Chrono-chemodynamical analysis of the globular cluster NGC 6355: Looking for the fundamental bricks of the Bulge

TL;DR

This study analyzes the chemodynamical properties of globular cluster NGC 6355 to understand its origin and the early formation of the Galactic bulge, combining spectroscopy, photometry, and dynamical modeling.

Contribution

It provides a detailed chemodynamical analysis of NGC 6355, offering insights into its in-situ formation and its role in the early Galactic bulge evolution.

Findings

NGC 6355 is approximately 13.2 Gyr old with [Fe/H] ≈ -1.39.

The cluster's chemical pattern aligns with bulge field RR Lyrae stars and in-situ globular clusters.

Orbital data suggest NGC 6355 is confined within the bulge and likely formed in the main bulge progenitor.

Abstract

The information on Galactic assembly time is imprinted on the chemodynamics of globular clusters. This makes them important probes that help us to understand the formation and evolution of the Milky Way. Discerning between in-situ and ex-situ origin of these objects is difficult when we study the Galactic bulge, which is the most complex and mixed component of the Milky Way. To investigate the early evolution of the Galactic bulge, we analysed the globular cluster NGC 6355. We derived chemical abundances and kinematic and dynamic properties by gathering information from high-resolution spectroscopy with FLAMES-UVES, photometry with the Hubble Space Telescope, and Galactic dynamic calculations applied to the globular cluster NGC 6355. We derive an age of $13.2\pm1.1$ Gyr and a metallicity of [Fe/H]$=-1.39\pm0.08$ for NGC 6355, with $\alpha$-enhancement of [$\alpha$/Fe]$=+0.37\pm0.11$. The abundance pattern of the globular cluster is compatible with bulge field RR Lyrae stars and in-situ well-studied globular clusters. The orbital parameters suggest that the cluster is currently confined within the bulge volume when we consider a heliocentric distance of $8.54\pm0.19$ kpc and an extinction coefficient of $R_V = 2.84\pm0.02$. NGC 6355 is highly likely to come from the main bulge progenitor. {Nevertheless, it still} has a low probability of being formed from an accreted event because its age is uncertain and because of the combined [Mg/Mn] [Al/Fe] abundance. Its relatively low metallicity with respect to old and moderately metal-poor inner Galaxy clusters may suggest a low-metallicity floor for globular clusters that formed in-situ in the early Galactic bulge.