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

A major uncertainty in the spectroscopic dating of extragalactic globularclusters concerns the degenerate effect that age and horizontal branchmorphology have on the strength of Balmer lines. In this Letter we show thatthe ratio between the equivalent widths of Hdelta and Hbeta is far moresensitive to horizontal branch morphology than to age, thus making it possibleto break the degeneracy. We show that it is possible to distinguishintermediate-age globular clusters from those whose Balmer lines arestrengthened by the presence of blue horizontal branch stars, purely on thebasis of the clusters' integrated spectra. The degeneracy between age andhorizontal branch morphology can be lifted with Hbeta and Hdelta line strengthsfrom spectra with S/N >= 30 per Angstrom, which is typical of current studiesof integrated spectroscopy of extragalactic globular clusters.Comment: 4 pages. To appear in the Astrophysical Journal Letter