A Tully-Fisher Relation for S0 Galaxies

We present an I-band Tully-Fisher relation (TFR) for 18 nearby S0 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 random motions through an asymmetric drift correction. Uniform and accurate distance 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. This result is surprising, because S0 galaxies appear to have both the simple kinematics of disk galaxies and the simple stellar populations of early-type galaxies. Remarkably, in this sample of overall rotation-dominated galaxies, the central stellar velocity dispersion is a better predictor of the total I-band luminosity (through the fundamental plane relations) than the circular speed at several exponential scale lengths. Furthermore, the TFR zero point, or the mean stellar I-band luminosity at a given V_c, differs by only 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 scale. This offset is less than expected if S0s are former spiral galaxies with prematurely truncated star formation (≳4 Gyrs ago).