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Astrophysics Group » 7C(G)

Astrophysics Group

Cavendish Laboratory

7C(G)

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, 294, 607).

Summary

The 7C(G) survey covers approximately the region 80ÒÀ < l < 180ÒÀ, |b|<5ÒÀ, with a resolution of about 1.2 Ó? 1.2 cosec(declination) arcmin2 (EWÓ?NS). The survey consisted of 24 fields, spaced 3ÒÀ.5 in l, and alternating between b = +3ÒÀ and -3ÒÀ, but with a gap near l = 110ÒÀ. There is some coverage to |b|<9ÒÀ. 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 = 80ÒÀ (near Cyg A), near the gap at l = 110ÒÀ (near Cas A), or l = 185ÒÀ (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. Please refer to the source list description for details of the entries ; the catalogue itself is available as:

The 7C(G) image archive

The 7C(G) images (B1950 coordinate frame) are available. These images are in FITS format, with headers that include some text describing the main properties of the images and some recommendations about their use.

Each of the Galactic longitudes listed below is the centre of a 7C(G) field. For each field a set of four images (ne, nw, se, sw) at high resolution (1.2 Ó? 1.2 cosec(declination) arcmin2) and a single image (low) embracing the whole field at low resolution (4 Ó? 4 cosec(declination) arcmin2) is available. Each set of 4+1 images can be retrieved as a gzipped tar bundle (about 5 to 10 Mbytes) by selecting the appropriate Galactic longitude below. (Use shift click should to access the save option if your browser tries to display the file as text).

Also available are gzipped tar bundles for specially observed fields centred on:

  • Cygnus Loop (one high-resolution and one low-resolution image);
  • IC 443 (one high-resolution image).

Usage note

As recommended in the header texts of all the images, data for any reasonably compact sources should be taken from the catalogue, as values given there will include all the required corrections. If you wish to estimate an integrated flux for a more extended source than can be described in the catalogue (eg: a large supernova remnant or H II region) you can get an approximate total flux density by taking a sum of pixels and dividing by 4.9cosec(delta), where delta is the declination of the field centre of the relevant image set. The field centre is the reference pixel in the FITS header. Please contact us at the e-mail address below if you need more advice or greater accuracy than this ‘rule of thumb’ for a rough estimate.

Last changed: 2000 April 5th

Dave Green/MRAO, Cambridge, UK/D.A.Green@mrao.cam.ac.uk