Chemical history of isolated dwarf galaxies of the Local Group: I. dSphs: Cetus and Tucana

TL;DR

This study develops chemical evolution models for the isolated dwarf spheroidal galaxies Cetus and Tucana, revealing complex chemical histories with phases of closed-box evolution and gas accretion or metal loss, based on star formation and metallicity data.

Contribution

First chemical evolution models for Cetus and Tucana derived from independent star formation and metallicity histories, highlighting their complex, multi-epoch chemical evolution.

Findings

Galaxies behaved as closed boxes during 75% of their history.

Both galaxies experienced significant gas loss around 4.5 Gyr ago.

Cetus is twice as massive as Tucana, yet their chemical histories are similar.

Abstract

For the first time, we obtain chemical evolution models (CEMs) for Tucana and Cetus, two isolated dwarf spheroidal galaxies (dSphs) of the Local Group. The CEMs have been built from the star formation histories (SFHs) and the metallicity histories, both obtained independently by the LCID project from deep color-magnitude dia- grams. Based on our models, we find that the chemical histories were complex and can be divided into different epochs and scenarios. In particular, during 75 percent of the SFH, the galaxies behaved as closed boxes and, during the remaining 25 percent, either received a lot of primordial gas by accretion or they lost metals through metal-rich winds. In order to discriminate between these two scenarios, abundances ratios in old stars are needed. At t approximately 4.5 Gyr, the galaxies lost most of their gas due to a short-strong, well-mixed wind. We obtain very similar CEMs for both galaxies, although Cetus is twice as massive as Tucana. We conclude that the star formation in both galaxies began with only 1.5 percent of the baryonic mass fraction predicted by lambda CDM.