... galaxies

Tables 3, 4 and Figs. 8-10, 13, 15, 16, 18 and 22 are only available in electronic form at http://www.edpsciences.org

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... NRAO

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

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... ATCA

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

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... brightness

In the following we will also use the usual expression "intensity''.

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... Surveys

The Digitized Sky Surveys were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions.

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... upstream

As the gas moves faster than the pattern speed of the bar, upstream is defined as the region in front of the shock which lags behind with respect to the galaxy's rotation.

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