... data

Complete Fig. 2 is only available in electronic form at http://www.edpsciences.org

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... NRAO

National Radio Astronomy Observatory.

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... VLA

Very Large Array, operated by the NRAO.

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...$\ge$1 Mpc

We assume that H₀=50 kms^-1Mpc^-1and q₀=0.5throughout this paper.

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... AIPS

Astronomical Image Processing System, developed and maintained by the NRAO.

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