7.2.5 Sample Selection
For the TF relations to be successfully applied, the galaxies must have detectable gas in essentially circular motion; otherwise, the estimate of Vmax will be inaccurate. Although there is occasionally detectable HI in lenticular systems (van Driel and van Woerden 1991), defining a TF relation for E/S0 galaxies involves several complications: the origin of the gas is uncertain, the gas is usually patchy, its detection is difficult, and its motion may not be circular. Because of these difficulties (and the potential for significant M/L variations among early-type galaxies as discussed in Sec. 7.3), it is best to limit samples of galaxies to types Sb - Irr having no signs of peculiar morphology, due, for example, to recent interactions. In addition, the galaxies must have enough gas in their exterior regions such that the observable rotation curve reaches a peak and Vmax can be adequately sampled. It is now clear that disk galaxies within the cores of even moderately poor clusters like Virgo suffer significant stripping of their outer envelope of HI gas (e.g., Haynes and Giovanelli 1986; Cayatte et al. 1990). Fortunately, only a few systems in Virgo are gas-stripped to radii within the turn-over in their rotation curves. The most serious cases also have morphological peculiarities and would likely have been excluded on this basis alone (e.g., NGC 4438). However, this remains an uncertainty for galaxies in the cores of more distant clusters; for these galaxies a full rotation curve may be necessary to assure that the rotation curves do indeed turn over.
The modest morphological type dependence mentioned earlier (i.e., earlier types having low luminosities for their line-widths, Sec. 7.2) is a consequence of late-type galaxies being systematically bluer than those of earlier type, coupled with the historical use of the B-band TF relation. Because the effect decreases dramatically toward longer wavelengths, a likely cause is the smaller bulge-to-disk ratio and the larger fraction of young stars in the low mass, gas-rich systems. Some of the effect may also be the result of a decrease in dust content for the later morphological types (e.g., van den Bergh and Pierce 1990). If so, the assumption of a mean optical depth for all morphological types could contribute to a morphological type dependence, especially at the shorter wavelengths where extinction estimates would be significantly overestimated.
As mentioned above, the kinematics of lower luminosity irregular galaxies become progressively more dominated by turbulence and less by rotational motion. Consequently, the TF relations must progressively break down below some luminosity. Tully and Fouqué (1985) discuss the line-width corrections for extending the TF relations to low luminosity systems. Some of the motivation for this was to increase the number of local calibrators available (e.g., Richter and Huchtmeier 1984). However, the TF relations for systems fainter than MB ~ -15.0 become rather poorly defined as the corrections to the line-widths become large and the inclinations become more uncertain. With these factors in mind it is recommended that samples be limited to galaxies brighter than MB ~ -16.0 for which the kinematics are dominated by rotation.