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1981AJ.....86.1010Ulvestad+

A VLA Survey of strong radio sources

(OCR by H.Andernach 10/96, proof-read and corrected by S. Trushkin 03/97)

J. ULVESTAD, AND K. JOHNSTON, R. PERLEY AND E. FOMALONT

Abstract

The VLA has been used to survey the structures of 360 strong high-frequency radio sources at 1480 and 4900 MHz. Of those sources, 250 are found to contain at least 90 % of their flux density in an unresolved component ( < 1 arcsec), while the remaining 110 sources are considered to be resolved. Positions and structural information for these sources are given, and maps of 20 of the resolved objects are shown. The VLA data have been combined with other structural information available for sources listed in the Parkes and S surveys in order to study a complete sample of 444 extragalactic objects having flux densities greater than 1 Jy at 5 GHz. The unresolved radio sources in the flux-limited sample have a median spectral index of a = - 0.09 (Sv ~ v^a) at centimeter wavelengths, while the resolved sources have a median index of a = - 0.73. The unresolved radio galaxies, in general, have steeper spectra than both the resolved and the unresolved quasars. Finally, there seems to be a lack of quasars having radio sources with large linear diameters beyond z =~ 1.

I. INTRODUCTION

We have used the partially completed Very Large Array (VLA) to conduct a survey of the structure of strong extragalactic radio sources. The sources were selected from the S surveys made at 5 GHz (Pauliny-Toth et al. 1978 and references therein) and the Parkes 2.7-GHz surveys (Savage, Bolton, and Wright 1977 and references therein). These surveys are virtually complete for sources having flux densities greater than ~0.6 Jy at 5 GHz and Iying more than 10^o from the galactic plane. Figure 2 of Pauliny-Toth et al. (1978) gives an outline of the surveyed regions.

Two basic criteria were used in selecting sources for our observations. First, we chose objects whose 5-GHz flux densities were reported to be greater than I Jy in the surveys mentioned above. Since many strong extragalactic sources are variable, this constraint does not mean that all our sources exceeded the flux density limit at the time of our observations. Second, we attempted to choose sources for which no high-resolution maps (e.g., Cambridge 5-km telescope maps) were available in the literature. The first criterion was relaxed for ~25 objects in order to fill in gaps in the observing program, while the second criterion was ignored for a few wellknown sources that we observed as a test to ensure that our reduction methods gave correct results.

The results of our study are given in the text and ta bles in the following sections. In Sec. II, we describe our VLA observations, tabulate the new data, and present maps of some of the more interesting sources. Then, in Sec. III, we construct a complete sample of high-frequency radio sources and present some analysis of the properties of that sample. Finally, Sec. IV contains a summary of our major conclusions.

Il. VLA OBSERVATIONS AND RESULTS

We observed a total of 288 sources, including calibrators, on the partially constructed VLA during the period 3-6 February 1979. The telescope was divided into two sub-arrays operating in parallel at frequencies of 1480 and 4900 MHz (20- and 6-cm wavelength, respectively). Seven antennas were used in each sub-array, with spacings ranging from 100 m to 13.6 km at the lower frequency and 1 to 15 km at the higher frequency. The longest spacings gave resolutions of ~3" at 1480 MHz and ~0.8" at 4900 MHz. Almost all sources were observed at a minimum of three hour angles. However, scheduling problems, holes in our observing run, and unusable data limited a small number of sources to two observations or, in rare cases, only one observation. Table I shows the approximate number of scans made as a function of source declination. Individual scans were 23 min in length, a time adequate to establish a sufficient signal-to-noise ratio for the strong sources, and were scheduled in order to maximize the u-v plane coverage in each declination band.

The interferometer phases and amplitudes were calibrated using the sources listed in Table II. In that table, column I gives the calibrator's IAU name and column 2 gives the initially assumed flux density in Jansky at 1480 MHz (L-band) for all sources used as calibrators at that frequency. The letter P denotes primary calibrators, objects for which VLA survey observations show no resolution on our baselines. Column 3 gives the initial assumed flux densities for sources used as calibrators at 4900 MHz, with the letter P having the same meaning as in column 2. Maps of all sources wore made at 4900 MHz in order to establish their positions relative to the primary calibrators, whose positions are known with an accuracy of ~0.1". After the positions of all sources were derived, both the primary and secondary calibrators were used in the final calibration of the data. The amplitude scale was set by adopting the flux density of 3C 286 (1328 + 307) as 14.44 and 7.41 Jy at 1480 and 4900 MHz, respectively (from scale of Baars et al. 1977).

