The Earliest Phases of Galaxy Evolution

Open Access
Publisher Website
Google Scholar
Abstract
In this paper we study the very early phases of the evolution of our Galaxy by means of a chemical evolution model that reproduces most of the observational constraints in the solar vicinity and in the disk. We have restricted our analysis to the solar neighborhood, and we present the predicted abundances of several elements (C, N, O, Mg, Si, S, Ca, and Fe) over a more extended range of metallicities, [Fe/H]=-4.0 to [Fe/H]=0.0, than previous models. We adopt the most recent yield calculations for massive stars taken from two different works, and compare the results with a very large sample of data, one of the largest ever used for this purpose. We have obtained this data set by selecting the most recent and higher quality abundance data from a number of sources and renormalizing them to the same solar abundances. These data have been analyzed with a new and powerful statistical method that allows us to quantify the observational spread in measured elemental abundances and to obtain a more meaningful comparison with the predictions from our chemical evolution model. Our analysis shows that the "plateau" observed for the [α/Fe] ratios at low metallicities (-3.0<[Fe/H]<-1.0) is not perfectly constant, but shows a slope, especially for oxygen. This slope is very well reproduced by our model with both sets of yields. This is not surprising, since realistic chemical evolution models, taking stellar lifetimes into account in detail, never predicted a completely flat plateau. This is due either to the fact that massive stars of different mass produce a slightly different O/Fe ratio or to the often forgotten fact that supernovae of type Ia, originating from white dwarfs, already start appearing at a Galactic age of 30 Myr and reach their maximum at 1 Gyr. For lower metallicities (-4.0<[Fe/H]<-3.0), the two sets of adopted yields differ, especially for iron. In this range, the "plateau" is almost constant, since at such low metallicities there is almost no contribution from type Ia supernovae. However, there are not enough data in this domain to significantly test this point. Finally, we show the evolution with redshift of the [O/Fe] ratio for different cosmologies and conclude that a sharp rise of this ratio should be observed at high redshift, irrespective of the adopted yields. The same behavior is expected for the [O/Zn] ratio, which should be easier to compare with the abundances observed in high-redshift damped Lyα systems, since these elements are not likely to be affected by dust. Future measurements of either [α/Fe] or [α/Zn] ratios in very metal poor stars will be useful to infer the nature and the age of high-redshift objects.
All Related Versions

This publication has 49 references indexed in Scilit: