... Teuben, P. J. 2003, in ASP Conf. ... The observed light distribution gives many clues about the different components disk galaxies seem to be made up of (bulge, disk, bars etc.), though it was not until the velocity field of tracer material such as HI in the outer parts of disk galaxies was studied (see Sofue & Rubin, 2001, for an excellent recent review), that an unseen component was postulated to exist. ... Velocity fields are commonly derived from a datacube by fitting gaussians or taking moments...