Age Estimation of Extragalactic Globular Cluster Systems Using H beta Index

Abstract

After taking into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity, our population synthesis models show that the integrated H beta index is significantly affected by the presence of blue HB stars. Our models indicate that the strength of the H beta index increases as much as 0.75 A due to blue HB stars. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the H beta vs. Mg2 diagram is understood if globular cluster systems in giant elliptical galaxies are couple of billion years older, in the mean, than the Galactic counterpart.