Presence Of Blue Horizontal Branch Stars Research Articles

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 HδF 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 HδF line strengths from spectra with S/N 30 A-1, which is typical of current studies of integrated spectroscopy of extragalactic globular clusters.

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. M31 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.

The Hβ Index As a Chronometer of Extragalactic Globular Cluster Systems

For the first time, we have taken into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity in our population synthesis models and they result that the integrated Hβ index is significantly affected by the presence of blue HB stars. As a matter of fact, due to the systematic HB morphology variation, it is found that Hβ does not monotonically decrease as metallicity increases at given ages, but shows a kind of wavy feature. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the Hβ vs. Mg2 plane is explained if globular cluster systems in giant elliptical galaxies are a couple of billion years older, in the mean, than the Galactic counterpart.

The Hβ Index as an Age Indicator of Old Stellar Systems: The Effects of Horizontal-Branch Stars

The strength of the H? index is computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. For the first time, these models take into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the H? index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the H? does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature at which the H? index becomes strongest. Our models indicate that the strength of the H? index increases as much as 0.75 ? because of the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way with those in the giant elliptical galaxies NGC 1399 and M87 shows a systematic shift in the H?-metallicity plane. Our models suggest that this systematic difference is explained if the mean age of globular cluster systems in giant elliptical galaxies is several billion years older than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable us to clarify whether this difference is indeed due to the age difference or whether other explanations are also possible.