We thank Adwin Boogert, Jos de Bruyne and Lodewijk Bonebakker for obtaining the data. We thank Roelof de Jong for making his data of a large sample of spirals available to us. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999