Eduardo Bica

The distinct stellar populations of the Galactic bulge can be disentangled through detailed analysis of their chemical abundances and kinematical properties. Recent studies have suggested that globular clusters (GCs) located in the Galactic bulge with metallicities around [Fe/H] ≈ −0.7 may represent some of the oldest systems in the Milky Way, potentially tracing the early spheroidal bulge. The coincidence of a metallicity peak at [Fe/H] ≈ −0.7 in both field stars and GCs, together with the presence of phosphorus-rich (P-rich) stars, may provide important clues to the nature of the first generations of stars formed in the Galaxy. In this work, we investigate the odd- Z elements Na, Al, and particularly P in the bulge GCs NGC 6539 ([Fe/H] ∼ −0.75) and NGC 6569 ([Fe/H] ∼ −0.85) using APOGEE spectra. We also examine the clusters Tonantzintla-1 and NGC 6316, which exhibit evidence of phosphorus enhancement. Our analysis confirms that NGC 6539 is a cluster of interest, with one clearly P-rich star, whereas NGC 6569 shows a lower level of P enhancement. This again suggests that there might have been an early bulge building block with the metallicity of [Fe/H] ∼ −0.75, which NGC 6539 would be part of. The observed abundance patterns indicate that the production of Na and Al is consistent with nucleosynthesis in massive stars. However, the origin of the phosphorus enrichment remains uncertain, suggesting that additional nucleosynthetic channels may be required to explain the observed abundances. These findings provide new constraints on the chemical evolution of the Galactic bulge and the nature of its earliest stellar populations.

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 _⊙ = 7.38 0.08 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.

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.

Context. In the Large Magellanic Cloud (LMC), only seven star clusters have been discovered to be older than ∼4 Gyr and younger than ∼10 Gyr, placing them in what is known as the age gap. Aims. We aim to analyze the photometric data from the VISCACHA survey in the V and I bands to determine, for the first time, the astrophysical parameters of SL 2, revealing that the cluster is indeed situated within the age gap. Methods. We used our newly developed SIESTA code to carry out a statistical isochrone fitting with synthetic color-magnitude diagrams (CMDs) to determine the cluster age, metallicity, distance, color excess, and binary fraction with two grids of stellar evolution models. In addition, the cluster mass was estimated based on its integrated magnitude. Results. The ages obtained from isochrone fitting are compatible with the age gap, amounting to (7.17 ± 0.35) Gyr when using PARSEC-COLIBRI isochrones and (8.02 ± 0.45) Gyr when using MIST. Notably, SL 2 is the first age gap cluster discovered in the southern region of the LMC. The mass of the cluster is considerably smaller than that of the group of older LMC clusters. Conclusions. SL 2 has a comparable metallicity to the other two age gap clusters with similar ages, namely, ESO 121-03 and KMHK 1592, as well as the LMC field star population. While the discovery of a new cluster with such characteristics could be seen as evidence that age gap clusters were formed in situ, the heliocentric distance of SL2 locates it far from the LMC center, akin to the SMC distance. Therefore, the question of its origin, alongside that of other age gap clusters, remains unresolved and open to further investigation.

Context. The ESO public survey VISTA Variables in the Vía Láctea (VVV) surveyed the inner Galactic bulge and the adjacent southern Galactic disk from 2009–2015. Upon its conclusion, the complementary VVV extended (VVVX) survey has expanded both the temporal as well as spatial coverage of the original VVV area, widening it from 562 to 1700 sq. deg., as well as providing additional epochs in JHK_s filters from 2016–2023. Aims. With the completion of VVVX observations during the first semester of 2023, we present here the observing strategy, a description of data quality and access, and the legacy of VVVX. Methods. VVVX took ~2000 h, covering about 4% of the sky in the bulge and southern disk. VVVX covered most of the gaps left between the VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV time baseline in the obscured regions affected by high extinction and hence hidden from optical observations. Results. VVVX provides a deep JHK_s catalogue of ≳1.5 × 10⁹ point sources, as well as a K_s band catalogue of ~10⁷ variable sources. Within the existing VVV area, we produced a 5D map of the surveyed region by combining positions, distances, and proper motions of well-understood distance indicators such as red clump stars, RR Lyrae, and Cepheid variables. Conclusions. In March 2023 we successfully finished the VVVX survey observations that started in 2016, an accomplishment for ESO Paranal Observatory upon 4200 h of observations for VVV+VVVX. The VVV+VVVX catalogues complement those from the Gaia mission at low Galactic latitudes and provide spectroscopic targets for the forthcoming ESO high-multiplex spectrographs MOONS and 4MOST.

