Cosmological Implications of Dwarf Spheroidal Chemical Evolution

Abstract

The chemical properties of dwarf spheroidals in the local group are shown to be inconsistent with star formation being truncated after the reionization epoch (z ∼ 8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like Sculptor indicate strong s-process production from low-mass stars whose lifetimes are comparable with the duration of the pre-reionization epoch. The chemical evolution of Sculptor is followed using a model with SNe II and SNe Ia feedback and mass-and metallicitydependent nucleosynthetic yields for elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the interstellar medium with s-elements. This robust result challenges the suggestion that most of the local group dwarf spheroidals are fossils of reionization and supports the case for large initial dark matter halos. 1 here we have assumed λCDM cosmology with present-day ωm = 0.34, ω λ = 0.66, Hubble parameter = 70 km s −1 Mpc −1 , and a 13 Gyr age for the universe

About the Chemical Evolution of dSphs (and the peculiar Globular Cluster ω Cen)

“…We thus conclude that, at least in our models, only a prolonged star formation history (> 2.0 Gyr) can account for the chemical differences between the Galactic halo and the dSphs as shown by Shetrone et al (2001). Such a long SFH for dSphs is also found in the chemical models by Fenner et al (2006) who are unable to reproduce the Ba/Y ratio unless stars formed over an interval long enough for the low-mass stars to pollute the ISM with s-elements.…”

Two distinct halo populations in the solar neighborhood

“…3 support the idea that the low-α stars have been formed in systems with a slower chemical evolution than the regions from which the high-α and thick-disk stars originated. Detailed models for the chemical evolution of dSph galaxies including mass-loss in SNe winds (Fenner et al 2006) confirm that an increase in Ba/Y for the metallicity range −2 < [Fe/H] < −1 is obtained, when the star formation history extends over more than one Gyr.…”

Chemical evolution models for the dwarf spheroidal galaxies Leo 1 and Leo 2

“…Local dwarf spheroidal (dSph) galaxies have been the subject of a series of studies both from the observational and theoretical point of view in the past few years Tolstoy et al 2003;Venn et al 2004;Lanfranchi & Matteucci 2003Bonifacio et al 2004;Monaco et al 2005;Geisler et al 2005;Fenner et al 2006;Helmi et al 2006;Marcolini et al 2006;Koch et al 2006;Sbordone et al 2007;Shetrone et al 2009;Revaz et al 2009). From all these works emerged a scenario in which these galaxies are characterized by low metallicities, a sharp decrease of [α/Fe] ratios at high metallicities, and by a stellar metallicity distribution (SMD) with a peak at low [Fe/H], a low number of metal-poor stars, and a sharp decline at the high-metallicity tail.…”

“…In particular, Carigi et al (2002), Ikuta & Arimoto (2002) and Fenner et al (2006) also adopted chemical evolution models in their analysis of local dSph galaxies. Neither one of them, however, compared their results with the stellar metallicity distributions observed, which LM consider one of the strongest constraint in the chemical evolution studies, specially when one is analyzing the effects of the galactic winds on the evolution of these galaxies.…”

“…Besides that, Carigi et al (2002) and Ikuta & Arimoto (2002) compared their results to a much more limited sample of stars and did not consider the removal of metals by galactic winds. Fenner et al (2006) on the other hand adopted a more detailed chemical evolution model compared to a more complete data sample. They argue that the evolution of several abundance ratios could be reproduced by models with moderate galactic winds (less efficient than LM04) which are not able to remove the remaining gas content of the galaxy.…”