Ancient relic moderately metal-rich bulge cluster Tonantzintla 2

TL;DR

This work presents a detailed characterization of Tonantzintla 2, a prime relic of the Milky Way's primordial bulge. By combining proper-motion-cleaned HST CMDs with isochrone fitting on multiple photometric bands and a complementary high-resolution APOGEE abundance analysis of seven members, the authors derive $t_{age}=13.58^{+0.72}_{-1.00}$ Gyr, $[M/H]=-0.68^{+0.04}_{-0.08}$, $d_igodot=7.38^{+0.13}_{-0.08}$ kpc, and $E(B-V)=1.44\pm0.02$, with $R_V=2.9\pm0.1$. The orbit is confined to the bulge, and chemical signatures (enhanced $\alpha$-elements, high $\mathrm{Al}$, $\mathrm{Ni}$, $\mathrm{N}$, $\mathrm{O}$, and Mn) indicate an in situ bulge origin, consistent with Ton 2 tracing the early assembly of the inner Galaxy. Ton 2 sits at the peak metallicity of the primordial bulge MDF ($[Fe/H]\approx-0.7$) and is among the oldest bulge clusters, providing direct constraints on the onset of bulge formation within ~0.2 Gyr after the Big Bang. Overall, the paper links Ton 2 to the proto-bulge's earliest chemical enrichment, offering a local fossil record of Milky Way assembly.

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.

Figures (5)

• Figure 1: Ton 2 F160W vs. F606W-F160W and F160W vs. F110W-F160W CMDs (right panels), and corner plot of the parameters (left panels). A best fit was found for age of 13.56$^{+0.85}_{-0.75}$ Gyr, [M/H]=-0.69$^{+0.05}_{-0.04}$, d$_{\odot}$ = 7.39$^{+0.09}_{-0.19}$ kpc, and E(B-V)=1.44$\pm$0.02.
• Figure 2: Age vs. metallicity plane for bulge-volume clusters, highlighting the location of Ton 2. The values are from bica24ortolani25massari25 . The green line represents the age of the universe of 13.801$\pm$0.024 Gyr according to planck20, with width showing the uncertainty.
• Figure 3: [Mg/Mn] vs. [Al/Fe] (upper panel) and [Ni/Fe] vs. [(C+N)/O]. Symbols: black-filled stars: star-members of Ton 2; brown lines: Heracles; dark blue lines: GSE.
• Figure 4: BaSTI isochrones of 13.5 and 12.5 Gyr, fitted with DSED isochrones, and corresponding age.
• Figure 5: Fit to Mn lines for star Ton2: 2M17361421-383431.