... data

Figures 1, 5, 6, 10, 14, 15, 19, 20, 24, 28 and Appendix A are only available in electronic form at http://www.edpsciences.org

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... (ATCA

The Australia Telescope is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.

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... (VLA

The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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... (WSRT

The Westerbork Synthesis Radio Telescope is operated by ASTRON (the Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO).

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... package

The Groningen Image Processing System; van der Hulst et al. (1992).

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