The metal-poorest tail of the Galactic halo: hypothesis on its origin from precise spectral analysis

Abstract

The origin of the Galactic halo is one of the fundamental topics linking the study of galaxy formation and evolution to cosmology. We aim at deriving precise and accurate stellar parameters, Mg abundances, and ages for a sample of metal-poor stars with [Fe/H] $<$ -2 dex from high signal-to-noise and high resolution spectra. We derive effective temperatures from H$\alpha$ profiles using three-dimensional non local thermodynamic equilibrium (3D NLTE) models, and surface gravities and ages from isochrone fitting based on Gaia data. Iron abundances were derived in one-dimensional (1D) NLTE, while Mg abundances were derived in 1D LTE, 1D NLTE, 3D LTE, and 3D NLTE to show the increasing level of accuracy. The stars show a tight trend in the [Mg/Fe] vs [Fe/H] plane with a knee at [Fe/H]$\sim$ -2.8 dex, which indicates a low level of stochasticity. Their location in the Lindblad diagram confirms their belonging to the Galactic halo, but does not show a distinct clustering that might be expected for a merger with a single low-mass galaxy. Comparison with chemical evolution models is also not definitive on whether the sample stars were born in-situ or in accreted low-mass galaxy mergers. We find two plausible explanations for the chemical sequence traced by the stars in the [Mg/Fe] vs [Fe/H] plane. One is that the sample stars originated in the already formed Milky Way, which at that time (12.5 Gyr ago) was already the main galaxy of its Local Group surroundings. Another one is that the sample stars originated in several small galaxies with similar properties, which later merged with the Galaxy. Only accurate spectroscopic analysis such as that done here can reveal trustworthy chemical diagrams required to observe the traces of the Galaxy evolution. Other elements are required to discern between the two hypotheses.

Accepted in A&A

References in corpus (42)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The mass distribution and gravitational potential of the Milky Way
• Galactic chemical evolution: Carbon through Zinc
• The GALAH Survey: Scientific Motivation
• Nucleosynthetic signatures of the first stars
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• First Stars VIII -- Enrichment of the neutron-capture elements in the early Galaxy
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• HE 0557-4840 - Ultra-Metal-Poor and Carbon-Rich
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• Both accurate and precise gf-values for FeII lines
• Atomic data for the Gaia-ESO Survey
• The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies
• The evolution of CNO isotopes: a new window on cosmic star-formation history and the stellar IMF in the age of ALMA
• Ultra faint dwarfs: probing early cosmic star formation
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• The Gaia-ESO Public Spectroscopic Survey: Motivation, implementation, GIRAFFE data processing, analysis, and final data products
• High-Resolution Spectroscopic Study of Extremely Metal-Poor Star Candidates from the SkyMapper Survey
• A spectroscopic confirmation of the Bootes II dwarf spheroidal
• Timing the Early Assembly of the Milky Way with the H3 Survey
• A new formulation of the Type Ia SN rate and its consequences on galactic chemical evolution
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• The GALAH Survey: Non-LTE departure coefficients for large spectroscopic surveys
• A Sequoia in the Garden: FSR 1758 - Dwarf Galaxy or Giant Globular Cluster?
• APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods
• Chemical Abundances of Main-Sequence, Turnoff, Subgiant and red giant Stars from APOGEE spectra II: Atomic Diffusion in M67 Stars
• Experimental and theoretical oscillator strengths of Mg I for accurate abundance analysis
• Chemical evolution of the Milky Way: constraints on the formation of the thick and thin discs
• The Gaia-ESO Survey: a new approach to chemically characterising young open clusters
• Back to the Lithium Plateau with J0023+0307 with [Fe/H]<-6
• Non-LTE Balmer line formation in late-type spectra: Effects of atomic processes involving hydrogen atoms
• Ultra-faint dwarf galaxies: unveiling the minimum mass of the first stars
• Carbon and oxygen in metal-poor halo stars
• Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars
• The Gaia-ESO Survey: A new approach to chemically characterising young open clusters II. Abundances of the neutron-capture elements Cu, Sr, Y, Zr, Ba, La, and Ce
• On the variation of carbon abundance in galaxies and its implications
• Discovery of a thin lithium plateau among metal-poor red giant branch stars
• Chronology of the chemical enrichment of the old Galactic stellar populations
• Understanding the origin of CEMP-no stars through ultra-faint dwarfs
• Espresso observations of HE 0107$-$5240 and other CEMP-no stars with $[Fe/H]\le -4.5$
• Titans metal-poor reference stars II. Red giants and CEMP stars