Measuring the fading of S0 galaxies using globular clusters

Aims. We test the hypothesis that S0 galaxies are the descendants of fading spirals whose star formation has been shut down, by using the properties of their globular cluster systems. Methods. We estimate the amount by which the globular cluster specific frequency (number of globular clusters per unit V -band luminosity) is enhanced in S0s relative to spirals. If the transformation hypothesis is correct, and no clusters are created or destroyed in the process, then this difference provides a measure of the degree to which the S0's V -band luminosity has faded relative to that of its spiral progenitor, which we can compare with the independent values estimated from stellar population synthesis and the S0 Tully–Fisher relation. We also explore whether the degree to which the globular cluster specific frequency is enhanced in S0s correlates with the colour of the stellar population, as also predicted by this hypothesis in which galaxies become redder as they fade. Results. We find that, on average, the globular cluster specific frequency is a factor ∼ 3 larger for S0s than for spirals, which can be interpreted as meaning that passively-evolving S0s have faded on average by about a factor of three from their spiral progenitors. This value fits remarkably well with the predictions of stellar population synthesis calculations, and the offset between the S0 and spiral Tully–Fisher relations, where the S0 V -band relation lies ∼ 1.2 mag, or a factor of three, below the spiral relation. We also find that the global colours of S0 galaxies are strongly correlated with their globular cluster specific frequencies: the redder the stellar population of an S0, the larger its specific frequency, as we might expect if we are catching different S0s at different stages of passively fading and reddening. Comparison to the predictions of stellar population synthesis models show that this explanation works quantitatively as well as qualitatively. Conclusions. These tests strongly support the hypothesis that S0 galaxies were once normal spirals, whose star formation was cut off, presumably due to a change of environment. We are now in a position to start to make quantitative measurements of when this life-changing event occurred in different galaxies.