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We test the hypothesis that S0 galaxies are the descendants of fading spiralswhose star formation has been shut down, by using the properties of theirglobular cluster (GC) systems. We estimate the amount by which the GC specificfrequency (number of GCs per unit V-band luminosity) is enhanced in S0srelative to spirals. If the transformation hypothesis is correct, and no GCsare created or destroyed in the process, then this difference provides ameasure of the degree to which the S0's V-band luminosity has faded relative tothat of its spiral progenitor. We also explore whether the degree to which theGC specific frequency is enhanced in S0s correlates with the colour of thestellar population, as also predicted by this hypothesis in which galaxiesbecome redder as they fade. We find that, on average, the GC specific frequencyis a factor ~3 larger for S0s than for spirals, which can be interpreted asmeaning that passively-evolving S0s have faded by about a factor of 3 fromtheir spiral progenitors. This value fits remarkably well with the predictionsof stellar population synthesis calculations, and the offset between the S0 andspiral Tully-Fisher relations. We also find that the global colours of S0galaxies are strongly correlated with their GC specific frequencies: the redderthe stellar population of an S0, the larger its specific frequency, as we mightexpect if we are catching different S0s at different stages of passively fadingand reddening. Comparison to the predictions of stellar population synthesismodels show that this explanation works quantitatively as well asqualitatively. These tests strongly support the hypothesis that S0 galaxieswere once normal spirals, whose star formation was cut off, presumably due to achange of environment.Comment: 7 pages, 2 figures, accepted for publication in A&

Measuring the fading of S0 galaxies using globular clusters