[Paper Review] Neutron-capture elements in dwarf galaxies III: A homogenized analysis of 13 dwarf spheroidal and ultra-faint galaxies

Moritz Reichert, C. J. Hansen+4 more|arXiv (Cornell University)|Apr 2, 2020

Stellar, planetary, and galactic studiesPhysics and Astronomy263 references68 citations

TL;DR

This study presents a homogeneous analysis of stellar parameters and abundances for 380 stars across 13 dwarf spheroidal and ultra-faint dwarf galaxies, using high-resolution spectroscopy from Keck and VLT. It reveals a universal mass-metallicity relation for α-elements and finds no dependence of r-process element production on galaxy mass between 2.1×10⁷M⊙ and 2.9×10⁵M⊙, suggesting rare core-collapse supernovae as the dominant r-process site in these systems.

ABSTRACT

We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving $13$ classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total this study includes $380$ stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies. We investigate the mass dependence of the individual systems on the production of $\alpha$-elements, but also try to shed light on the long-standing puzzle of the dominant production site of r-process elements. We use data from the Keck observatory archive and the ESO reduced archive to reanalyze stars from these $13$ dSph galaxies. We automatize the step of obtaining stellar parameters, but run a full spectrum synthesis to derive all abundances except for iron. The homogenized set of abundances yielded the unique possibility to derive a relation between the onset of type Ia supernovae and the stellar mass of the galaxy. Furthermore, we derived a formula to estimate the evolution of $\alpha$-elements. Placing all abundances consistently on the same scale is crucial to answer questions about the chemical history of galaxies. By homogeneously analysing Ba and Eu in the 13 systems, we have traced the onset of the s-process and found it to increase with metallicity as a function of the galaxy's stellar mass. Moreover, the r-process material correlates with the $\alpha$-elements indicating some co-production of these, which in turn would point towards rare core-collapse supernovae rather than binary neutron star mergers as host for the r-process at low [Fe/H] in the investigated dSph systems.

Motivation & Objective

• To create a consistent, homogeneous catalog of stellar parameters and elemental abundances across 13 dwarf spheroidal and ultra-faint dwarf galaxies for comparative chemical evolution studies.
• To investigate the dependence of α-element and neutron-capture element production on galaxy stellar mass across a wide mass range.
• To resolve the long-standing puzzle of the astrophysical site of the r-process by analyzing Ba and Eu abundances in low-metallicity systems.
• To determine whether the production of heavy r-process elements correlates with α-elements, which could distinguish between binary neutron star mergers and core-collapse supernovae as r-process hosts.
• To establish a reliable relation between the onset of type Ia supernovae and galaxy stellar mass using a consistent abundance analysis framework.

Proposed method

• Re-analyzed high-resolution spectroscopic data from the Keck Observatory and ESO archives for 13 dSph and UFD galaxies.
• Automated the derivation of stellar parameters (Teff, log g, [Fe/H], vsini) using spectral synthesis in 1D and local thermodynamic equilibrium (LTE).
• Performed full-spectrum synthesis for all elements except iron, applying non-LTE corrections where possible to improve accuracy.
• Used a consistent atomic line list and abundance synthesis method across all 13 galaxies to ensure homogeneity in the final catalog.
• Calculated surface gravities from parallaxes using Bayesian distance estimation to test membership and consistency with galactic distances.
• Quantified the probability of stellar membership in each dSph by comparing derived parallaxes with expected galactic distances, accounting for measurement uncertainties.

Experimental results

Research questions

• RQ1 Is there a universal relation between galaxy stellar mass and the evolution of α-elements in dwarf spheroidal galaxies?
• RQ2 Does the production of r-process elements (traced by Eu) depend on the stellar mass of the host galaxy in low-metallicity systems?
• RQ3 What is the connection between α-element production and r-process element enrichment in dSphs, and what does this imply for the r-process site?
• RQ4 Can the onset of type Ia supernovae be reliably linked to galaxy mass using a homogeneous abundance analysis?
• RQ5 How do the abundances of Ba (s-process tracer) and Eu (r-process tracer) vary with metallicity and galaxy mass, and what does this reveal about early chemical enrichment?

Key findings

• A universal relation between galaxy stellar mass and α-element evolution was derived across 13 dSph and UFD galaxies, spanning a wide range in mass and metallicity.
• No dependence of r-process element production on galaxy stellar mass was found between 2.1×10⁷M⊙ and 2.9×10⁵M⊙, indicating that r-process yields are independent of host mass in this range.
• The correlation between r-process elements (Eu) and α-elements suggests coproduction, pointing toward rare core-collapse supernovae rather than binary neutron star mergers as the dominant r-process site in these low-metallicity systems.
• The onset of type Ia supernovae was successfully linked to galaxy stellar mass through the homogenized abundance catalog, enabling a quantitative mass-dependent analysis.
• The study revealed a significant discrepancy in Ba/Fe ratios compared to previous studies, primarily due to improved line weighting and correction for Fe I blending at λ6141.73 Å.
• Parallax-based surface gravity estimates were found to be unreliable for distant stars due to large uncertainties, with Bayesian distance estimation showing that 1/π often overestimates distance and leads to inconsistent log g values.