Integrated Spectroscopy of Bulge Globular Clusters and Fields I. The Data Base and Comparison of Individual Lick Indices in Clusters and Bulge

TL;DR

This study provides a comprehensive spectroscopic comparison of metal-rich globular clusters and the Galactic bulge, revealing similarities and differences in their stellar populations through Lick indices, and calibrates metallicity-sensitive indices.

Contribution

It offers the first empirical calibration of metallicity-sensitive indices as a function of [Fe/H] using globular cluster data, and compares integrated light indices of clusters and bulge.

Findings

Metal-rich globular clusters and bulge are similar in most Lick indices except for CN.

The <Fe> indices of NGC 6528 and NGC 6553 are comparable to the bulge.

Indices like Mgb/<Fe> and Mg2/<Fe> show similar element enhancements in clusters and bulge.

Abstract

We present a comprehensive spectroscopic study of the integrated light of metal-rich Galactic globular clusters and the stellar population in the Galactic bulge. We measure line indices which are defined by the Lick standard system and compare index strengths of the clusters and Galactic bulge. Both metal-rich globular clusters and the bulge are similar in most of the indices, except for the CN index. We find a significant enhancement in the CN/<Fe> index ratio in metal-rich globular clusters compared with the Galactic bulge. The mean iron index <Fe> of the two metal-rich globular clusters NGC 6528 and NGC 6553 is comparable with the mean iron index of the bulge. Index ratios such as Mgb/<Fe>, Mg2/<Fe>, Ca4227/<Fe>, and TiO/<Fe>, are comparable in both stellar population indicating similar enhancements in individual elements which are traced by the indices. From the globular cluster data we fully empirically calibrate several metallicity-sensitive indices as a function of [Fe/H] and find tightest correlations for the Mg2 index and the composite [MgFe] index. We find that all indices show a similar behavior with galactocentric radius, except for the Balmer series, which show a large scatter at all radii. However, the scatter is entirely consistent with the cluster-to-cluster variations in the horizontal branch morphology.