The formation of S0 galaxies:: evidence from globular clusters

Aims. We devise a simple experiment to test the theory that lenticular (S0) galaxies form from spirals whose star formation has been shut down. An individual galaxy's fading is measured using the globular cluster specific frequency (S (N)), defined as the number of globular clusters normalised by the galaxy luminosity. This is compared with a spectroscopically-derived age estimate. Methods. We make NTT/EMMI long-slit spectroscopic observations of 11 S0 galaxies at z < 0.006. We measure the absorption-line indices, H delta, H gamma, Mgb, Fe5270 and Fe5335 within the central r(e)/8. By inverting single- stellar population models, luminosity-weighted mean ages, metallicities and alpha-element abundance ratios are derived. We estimate the amount of fading a galaxy has undergone by comparing each galaxy's S (N) with its deviation from the mean spiral S (N). Results. Galaxies with higher S (N) have older stellar populations. Moreover, we find that the zero-point and amount of fading is consistent with a scenario where lenticulars are formed by the quenching of star formation in spiral galaxies. Our data also rule out any formation method for S0s which creates a large number of new globular clusters. We confirm that previous results showing a relationship between S (N) and color are driven by the S (N)-Age relation. Five galaxies show detectable H beta, [O III], H alpha or [N II] emission lines. However, only in the two youngest galaxies is this emission unambiguously from star formation. Conclusions. Our results are consistent with the theory that S0 galaxies are formed when gas in normal spirals is removed, possibly as a result of a change in environment. The on-going star formation in the youngest galaxies hints that the timescale of quenching is less than or similar to 1 Gyr. We speculate, therefore, that the truncation of star formation is a rather gentle process unlikely to involve a rapid burst of star formation.