Ancient relic moderately metal-rich bulge cluster Tonantzintla 2

TL;DR

This paper presents a detailed study of the ancient globular cluster Tonantzintla 2, revealing its age, composition, and implications for the early formation of the Milky Way's bulge, making it a key relic of primordial galaxy assembly.

Contribution

It provides the most detailed characterization of Tonantzintla 2, including age, metallicity, and chemical abundance, establishing it as the oldest bulge globular cluster and constraining early Galactic formation.

Findings

Tonantzintla 2 is approximately 13.58 Gyr old.

Its chemical pattern suggests an in-situ origin.

Star formation in the inner Galaxy began within ~0.2 Gyr of the Big Bang.

Abstract

The assembly history of the Galactic bulge is intimately tied to the formation of the proto-Milky Way, yet reconstructing this early phase is difficult because mergers and secular evolution have erased most of its original structure. Among present-day stellar systems, only globular clusters retain the ancient signatures needed to trace these primordial building blocks. Here we present the most detailed characterization to date of Tonantzintla 2, a prime candidate for a relic of the Milky Way's primordial bulge. It is a moderately metal-rich globular cluster projected onto the bulge that has remained largely unexplored despite its potential to constrain the early formation of the inner Milky Way. We derive its fundamental parameters using proper motion-corrected Hubble Space Telescope WFC3 and ACS photometry. By applying an isochrone fitting to very clean data, we obtain an age of 13.58 Gyr, a reddening E(B-V) = 1.44, a metallicity [M/H]=-0.68, and a heliocentric distance of d = 7.38 kpc. A complementary chemical-abundance analysis of seven member stars from APOGEE high-resolution spectroscopy reveals an enrichment pattern consistent with an in-situ origin. Tonantzintla 2 is among the oldest globular clusters studied in the literature, and the oldest so far analyzed in the Galactic bulge. Its age places a stringent constraint on the onset of the bulge formation, implying that star formation in the inner Galaxy began within ~0.2 Gyr of the Big Bang and that Tonantzintla 2 represents an exceptional relic of the Milky Way's earliest chemical enrichment.