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

Thomas H. Puzia, R. P. Saglia+5 more|ArXiv.org|Sep 12, 2002

Stellar, planetary, and galactic studies70 references73 citations

TL;DR

This study presents the first comprehensive measurement of Lick indices in metal-rich Galactic bulge globular clusters, establishing empirical calibrations of metallicity-sensitive indices (e.g., Mg2, [MgFe]) against [Fe/H] from −2.29 to −0.17 dex. Key findings show that while most indices (e.g., Mgb/⟨Fe⟩, Ca4227/⟨Fe⟩) are similar between clusters and the bulge, the CN/⟨Fe⟩ ratio is significantly enhanced in clusters, suggesting cluster-specific pollution from AGB stars.

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/ index ratio in metal-rich globular clusters compared with the Galactic bulge. The mean iron index 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/, Mg2/, Ca4227/, and TiO/, 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.

Motivation & Objective

• To measure and compare Lick indices in metal-rich globular clusters and the Galactic bulge for the first time.
• To establish empirical calibrations of Lick indices against [Fe/H] for old stellar populations across a wide metallicity range.
• To investigate differences in integrated light between metal-rich clusters and the bulge, particularly in α-element and CN indices.
• To assess the role of cluster-specific processes (e.g., AGB pollution) in shaping index variations.
• To provide a benchmark dataset for testing population synthesis models and estimating ages/abundances in unresolved systems.

Proposed method

• Obtained integrated spectroscopy of metal-rich globular clusters (NGC 5927, 6528, 6553) and bulge fields using the Lick standard system.
• Measured 15 Lick line indices, including Fe, Mg, Ca, TiO, and CN indices, from high signal-to-noise spectra.
• Calibrated metallicity-sensitive indices (e.g., Mg2, [MgFe]) against [Fe/H] using cluster data with independently measured metallicities.
• Compared index ratios (e.g., Mgb/⟨Fe⟩, Ca4227/⟨Fe⟩) between clusters and the bulge to assess α-element enhancements.
• Analyzed radial trends in indices, particularly Hβ, to link scatter to horizontal branch morphology.
• Used Spearman rank correlation to quantify index-metallicity relationships and assess reliability of indicators.

Experimental results

Research questions

• RQ1 How do the Lick indices of metal-rich globular clusters compare with those of the Galactic bulge field?
• RQ2 To what extent do metal-rich clusters and the bulge share similar α-element enhancements, as traced by index ratios?
• RQ3 Why is the CN/⟨Fe⟩ index ratio significantly higher in clusters than in the bulge?
• RQ4 How well can Lick indices be calibrated against [Fe/H] in old stellar populations spanning from metal-poor to near-solar metallicities?
• RQ5 What causes the observed scatter in the Hβ index as a function of galactocentric radius in clusters?

Key findings

• The Mg2 and [MgFe] indices show the tightest correlation with [Fe/H], with a root-mean-square scatter of 0.15 dex, making them the most reliable metallicity indicators in the sample.
• The mean ⟨Fe⟩ index of NGC 6528 and NGC 6553 is indistinguishable from that of the Galactic bulge, indicating similar overall [Fe/H].
• Index ratios such as Mgb/⟨Fe⟩, Mg2/⟨Fe⟩, Ca4227/⟨Fe⟩, and TiO/⟨Fe⟩ are consistent between clusters and the bulge, implying similar α-element enhancements.
• The CN/⟨Fe⟩ index ratio is significantly higher in metal-rich clusters than in the bulge, suggesting cluster-specific enrichment processes.
• The Hβ index shows large scatter with galactocentric radius, which is fully explained by variations in horizontal branch morphology across clusters.
• All indices except Balmer lines show a monotonic decrease with galactocentric radius and stabilize beyond ~10 kpc, indicating a radial gradient in stellar population properties.