A Tully-Fisher relation for S0 galaxies

We present an I-band Tully-Fisher relation (TFR) for 18 nearby SO galaxies using kinematics derived from long-slit spectroscopy of stellar absorption lines. Our estimates of the circular velocity, V-c, at 2-3 exponential disk scale lengths account for line-of-sight projection and for the stellar ran dom motions through an asymmetric drift correction. Uniform and accurate di stance calibration is available from recent surface brightness fluctuation measurements. Despite the care taken in estimating both V-c and M-I the TFR shows an intrinsic scatter of about 0.7 mag in M-I or 0.15 in log V-c. Thi s result is surprising, because S0 galaxies appear to have both the simple kinematics of disk galaxies and the simple stellar populations of early-typ e galaxies. Remarkably, in this sample of overall rotation-dominated galaxi es, the central stellar velocity dispersion is a better predictor of the to tal I-band luminosity (through the fundamental plane relations) than the ci rcular speed at several exponential scale lengths. Furthermore, the TFR zer o point, or the mean stellar I-band luminosity at a given V-c differs by on ly about 0.5 mag between our sample of S0s and a comparison sample of late- type spirals, once both data sets are brought onto a consistent distance sc ale. This offset is less than expected if S0s are former spiral galaxies wi th prematurely truncated star formation (greater than or similar to 4 Gyrs ago).