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1998MNRAS.294..607Vessey & Green
The 7C(G) survey of radio sources at 151 MHz in the Galactic Plane
The 7C(G) survey covers much of the northern Galactic Plane Survey at 151 MHz.
The ``G'' stands for Galactic, to distinguish this survey from
other 7C surveys away from the Galactic plane that
have also been made with the Cambridge Low Frequency Synthesis Telescope (CLFST).
Full details of the observations, data reduction and calibration of this
survey, and the production of the 7C(G) compact source list, are described in
Vessey & Green (1998, MNRAS, in press).
The 7C(G) survey covers approximately the region 80° < l <
180°, |b|<5°, with a resolution of about 1.2 × 1.2
cosec(declination) arcmin2 (EW×NS). The survey consisted of 24
fields, spaced 3°.5 in l, and alternating between b =
+3° and -3°, but with a gap near l = 110°. There is some
coverage to |b|<9°. As the CLFST has a wide field of view, and
operates at a low-frequency so that ionospheric fluctuations are significant,
the quality of images synthesized near bright sources is poor. Therefore the
sensitivity of the survey varies considerably with l, but is poorer
towards l = 80° (near Cyg A), near the gap at l = 110°
(near Cas A), or l = 185° (near Tau A, the Crab Nebula).
The 7C(G) compact source list
The compact source list derived from this survey -- the 7C(G) catalogue --
consists of 6262 compact sources, with a completeness limit of approximately
0.25 Jy over most of the survey region. The catalogue has an rms position
accuracy of better than 10 arcsec, and the flux densities have an accuracy of
better than 10 per cent. This catalogue is available as:
The entries in the source list are as follows:
- Right ascension and declination of the beam fitted position of the source
at epoch B1950.0; the rms accuracies are 5 and 7 arcsec respectively.
- Right ascension and declination of the centroid position of the source at
epoch B1950.0. Multiple peaks integrated together have the same centroid
- The Galactic longitude and latitude of the beam fitted position of the
- The beam-fitted and integrated flux densities, in Jy. The beam-fitted flux
is the preferred flux density unless the source is clearly extended, in which
case the integrated flux density should be used.
- The signal-to-noise ratio, i.e. the interpolated peak height of the source
on the map (after subtraction of the local zero level), divided by the local
- The extent of the source size in units of the local beam area. This
estimate is influenced by residual ionospheric smearing of the synthesised
beam. Another measure of whether a source is resolved or not, is the ratio of
integrated/beam-fitted flux densities. The flux density calibration is
performed in such a way as to ensure that the median value of this ratio for
unresolved sources is unity. Thus, any source with this ratio >>1 is
resolved, and for this survey a ratio of integrated/beam-fitted flux densities
greater than about 1.2 is a good indication that the source is resolved.
- The number of individual peaks that were included in the flux integration
is given, together with a single character flag indicating whether the source
is the main (`M') or a component (`C') peak in the integration. Usually no
number and flag are given, indicating the source has a single peak only. For
multiple sources each peak has a separate entry in the source list, with its
own beam fitted flux density and position, although the integrated flux and the
centroid position are the same for each. Some multiple peaked sources appear
with different numbers of components in overlapping fields. These entries have
not been averaged together, and care should be taken with such entries.
Further data from the 7C(G) survey, including access to the images, will be
made available as and when time permits.
Last changed: 1998 January 21st
Dave Green/MRAO, Cambridge, UK/D.A.Green@ mrao.cam.ac.uk