2.3 Conspiracies ?
In the late seventies, the emphasis on the rotation curve shapes concerned their flatness: the absence of the expected Keplerian decline of the rotation curve beyond the optical image implied the presence of a large amount of mass with high mass-to-light ratios (Bosma 1978). The systematics of the rotation curves as function of Hubble type and luminosity were determined by Rubin and her collaborators in a series of papers (Rubin et al. 1978, 1980, 1982, 1985).
In the mid-eighties arguments centered on the flat and featureless nature of the curves (Bahcall & Casertano 1985). The question how a thin disc and a presumably round dark halo conspire to keep the rotation curve flat without showing any marked feature was raised, and an answer was found in the process of adiabatic compression of the halo material when the disc was formed (cf. Barnes 1987, Blumenthal et al. 1986).
However, some galaxies have rotation curves which decline just beyond the optical image, and stay more or less flat thereafter. Early examples are NGC 5033 and NGC 5055 (Bosma 1978, 1981a), and also NGC 5908 (Van Moorsel 1982). Thus the notion of featureless rotation curves never corresponded to hard reality. Two more examples, NGC 2683 and NGC 3521, were given by Casertano & Van Gorkom (1991), who speculated that declining curves are linked with discs having short scalelengths. However, Broeils (1992) finds cases of declining curves for galaxies with large disc scalelengths.
In any case, all these examples show that the process of adiabatic compression is not fully operational for all spiral galaxies. In particular, a partial decoupling between disc and halo is seen in a non-negligible fraction of the brighter spirals.