3.4. Future Directions

An intriguing recent development been application of the TF relation to relatively high-redshift galaxies. This has been made possible by the advent of large-aperture telescopes capable of measuring rotation curves out to redshifts of z 1. Vogt et al. (1996) measured rotation curves and magnitudes for nine field galaxies in the redshift range 0.1 z 1 using the Keck 10-meter telescope. They found such objects obey a TF relation similar to that of local objects, with only a modest shift (M_B 0.6 mag) toward brighter magnitudes. This is illustrated in Figure 3, in which the Vogt et al. data are plotted along with the TF relation derived by Pierce & Tully (1992). However, a very different conclusion has been reached by Rix et al. (1996), who combined photometry with fiber-optic spectroscopy of spirals at moderate (z 0.25) redshift. Rix et al. conclude that even at such modest look-back times, spiral galaxies are significantly (~ 1.5 mag) brighter than their local counterparts. If the TF relation is to be applied to problems such as peculiar velocities at high redshift or estimation of q₀, its evolution with redshift will have to be understood. This is an observatonal problem which deserves, and will undoubtedly receive, considerably more attention in the near future.

Figure 3. Rotation velocity versus absolute magnitude for spiral galaxies at a median redshift of ~ 0.5. This figure has been adapted from Vogt et al. (1996).