arXiv subjects

Stefano Souza

Publications and source records attributed to Stefano Souza.

5 recordsLinked to original sources

arXiv2026

oMEGACat. X. Shedding light on the disrupted dwarf galaxy of Omega Centauri

Omega Centauri ($\omega\,$Cen) is the most massive and chemically complex star cluster in the Milky Way and is widely regarded as the surviving nuclear star cluster of an accreted dwarf galaxy. However, its parent host remains uncertain. Here, we investigate a scenario in which Sequoia, Thamnos, and Gaia--Enceladus (GE) are debris from a single disrupted progenitor, the $\omega\,$Dwarf, whose nucleus survives today as $\omega\,$Cen. Using APOGEE and GALAH abundances together with Gaia astrometry, we reconstruct the chemical structure across this progenitor adopting orbital energy as a proxy for pre-merger radius. We find that the chemically evolved (younger Al-N-He-rich) population is strongly concentrated toward the inner regions, representing a population formed after/during the merger, while the primordial population represents a dwarf-galaxy-like population, supporting a common dwarf-galaxy origin for its components. The metallicity profile shows an inverted U-shaped gradient similar to those observed in present-day nucleated dwarf galaxies. At the same time, the inner regions ($\omega\,$Cen+Thamnos) are more $\alpha$-enhanced than the outskirts, pointing to shorter and more efficient star formation and indicating that the nucleus may have assembled through the merger of inspiraling globular clusters. Neutron-capture abundances reveal a Eu-rich, r-process-dominated outskirts and inner regions enhanced in [Ba/Eu] and [La/Eu], requiring delayed enrichment and more complex chemical evolution. Finally, our analysis shows that Sequoia and Thamnos naturally fit an outside-in stripping sequence around $\omega\,$Cen, whereas the connection with GE remains unsure.

astro-ph.GA↗

arXiv2026

Variations in the Milky Way's Stellar Mass Function at [Fe/H] < -1

We present the first determination of the Galactic stellar mass function (MF) for low-mass stars (0.2-0.5 M_sun) at metallicities [Fe/H] < -1. A sample of ~53,000 stars was selected as metal-poor on the basis of both their halo-like orbits and their spectroscopic [Fe/H] from Gaia DR3 BP/RP (XP) spectra. These metallicity estimates for low-mass stars were enabled by calibrating Gaia XP spectra with stellar parameters from SDSS-V. For -1.5 < [Fe/H] < -1, we find that the MF below 0.5 M_sun exhibits a "bottom-heavy" power-law slope of alpha ~ -1.6. We tentatively find that at even lower metallicities, the MF becomes very bottom-light, with a near-flat power-law slope of alpha ~ 0 that implies a severe deficit of low-mass stars. This metallicity-dependent variation is insensitive to the adopted stellar evolution model. These results show that the Galactic low-mass MF is not universal, with variations in the metal-poor regime. A further calibration of XP metallicities in the regime of M < 0.5 M_sun and [Fe/H] < -1.5 will be essential to verify these tentative low-metallicity trends.

astro-ph.GA↗

arXiv2025

The VISCACHA Survey: XIII. The extended main-sequence turn-off in intermediate-age low-mass clusters

The extended main-sequence turn-off (eMSTO) is a well-known feature observed in young and intermediate-age star clusters, characterized by a significant broadening of the main-sequence turn-off region. Although prolonged star formation and stellar rotation have been proposed as possible explanations, no consensus has yet been reached. Most previous studies have focused on high-mass clusters. In this work, we extend the analysis to the less-explored low-mass regime by investigating star clusters in the Magellanic Clouds using data from the VISCACHA survey. We employed a widely used method to quantify the MSTO width in terms of age spread. Additionally, to validate our approach, we used a cluster also observed with HST. Our analysis confirms that the eMSTO phenomenon is also present in low-mass clusters, following the known age/mass-MSTO width relations. In particular, the less massive cluster in our sample does not show an eMSTO, supporting the proposed link between the eMSTO and the escape velocity of the cluster, providing a new lower limit to the age spread of $88\pm40$ Myr for the presence of the eMSTO. The consistent MSTO width measurements between the VISCACHA and HST photometries confirm the robustness of our method and demonstrate that the age spread determination is independent of the photometric system, showing also the power of ground-based observations to investigate the eMSTO phenomenon.

astro-ph.GA↗

arXiv2025

The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar

Studying the composition and origin of the inner region of our Galaxy -- the "Galactic bulge" -- is crucial for understanding the formation and evolution of the Milky Way and other galaxies. We present new observational constraints based on a sample of around 18,000 stars in the inner Galaxy, combining Gaia DR3 RVS and APOGEE DR17 spectroscopy. Gaia-RVS complements APOGEE by improving sampling of the metallicity, [Fe/H] in the -2.0 to -0.5 dex range. This work marks the first application of Gaia-RVS spectroscopy to the bulge region, enabled by a novel machine learning approach (hybrid-CNN) that derives stellar parameters from intermediate-resolution spectra with precision comparable to APOGEE's infrared data. We performed full orbit integrations using a barred Galactic potential and applied orbital frequency analysis to disentangle the stellar populations in the inner Milky Way. For the first time, traced by the field stars, we are able to robustly identify the long-sought pressure supported bulge. We show this stellar population to be chemically and kinematically distinct from the other main components co-existing in the same region. The spheroidal bulge has a metallicity distribution function (MDF) peak at around -0.70 dex extending to solar value, is dominated by a high-[alpha/Fe] population with almost no dependency on metallicity, consistent with very rapid early formation, predating the thick disc and the bar. We find evidence that the bar has influenced the dynamics of the spheroidal bulge, introducing a mild triaxiality and radial extension. We identify a group of stars on X4 orbits, likely native to the early spheroid, as this population mimics the chemistry of the spheroidal bulge, with a minor contamination from the more metal-poor ([Fe/H] < -1.0) halo. We find the inner-thick disc to be kinematically hotter (mean Vphi ~125 km/s) than the local-thick disc. ...

astro-ph.GA↗

arXiv2025

oMEGACat. VI. Analysis of the overall kinematics of Omega Centauri in 3D: velocity dispersion, kinematic distance, anisotropy, and energy equipartition

Omega Centauri ($ω$ Cen) is the Milky Way's most massive globular cluster and is likely the stripped nucleus of an accreted dwarf galaxy. In this paper, we analyze $ω$ Cen's kinematics using data from oMEGACat, a comprehensive catalog of $ω$ Cen's central regions, including 1.4 million proper motion measurements and 300,000 spectroscopic radial velocities. Our velocity dispersion profiles and kinematic maps are consistent with previous work but improve on their resolution, precision, and spatial coverage. The cluster's 3D dispersion is isotropic in the core, with increasing radial anisotropy at larger radii. The 2D kinematic maps show an elongation of the velocity dispersion field comparable to the flattening observed photometrically. We find good agreement between proper motions and line-of-sight velocity dispersion and measure a kinematic distance of 5494$\pm$61 pc, the most precise kinematic distance to $ω$ Cen available. The subset of data with precise metallicity measurements shows no correlation between metallicity and kinematics, supporting the picture of well-mixed stellar populations within the half-light radius of $ω$ Cen. Finally, we study the degree of energy equipartition using a large range of stellar masses. We find partial energy equipartition in the center that decreases towards large radii. The spatial dependence of the radial energy equipartition is stronger than the tangential energy equipartition. Our kinematic observations can serve as a new reference for future dynamical modeling efforts that will help to further disentangle the complex mass distribution within $ω$ Cen.

astro-ph.GA↗