Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations

Theoretical integrated broadband colors ranging from far-UV to near-IR have been computed for old stellar systems from our evolutionary population synthesis code. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that some temperature-sensitive color indices are significantly affected by the presence of blue HB stars. In particular, B-V does not become monotonically redder as metallicity increases at given ages, but becomes bluer by as much as ∼0.15 mag because of the contribution from blue HB stars. Similar trends are also found in the Washington photometric system. In addition to appropriate age-sensitive spectrophotometric indices, the use of far-UV to optical colors is proposed as a powerful age diagnostic for old stellar systems with differing HB morphologies. Our models are calibrated in the B-V, V-I, C-T1, and M-T1 versus [Fe/H] planes, using low-reddened Galactic globular clusters (GCs) [E(B- V) < 0.2], and the relative age difference between the older inner halo Galactic GCs and younger outer halo counterparts is well reproduced. Several empirical linear color-metallicity transformation relations are assessed with our models, and it is noted that they may not be safely used to estimate metallicity if there are sizable age differences among GCs within and between galaxies. M3I GCs are found to be fundamentally similar to those in the Milky Way, not only in the optical to near-IR range, but also in the UV range. For globular cluster systems in two nearby giant ellipticals, M87 and NGC 1399, the current available photometric data in the literature do not appear sufficient to provide robust age discrimination. It is anticipated, however, that the detailed population models presented here, coupled with further precise spectrophotometric observations of globular cluster systems in external galaxies from the large ground-based telescopes and space UV facilities, will enable us to accurately estimate their ages and metallicities.