Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)

TL;DR

This study combines infrared photometry, proper motions, and calcium triplet spectroscopy to analyze the properties of globular cluster M 28, confirming its old age, chemical homogeneity, and tidal mass loss, suggesting it may be a remnant of a primordial bulge building block.

Contribution

It provides a comprehensive multi-method analysis of M 28, confirming its age, metallicity uniformity, and tidal disruption, and proposes it as a potential remnant of early bulge formation.

Findings

M 28 is one of the oldest bulge globular clusters (13-14 Gyr).

No significant internal metallicity variation (Delta[Fe/H]<0.08 dex).

Evidence of tidal mass loss into the bulge field.

Abstract

Recent studies show that the inner Galactic regions host genuine bulge globular clusters, but also halo intruders, complex remnants of primordial building blocks, and objects likely accreted during major merging events. In this study we focus on the properties of M 28, a very old and massive cluster currently located in the Galactic bulge. We analysed wide-field infrared photometry collected by the VVV survey, VVV proper motions, and intermediate-resolution spectra in the calcium triplet range for 113 targets in the cluster area. Our results in general confirm previous estimates of the cluster properties available in the literature. We find no evidence of differences in metallicity between cluster stars, setting an upper limit of Delta[Fe/H]<0.08 dex to any internal inhomogeneity. We confirm that M 28 is one of the oldest objects in the Galactic bulge (13-14 Gyr). From this result and the literature data, we find evidence of a weak age-metallicity relation among bulge globular clusters that suggests formation and chemical enrichment. In addition, wide-field density maps show that M 28 is tidally stressed and that it is losing mass into the general bulge field. Our study indicates that M 28 is a genuine bulge globular cluster, but its very old age and its mass loss suggest that this cluster could be the remnant of a larger structure, possibly a primeval bulge building block.