Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5

TL;DR

This study provides the largest spectroscopic analysis of Terzan 5, revealing a complex, multi-modal metallicity distribution that challenges its classification as a simple globular cluster.

Contribution

It offers the first comprehensive metallicity distribution of Terzan 5 based on 135 stars, demonstrating its complex stellar population and distinct from typical globular clusters.

Findings

Three peaks in metallicity distribution: super-solar, sub-solar, and metal-poor populations.

Terzan 5 is a remnant of a complex stellar system, not a typical globular cluster.

Large spectroscopic sample covering entire radial extent of the system.

Abstract

We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this dataset, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at [Fe/H]$\simeq0.25$ dex, accounting for 29% of the sample, a dominant sub-solar population at [Fe/H]$\simeq-0.30$ dex, corresponding to 62% of the total, and a minor (6%) metal-poor component at [Fe/H]$\simeq-0.8$ dex. Such a broad, multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.