Template rotation curves for disk galaxies

A homogeneous sample of similar to 2200 low-redshift disk galaxies with both high sensitivity long-slit optical spectroscopy and detailed I-band photometry is used to construct average, or template, rotation curves in separate luminosity classes, spanning six magnitudes in I-band luminosity. The template rotation curves are expressed as functions both of exponential disk scale lengths r(d) and of optical radii R-opt, and extend out to 4.5r(d) - 6.5r(d), depending on the luminosity bin. The two parameterizations yield slightly different results beyond R-opt, because galaxies whose H alpha emission can be traced to larger extents in the disks are typically of higher optical surface brightness and are characterized by larger values of R-opt/r(d). By either parameterization, these template rotation curves show no convincing evidence of velocity decline within the spatial scales over which they are sampled, even in the case of the most luminous systems. In contrast to some previous expectations, the fastest rotators (most luminous galaxies) have, on average, rotation curves that are flat or mildly rising beyond the optical radius, implying that the dark matter halo makes an important contribution to the kinematics also in these systems. The template rotation curves and the derived functional fits provide quantitative constraints for studies of the structure and evolution of disk galaxies, which aim at reproducing the internal kinematics properties of disks at the present cosmological epoch.