Context. The number of known globular clusters in the Galactic bulge has been increasing steadily thanks to different new surveys. Aims. The aim of this study is to provide a census of the newly revealed globular clusters in the Galactic bulge, and analyze their characteristics. Methods. In recent years, many globular clusters have been discovered or identified. The stellar populations to which they belong are indicated in their original studies: they are mostly bulge clusters, with some identified as disk or halo members. We collected 41 new globular clusters revealed in the last decade and compared them to the known bulge clusters. Results. The new clusters are intrinsically faint with M_V of around −6.0 mag. The distance to the Sun of the ensemble of well-known and new bulge clusters is compatible with the Galactocentric distance measurements from the Galactic black hole location. The ensemble sample shows metallicity peaks at [Fe/H] ∼ −1.08 ± 0.35 and −0.51 ± 0.25 dex, confirming previous findings. The age–metallicity relation of the new clusters younger than 10 Gyr is compatible with that of the ex situ samples of the dwarf galaxies Sagittarius, Canis Majoris, and Gaia-Enceladus-Sausage. The clusters with ages between 11.5 and 13.5 Gyr show no age–metallicity relation, because they are all old. This is compatible with their formation in situ in the early Galaxy.

We present the analysis of three faint clusters of the Small Magellanic Cloud RZ 82, HW 42, and RZ 158. We employed the SOAR telescope instrument SAM with adaptive optics, allowing us to reach to V ∼ 23–24 mag, unprecedentedly, a depth sufficient to measure ages of up to about 10–12 Gyr. All three clusters are resolved to their centres, and the resulting colour–magnitude diagrams (CMDs) allow us to derive ages of 3.9, 2.6, and 4.8 Gyr, respectively. These results are significantly younger than previous determinations (7.1, 5.0, and 8.3 Gyr, respectively), based on integrated photometry or shallower CMDs. We rule out older ages for these clusters based on deep photometry and statistical isochrone fitting. We also estimate metallicities for the three clusters of |$\rm {[Fe/H]}=-0.68$|⁠, −0.57, and −0.90, respectively. These updated ages and metallicities are in good agreement with the age-metallicity relation for the bulk of SMC clusters. Total cluster masses ranging from |$\sim 7\!-\!11\cdot 10^3\, \mathit{ M}_\odot$| were estimated from integrated flux, consistent with masses estimated for other SMC clusters of similar ages. These results reduce the number of SMC clusters known to be older than about 5 Gyr and highlight the need of deep and spatially resolved photometry to determine accurate ages for older low-luminosity SMC star clusters.

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In the present work we analyzed seven globular clusters (GCs) selected from their location in the Galactic bulge and with metallicity values in the range −1.30 ≲ [Fe/H] ≲ −0.50. The aim of this work is first to derive cluster ages assuming single stellar populations and second to identify the stars from first (1G) and second generations (2G) from the main sequence, subgiant, and red giant branches, and to derive their age differences. Based on a combination of UV and optical filters used in this project, we apply the Gaussian mixture models to distinguish the multiple stellar populations. Applying statistical isochrone fitting, we derive self-consistent ages, distances, metallicities, and reddening values for the sample clusters. An average age of 12.3 ± 0.4 Gyr was obtained both using DSED and BaSTI (accounting atomic diffusion effects) isochrones, without a clear distinction between the moderately metal-poor and the more metal-rich bulge clusters, except for NGC 6717 and the inner halo NGC 6362 with ∼13.5 Gyr. We derived a weighted mean age difference between the multiple populations hosted by each GC of 41 ± 170 Myr adopting canonical He abundances; whereas for higher He in 2G stars, this difference reduces to 17 ± 170 Myr, but with individual uncertainties of 500 Myr.