A VLBI survey at 2.29 GHz

(OCR+proof by H.Andernach 8/96)

R. A. PRESTON, D. D. MORABITO, J. G. WILLIAMS, J. FAULKNER, D. L. JAUNCEY, G. D. NICOLSON VLBI observations at 2.29 GHz with fringe spacings of about 3 milliarcsec have been performed on 1398 radio sources spread over the entire sky. 917 sources were detected, including 93% of the identified BL Lacertae objects, 86% of the quasars, and 36% of the galaxies. The resulting catalog of compact radio sources is useful for various astrophysical studies and in the formation of VLBI celestial reference frames.

1. INTRODUCTION

The results of this survey are presently being utilized to form a VLBI reference frame of 100-200 sources by determining precise relative positions (0.001"-0.01") (see, for example, Fanselow et al. 1984). Such celestial reference frames (see also Ma et al. 1981) will be at least an order of magnitude more precise than previous stellar frames and are nearly inertial since the extragalactic sources are without measurable proper motions. They enable significant advances in various geodetic and astrometric studies (e.g., crustal plate dynamics, Earth rotational irregularities, planetary dynamics, interplanetary spacecraft navigation). A similar, but deeper, VLBI survey of the ecliptic zone has been previously published (Wehrle, Morabito, and Preston 1984).

The VLBI survey is also useful for studying the characteristics of compact radio sources. The detected survey sources coincide with the cores of quasars and galaxies. Understanding the nature of these energetic cores is crucial in unraveling the origin and evolution of the objects in which they reside. The catalog will not only serve as a reference list for observers, but it can be used in statistical studies of radio-source properties and cosmological theories. Toward these ends, the catalog has been supplemented with optical identifications, optical magnitudes, redshifts, and radio spectral indices derived from the literature.

2. SAMPLE SELECTION AND COMPLETENESS

681 weaker or steeper spectrum sources from the Parkes and NRAO-Bonn surveys, as well as from the general literature, were also observed. Our sample was intended to be purely extragalactic, and identified galactic sources were eliminated from the sample. However, some of the optically unidentified sources that met our sample criteria could be galactic. Such sources are highly unlikely to have been detected with VLBI at our angular resolution and sensitivity.

Completeness of the observed sample is difficult to estimate, not only due to temporal variability but also because the two finding surveys had different levels of completeness for different sky regions, lacked two-frequency information for all sources, and had different primary survey frequencies. Neglecting temporal variability, both finding surveys are nearly complete for S>=1.0 Jy and alpha>=-0.5, resulting in a combined completeness of more than 97% for the sky area covered. The spectral-index criterion is necessary because the NRAO-Bonn survey frequency was 5.0 GHz, not 2.7 GHz. Based on these sample criteria, the VLBI survey is estimated to be 93 % complete, again neglecting temporal variability, with a total of 312 sources observed. Because the flux-density limits of the finding surveys varied depending on sky region, estimates of completeness for sources with lower total flux densities do not apply to the entire sky (see Table 1).

3. THE OBSERVATIONS

The fringe spacing sampled ranged from 2.5 to 4.1 milliarcsec. For the mean fringe spacing of 3.3 milliarcsec, the normalized fringe visibility of a Gaussian source varies from 0.9 to 0.1 as the half-intensity diameter increases from 0.5 to 2.2 milliarcsec.

The 5 sigma detection limit in correlated flux density was generally ~0.1Jy. The corresponding random uncertainty in detected source strength is ~0.02 Jy, but systematic errors at about the 10% level dominate the random contribution for most sources. To ensure that few compact radio components would be missed due to a priori source-position errors, the sky was completely searched within 0.5 arcmin of all nominal source positions by cross-correlating over an appropriate range of delay and delay rate.

Total flux densities at 2.29 GHz were also measured for most sources at the time of VLBI measurement by means of on-off measurements with a noise-adding radiometer. The random uncertainties in total flux-density measurements typically range from 0.03 to 0.3 Jy, with systematic errors in antenna sensitivity being ~ 3%.

4. RESULTS

Detection statistics as a function of optical identification type appear in Table 4. Detection statistics as a function of general optical class appear in Table 5: 93% of identified BL Lacertae objects were detected, 86% of QSO's were detected, and 36% of galaxies were detected.

The survey results appear in tabular form in Table 6. Descriptions of the table entries appear below:

Column 1. Source name.

Columns 2 and 3. 1950.0 positions, Asterisked positions are determined from the VLBI-survey data and have typical uncertainties of ~1" (see Morabito et al. 1982; Morabito et al. 1983; and Morabito et al. 1985). Other positions are from the literature, and in most cases, errors are <30".

Column 4. Spectral indices between 2700 and 5000 MHz followed by corresponding reference number (see Table 7). A few existing compilations of redshifts, optical identifications, and optical magnitudes were useful aids in preparing our catalog (References 63, 84, 86, 111, and 232 in Table 7). However, in almost all cases we have drawn values for these quantities from original references to enhance accuracy. A star following the reference number indicates a questionable or conflicting value, and is explained in the notes to Table 6. For many Southern Hemisphere sources, the optical characteristics were obtained from an optical identification program which utilized the radio source positions determined by our survey (Jauncey et al. 1984 - Reference 173; Jauncey et al. 1985-Reference 174; Savage et al. 1983).

Column 5. Redshifts followed by corresponding reference number (see Table 7).

Column 6. Optical identifications followed by corresponding reference number (see Table 7). Optical identification codes are defined in Table 4.

Column 7. Optical magnitudes followed by the corresponding reference number (see Table 7). These values may be visual, blue, or red.

Column 8. Experiment codes as defined in Table 3.

Column 9. Measured 2.29-GHz total flux density (Jy).

Column 10. Measured 2.29-GHz correlated flux density (Jy). Values for seventeen ecliptic sources marked by asterisks are from Wehrle, Morabito, and Preston (1984).

Column 11. Visibility is defined as the correlated flux density divided by the total flux density.

Column 12. East-west (u) and north-south (u) spatial frequencies of the observations in units of 106 wavelengths.

Computer readable versions of the catalog are available upon request.

REFERENCES

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