arXiv subjects

André R. da Silva

Publications and source records attributed to André R. da Silva.

5 recordsLinked to original sources

arXiv2026

Evidence for the Typhon Stellar Stream as the Remnant of a Recently-disrupted Dwarf Galaxy

In the hierarchical model of galaxy formation, the Milky Way's stellar halo assembles through the successive accretion of dwarf galaxies. While many resulting substructures have been identified, the origin of the recently discovered Typhon stellar stream remains ambiguous, as dynamical data alone are insufficient to distinguish between a globular cluster and a dwarf galaxy progenitor. To resolve this, we combine astrometry from Gaia with spectroscopic data from available surveys to comprehensively analyze the stream's chemical and dynamical properties. We identify 107 candidate members, significantly expanding the known sample. Three of these candidates lie beyond $55\,\mathrm{kpc}$, with the farthest at $67\,\mathrm{kpc}$, lending further support to Typhon's inferred large apocentric distance. Our analysis yields a mean metallicity of $\rm[Fe/H]=-1.42\pm0.07$ and a metallicity dispersion of $σ_{\text{[Fe/H]}} = 0.26^{+0.06}_{-0.05}$ dex. This metallicity dispersion supports a dwarf-galaxy origin for the Typhon stream. Typhon's high orbital energy is consistent with a relatively recent accretion of a fully disrupted dwarf galaxy with a stellar mass in the range of $10^6 M_{\odot} < M_\star \lesssim 10^7 M_{\odot}$. Furthermore, Typhon is the only major dwarf-galaxy stellar stream in the Milky Way's halo to exhibit such an extreme orbital eccentricity of $e \gtrsim 0.85$. We conjecture that dwarf-galaxy streams from high-eccentricity mergers are fully dissolved much more rapidly than those on circular orbits, in line with Typhon's proposed recent accretion. Additional dwarf-galaxy stellar streams on high-eccentricity orbits should be discoverable with next-generation photometric surveys as well as future releases from the Gaia space mission.

astro-ph.GA↗

arXiv2026

The Metallicity Gradient of Sagittarius Dwarf Spheroidal Galaxy Prior to Infall Constrained by S-PLUS Observations of its Tidal Stream

We study the metallicity distribution along the Sagittarius (Sgr) stream using photometric metallicities from S-PLUS DR4, combined with Gaia DR3 kinematics and APOGEE DR17 spectroscopy. Our analysis confirms that the leading arm (Galactic latitude $b > 0$) is systematically more metal-poor than the trailing arm ($b < 0$) by 0.15--0.20 dex, and reveals a clear negative metallicity gradient along the leading arm. The trailing arm shows no significant overall gradient but displays distinct inner (negative) and outer (positive) trends. These features are consistently recovered across different photometric estimators and agree with spectroscopic data. We compare these results with predictions from an $N$-body simulation, in which metallicities were assigned according to a set of imprinted radial gradients in the progenitor. We were able to constrain the original metallicity gradient of the Sgr progenitor to be between $-0.38$ and $-0.24$ dex kpc$^{-1}$ based on photometric data, and $-0.42$ to $-0.10$ dex kpc$^{-1}$ from APOGEE. These values are consistent with gradients observed in other Local Group dwarf galaxies. Our findings demonstrate that metallicity-sensitive photometric surveys such as S-PLUS are powerful tools for reconstructing the chemodynamical evolution of disrupted satellites.

astro-ph.GA↗

arXiv2022

The CUBES Science Case

We introduce the scientific motivations for the development of the Cassegrain U-Band Efficient Spectrograph (CUBES) that is now in construction for the Very Large Telescope. The assembled cases span a broad range of contemporary topics across Solar System, Galactic and extragalactic astronomy, where observations are limited by the performance of current ground-based spectrographs shortwards of 400nm. A brief background to each case is presented and specific technical requirements on the instrument design that flow-down from each case are identified. These were used as inputs to the CUBES design, that will provide a factor of ten gain in efficiency for astronomical spectroscopy over 300-405nm, at resolving powers of R~24,000 and ~7,000. We include performance estimates that demonstrate the ability of CUBES to observe sources that are up to three magnitudes fainter than currently possible at ground-ultraviolet wavelengths, and we place its predicted performance in the context of existing facillities.

astro-ph.IM↗

arXiv2021

Titans metal-poor reference stars. I. Accurate effective temperatures and surface gravities for dwarfs and subgiants from 3D non-LTE H$α$ profiles and Gaia parallaxes

Several large stellar spectroscopic surveys are producing overwhelming amounts of data that can be used for determining stellar atmospheric parameters and chemical abundances. Nonetheless, the accuracy achieved in the derived astrophysical parameters is still insufficient, mainly because of the paucity of adequate calibrators, particularly in the metal-poor regime ([Fe/H] $\leq -$1.0). Here, we introduce the Titans metal-poor reference stars: a sample of 41 dwarf and subgiant stars with accurate parameters. Effective temperatures (Teff) were derived by fitting observed H$α$ profiles with synthetic lines computed using 3D hydrodynamic NLTE models. Surface gravities (logg) were computed using evolutionary tracks and parallaxes from Gaia EDR3. The same methods recover the Teff values of the Gaia benchmark stars, which are mostly based on interferometric measurements, with a 1$σ$ dispersion of $\pm 50$ K. We assume this to be the accuracy of the H$α$ profiles computed from 3D non-LTE models for metal-poor dwarfs and subgiants. We achieved an internal precision typically between 30-40 K, these errors dominated by instrumental effects. The final total uncertainty for the Teff values of the Titans are thus estimated to be of the order of $1\%$. The typical error for logg is $\leq$ 0.04 dex. In addition, we identified a few members of Gaia-Enceladus, of Sequoia, and of the Helmi stream in our sample. These stars can pave the way for the accurate chemical characterization of these Galactic substructures. Using the Titans as reference, large stellar surveys will be able to improve the internal calibration of their astrophysical parameters. Ultimately, this sample will help users of data from Gaia and large surveys in reaching their goal of redefining our understanding of stars, stellar systems, and the Milky Way.

astro-ph.SR↗

arXiv2020

Chemo-kinematic analysis of metal-poor stars with unsupervised machine learning

Metal-poor stars play an import role in the understanding of Galaxy formation and evolution. Evidence of the early mergers that built up the Galaxy might remain in the distributions of abundances, kinematics, and orbital parameters of the stars. In this work, we report on preliminary results of an on-going chemo-kinematic analysis of a sample of metal-poor ([Fe/H] $\leq$ -1.0) stars observed by the GALAH spectroscopic survey. We explored the chemical and orbital data with unsupervised machine learning (hierarchical clustering, k-means cluster analysis and correlation matrices). Our final goal is to find an optimal way to separate different Galactic stellar populations and stellar groups originating from merging events, such as Gaia-Enceladus and Sequoia.

astro-ph.GA↗