning the global kinematics of stars as a function of distance, but that significant differences exist in how these data are decomposed to derive the kinematics of individual Galactic components. The status and future prospects regarding the measurement of proper motions and orbits for Galactic globular clusters and satellites are also discussed.

AIST Project: Scientific and Technological Foundations

The results of an initial technical design of the AIST spacecraft and payload are presented. The spacecraft design is subjected to the main requirement: to supply the smooth rotation. Two telescopes are symmetrically placed around central beam combiner unit. The fine adjustment of the light pressure centre position by means of special-purpose sails is proposed. Scientific objectives of the project are the implementation of astrometric survey of the sky and obtaining a series of star position catalogues with milliarcsecond accuracy, including a second epoch of Hipparcos/Tycho catalogues. The survey list will consist of nearly 10–15 million stars brighter than 15th magnitude.

High-velocity stars toward the South Galactic CAP

Spectroscopic observations of stars identified in Murray's (1986, MNRAS, 223) astrometric survey of the South Galactic Cap as having unusual kinematics are presented. Most of these stas prove to be subdwarfs from the galactic halo with abundances from [Fe/H] ∼ −3.3 to ∼ −1.0, with a small number of more metal-rich (−0.5 ≤ [Fe/H] ≤ −1) high-velocity stars. The former group has a velocity dispersion perpendicular to the Plane of σ w ∼ 90 km/s and a local density of 0.2%-0.3% that of the old disk, parameters consistent with other surveys of the local subdwarf population. Seven stars in the sample appear to be on retrograde galactic orbits

Systematic Differences between Radio Astrometric Surveys

AbstractA radio astronomical reference frame has been established from four interferometric position surveys of extragalactic objects. The resulting systematic differences between surveys were studied with different weighting schemes and averaged systematic corrections were derived.

Measurements of the Cape Astrometric Survey of the Southern Hemisphere

SummaryA routine has been developed for the processing of the 5820 plates of the survey. The plates are measured on the automatic measuring machine, GALAXY, and the measures are subsequently processed by computer, to edit and then refer them to the SAO catalogue. A start has been made on measuring the plates, but the final selection of stars to be made is still a matter for discussion.