2.4 Universal Rotation Curves ?

Persic et al. (1996) propose that the shapes of rotation curves follow a systematic pattern as function of luminosity only. Their conclusions are based on a re-analysis of the data by Mathewson et al. (1992), although the notion of universal rotation curve is older (e.g. Rubin et al. 1980). The rotation curves for low luminosity galaxies are rising, those of intermediate luminosity are more or less flat, while those of the highest luminosity galaxies are falling.

However, a cursory inspection of the 12 curves with the highest rotation velocities in the sample of Persic et al. (1996, see also Persic & Salucci 1995 for more details) shows that their weighted average is not declining at all. Moreover, about 18% of the galaxies in their sample have inclinations larger than 85°, and about half of these are seen exactly edge-on. This has consequences for the shape of the resulting average rotation curves : as has been shown by Bosma et al. (1992) and Bosma (1995), the opacity in the inner parts of large spirals tends to make the inner slopes of H rotation curves too shallow compared to the true rotation curves. Thus the averaging procedure should be redone without taking the data from highly inclined galaxies into account.

Even so, Verheyen (1997) reexamined the problem of universal curves, and finds that for the 30 rotation curves in his sample of spiral galaxies in the Ursa Major cluster about 10 do not fit at all to the universal rotation curves of Persic et al. (1996). This means that although there is some merit to the scheme, at the 10% - 20% level the notion of universal rotation curves breaks down.