J/AJ/127/3609 VLBI observations of southern ICRF sources (Ojha+, 2004)
VLBI observations of southern hemisphere ICRF sources. I.
Ojha R., Fey A.L., Johnston K.J., Jauncey D.L., Reynolds J.E.,
Tzioumis A.K., Quick J.F.H., Nicolson G.D., Ellingsen S.P., Dodson R.G.,
Mcculloch P.M.
<Astron. J., 127, 3609-3621 (2004)>
=2004AJ....127.3609O 2004AJ....127.3609O
ADC_Keywords: Radio sources ; QSOs ; Morphology ; Redshifts
Keywords: astrometry - galaxies: active - quasars: general -
radio continuum: galaxies - reference systems - surveys
Abstract:
We present 8.4GHz very long baseline interferometry (VLBI)
observations of 69 southern hemisphere extragalactic sources in the
International Celestial Reference Frame. These are the first in a
series of observations intended to image all such sources at
milliarcsecond resolution in order to determine their continued
suitability for reference-frame use based on intrinsic structure. We
use the resultant images to calculate a core fraction, that is, the
ratio of core flux density to total flux density, for all observed
sources. The resulting distribution, with a mean value of 0.83,
suggests that most sources are relatively compact. However, just over
half the observed sources show significant extended emission in the
form of multiple compact components. These sources are probably poorly
suited for high-accuracy reference-frame use unless intrinsic
structure and potential variability can be taken into account. Our
observations represent the first large, comprehensive VLBI imaging
survey in the southern hemisphere, significantly extending the
existing limited VLBI surveys and, along with some well-known objects,
containing many sources that have never been imaged at milliarcsecond
resolution. The overlap with Very Long Baseline Array images of
sources between 0° and -35° declination helps determine the
limits to imaging with the southern hemisphere-accessible telescopes.
Description:
The observations reported in this paper were made using the five
telescopes that make up the Australian Long Baseline Array (LBA),5 as
well as telescopes in South Africa and Hawaii. Because of limited
mutual visibility between the last two sites, each observing epoch is
split into two sessions, one involving observation with the LBA and
South Africa and the other involving observation with the LBA and
Hawaii. The data from the two epochs are calibrated separately and
then combined in order to image with the best possible u-v plane
coverage. Images from two epochs are presented here. The first epoch
consisted of a 24hr session with the LBA and Kokee on 2002 July 16,
followed by a 48hr session with the LBA and Hartebeesthoek from 2002
July 18 through July 20. The second epoch consisted of a 24hr session
with the LBA and Hartebeesthoek on 2002 November 14, followed by a
48hr session with the LBA and Kokee on 2002 November 15 and 16.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
objects.dat 37 69 Objects position (from Simbad)
table2.dat 51 74 Parameters of Naturally Weighted Images
table3.dat 23 69 Physical Characteristics of Sources
table4.dat 43 121 Gaussian Models
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Byte-by-byte Description of file: objects.dat
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Bytes Format Units Label Explanations
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1- 3 A3 --- --- [PKS]
5- 12 A8 --- PKS PKS source name (based on B1950 position)
18- 19 I2 h RAh Simbad right ascension (J2000)
21- 22 I2 min RAm Simbad right ascension (J2000)
24- 27 F4.1 s RAs ? Simbad right ascension (J2000)
29 A1 --- DE- Simbad declination sign (J2000)
30- 31 I2 deg DEd Simbad declination (J2000)
33- 34 I2 arcmin DEm Simbad declination (J2000)
36- 37 I2 arcsec DEs ? Simbad declination (J2000)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- PKS PKS source name
10 A1 --- Obs [de] Epoch of observation (1)
12- 14 F3.1 mas aaxis Restoring beam FWHM of the major axis
16- 18 F3.1 mas baxis Restoring beam FWHM of the minor axis
20- 22 I3 deg PA [-90/90] Restoring beam position angle (2)
24- 27 F4.2 Jy Sp8.4GHz Peak flux at 8.4GHz in units of Jy/beam
29- 32 F4.1 mJy RMS Root-mean-square of final hybrid image
residuals in units of mJy/beam
34- 51 A18 mJy Level Contour levels in units of mJy/beam (3)
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Note (1): Epoch as follows:
d = Epoch 2002 July 16-20
e = Epoch 2002 Nov. 14-16
Note (2): Measured north through east.
Note (3): Contours levels are represented by the geometric series
1,...,2n, e.g. for n=5 the contour levels would be ±1,2,4,8,16,32
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- PKS PKS source name
10 A1 --- ID [QGLA] Optical identification (1)
12 A1 --- l_z Limit flag on z
13- 17 F5.3 --- z ? Redshift
19- 23 F5.2 mag Vmag ? Visual magnitude
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Note (1): Optical identification ad follows:
Q = QSO
G = galaxy
L = BL Lacertae
A = Other
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- PKS PKS source name
10 A1 --- Obs [bc] Epoch of observation (1)
12 I1 --- Comp [1/3] Component number
14- 18 F5.3 Jy S8.4GHz Flux density at 8.4GHz
20- 24 F5.1 mas Sep Component separation from arbitrary origin
26- 29 I4 deg PA1 [-180/180]? Position angle between component
and origin (2)
31- 34 F4.2 mas aaxis FWHM of the major axis
36- 39 F4.2 --- b/a Major to minor axis ratio
41- 43 I3 deg PA2 [-90/90]? Major axis position angle (2)
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Note (1): Epoch as follows:
b = Epoch 2002 July 16-20
c = Epoch 2002 Nov. 14-16
Note (2): Measured north through east.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Patricia Vannier [CDS] 13-Jan-2007