Abundance ratios of red giants in low mass ultra faint dwarf spheroidal galaxies

TL;DR

This study analyzes the chemical abundance ratios in 11 stars across five ultra faint dwarf spheroidal galaxies using VLT spectra, revealing unique abundance patterns and insights into their star formation histories and chemical evolution.

Contribution

First detailed chemical abundance analysis of stars in ultra faint dwarf spheroidal galaxies, highlighting differences from classical dwarf galaxies and identifying exceptionally high strontium levels.

Findings

[alpha/Fe] decreases with increasing [Fe/H], reaching solar levels at lower metallicity than in larger dwarf galaxies.

First measurement of strontium abundance in CVnI galaxy, showing an extremely high [Sr/Fe] and [Sr/Ba] ratio.

Fossil galaxies show lower [X/Fe] ratios at a given metallicity compared to galaxies with star formation discontinuities.

Abstract

Low mass dwarf spheroidal galaxies are key objects for our understanding of the chemical evolution of the pristine Universe and the Local Group of galaxies. Abundance ratios in stars of these objects can be used to better understand their star formation and chemical evolution. We report on the analysis of a sample of 11 stars belonging to 5 different ultra faint dwarf spheroidal galaxies (UfDSph) based on X-Shooter spectra obtained at the VLT. Medium resolution spectra have been used to determine the detailed chemical composition of their atmosphere. We performed a standard 1D LTE analysis to compute the abundances. Considering all the stars as representative of the same population of low mass galaxies, we found that the [alpha/Fe] ratios vs [Fe/H] decreases as the metallicity of the star increases in a way similar to what is found for the population of stars belonging to dwarf spheroidal galaxies. The main difference is that the solar [alpha/Fe] is reached at a much lower metallicity for the UfDSph than the dwarf spheroidal galaxies. We report for the first time the abundance of strontium in CVnI. The star we analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of barium which makes it a star with an exceptionally high [Sr/Ba] ratio. Our results seem to indicate that the galaxies which have produced the bulk of their stars before the reionization (fossil galaxies) have lower [X/Fe] ratios at a given metallicity than the galaxies that have experienced a discontinuity in their star formation rate (quenching).