The Identification of Blue Horizontal-Branch Stars in the Integrated Spectra of Globular Clusters

A major uncertainty in the spectroscopic dating of extragalactic globular clusters concerns the degenerate effect that age and horizontal-branch morphology have on the strength of Balmer lines. In this Letter we show that the ratio between the equivalent widths of Hδ F and Hβ is far more sensitive to horizontal-branch morphology than to age, thus making it possible to break the degeneracy. We show that it is possible to distinguish intermediate-age globular clusters from those whose Balmer lines are strengthened by the presence of blue horizontal-branch stars purely on the basis of the clusters' integrated spectra . The degeneracy between age and horizontal-branch morphology can be lifted with Hβ and Hδ F line strengths from spectra with S/N ≳ 30 Å -1 , which is typical of current studies of integrated spectroscopy of extragalactic globular clusters.