The sources showing less than 10 % of their total flux density in extended emission or secondary components were considered unresolved; those sources are listed in Table III. In addition to the sources studied during February 1979, Table III also contains all sources observed in the VLA calibration program by mid-1980 which have been found to have little or no resolved structure at ~ 1465 and ~4885 MHz. Not all of these sources have flux densities above I Jy at 5 GHz, but we report them here for the purposes of providing information to other radio observers. The subset of sources in Table III with observed 5-GHz flux densities above I Jy is a sample of unresolved radio sources, which we estimate to be '90% complete at a given time for declinations between + 70^o and -45^o (excluding regions where |b| < 10^o). This estimate is based on the fact that we have not observed 18 sources in the flux-limited sample, while 250 unresolved sources are listed in Table III (the surveys from which our sources have been drawn are estimated by their authors to be virtually complete to flux density limits below 1 Jy). A few sources having DEC > + 70^o are included in Table III, but that part of the list is far from being complete, since the S surveys from which the northern sources were drawn were not published at the time this research was conducted.

In Table III, column 1 gives the IAU source designation, while column 2 gives an alternative 3C or 4C name. Column 3 gives an optical identification (GAL = galaxy, QSS = quasi-stellar source) taken from references listed in the Appendix. Columns 4 and 5 give the 1950 epoch right ascension and declination, with positional accuracies < 0.2" for VLA calibrators and < 0.4" in most other cases. The source flux densities at 20, 6, and (where applicable) 2 cm are given in columns 6-8, respectively. For sources or frequencies not observed in the main survey, these values are taken from calibration runs made as close as possible in time to February 1979. Finally, column 9 specifies the sources which are VLA calibrators, with the letters L, C, and U denoting calibrators at 20, 6, and 2 cm, respectively.

All sources displaying structure were mapped at 1480 MHz. No maps at 4900 MHz were made for presentation because our minimum antenna spacing of 1 km at that frequency made the observations insensitive to structures > 12". Parameters for these resolved sources are listed in Table IV. Columns 1-3 give information similar to that listed in Table III. Columns 4 and 5 give 1950 epoch right ascensions and declinations, with positional accuracies of < 1". For sources with a single resolved component, the position refers to the radio peak, while multicomponent sources have positions listed for each of the radio peaks that we consider significant. Column 6 gives the total and peak flux densities at 4900 MHz (6 cm) for each source, with the flux density always referring to the dominant component. Column 7 gives total and peak flux densities at 1480 MHz (20 cm). Here, the top refers to the total emission from the entire source and the peak flux densities for the additional components whose positions are listed in columns 4 and 5. We note that our 4900 MHz measurements understimate the total flux density for a number of well resolved sources owing to the lack of short spacings mentioned above; previous single dish measurements are probably more accurate than our VLA values in such cases. In column 8, we comment on the source structures. The Gaussian sizes referred to are FWHM. Samples of the source maps are displayed in Figs. 1a-1v (22).

Table II. Calibration sources in VLA survey.
===========================================
Source    1480MHz   4900 MHz
-------------------------------------------
0201+113  1.01 P    1.00 P
0202+319  0.85
0235+164  1.53
0237-233            3 07 P
0316+161            2.65 P
0319+121  1.70
0400+258  1.65
0420-014            2.99
0426-380            1.08
0428+205  3.80
0457+024  1.40
0500+019  2.25
0518+165  8.62 P
0528-250  1.12
0528+134            2.50 P
0552+398  1.72 P    4.81 P
0711+356            1.14
0727-115            3.19 P
0738+313  2.10
0742+103  3.70 P    4.00 P
0748+126  1.63
0826-373            4.01
0919-260            2.37
1117+146  2.35
1127-145            4.75 P
1155-251  1.03 P    0.91 P
1245-197  5.14 P    2.34 P
1252+119  1.08 P    1.04
1311+678            0.86
1323+321  4.65
1328+254  6.91 P    3.17 P
1328+307 14.44 P    7.41 P
1511+238  1.63
1519-273            1.35
1555+001  1.46 P    1.26
1607+268  4.75
1741-038  1.91 P    2.24
2005+403  3.49 P    3.73 P
2128+048  3.85
2128-123            2.45
2134-004            9.72
2200+420  2.10 P    1.73 P
2234+282  0.90
2352+495  2.60 P    1.57 P
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