Ancient Relic Moderately Metal-rich Bulge Cluster Tonantzintla 2
(2026) In Astrophysical Journal Letters 998(1).- 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... (More)
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 Advanced Camera for Surveys photometry. By applying an isochrone fitting to very clean data, we obtain an age of 13.58− 1.0+ 0.72 Gyr, a reddening E(B – V) = 1.44 ± 0.02, a metallicity [M/H] = −0.68− 0.05+ 0.04, and a heliocentric distance of d ⊙ = 7.380.08 0.13 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.
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- author
- Ortolani, Sergio ; Souza, Stefano O. ; Nardiello, Domenico ; Barbuy, Beatriz ; Bica, Eduardo ; Ferreira, Bernardo P.L. ; Chiappini, Cristina ; Fernández-Trincado, José G. and Ernandes, Heitor LU
- organization
- publishing date
- 2026-02-10
- type
- Contribution to journal
- publication status
- published
- subject
- in
- Astrophysical Journal Letters
- volume
- 998
- issue
- 1
- article number
- L8
- publisher
- IOP Publishing
- external identifiers
-
- scopus:105033772148
- ISSN
- 2041-8205
- DOI
- 10.3847/2041-8213/ae3b3e
- language
- English
- LU publication?
- yes
- id
- b13f463e-df44-4b33-ae6d-9d050d3cc1ca
- date added to LUP
- 2026-06-11 13:54:54
- date last changed
- 2026-06-11 13:55:22
@article{b13f463e-df44-4b33-ae6d-9d050d3cc1ca,
abstract = {{<p>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 Advanced Camera for Surveys photometry. By applying an isochrone fitting to very clean data, we obtain an age of 13.58<sub>− 1.0</sub><sup>+ 0.72</sup> Gyr, a reddening E(B – V) = 1.44 ± 0.02, a metallicity [M/H] = −0.68<sub>− 0.05</sub><sup>+ 0.04</sup>, and a heliocentric distance of d <sub>⊙</sub> = 7.380.08 0.13 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.</p>}},
author = {{Ortolani, Sergio and Souza, Stefano O. and Nardiello, Domenico and Barbuy, Beatriz and Bica, Eduardo and Ferreira, Bernardo P.L. and Chiappini, Cristina and Fernández-Trincado, José G. and Ernandes, Heitor}},
issn = {{2041-8205}},
language = {{eng}},
month = {{02}},
number = {{1}},
publisher = {{IOP Publishing}},
series = {{Astrophysical Journal Letters}},
title = {{Ancient Relic Moderately Metal-rich Bulge Cluster Tonantzintla 2}},
url = {{http://dx.doi.org/10.3847/2041-8213/ae3b3e}},
doi = {{10.3847/2041-8213/ae3b3e}},
volume = {{998}},
year = {{2026}},
}