Age-metallicity relation

Finally Age-metallicity relation in the thick disk - this is a very tentative statement and, to our knowledge, there is only one study that claims the possibility of such an age-metallicity relation ([5]). This would suggest an extended period of star formation in the thick disk... 

Conclusions probably not a viable scenario (if an age-metallicity relation in thick disk is established this formation scenario is in serious trouble)... 

Based on currently available elemental abundance data and age determinations, the thick disk could have formed either through a violent, heating merger or through accretion of (substantial) satellites in a hierarchical galaxy formation scenario. The fast monolithic-like collapse is getting more and more problematic as data are gathered. It would be especially crucial to establish if there is an age-metallicity relation in the thick disk or not as in that case the thick disk could not have formed in that way (since the models indicate that the formation time-scale for the stars in the thick disk would be very short, see [7]). 

It is therefore among the disk stars of the solar neighbourhood that one looks for quantitative evidence for an age-metallicity relation some relevant data are shown in Figs. 8.16, 8.17 and 8.41. 

Fig. 8.17. Age-metallicity relation for disk stars using data from Edvardsson et al. (1993). Open circles, filled circles and crosses represent respectively stars with mean Galactocentric distances 7 to 9 kpc (like the Sun), stars from the inner Galaxy (under 7 kpc) and from the outer Galaxy (over 9 kpc). Model curves assume linear star-formation laws with

This, however, can be generalized (except for cases where actual time is significant, as in the age-metallicity relation or cosmochronology) by letting co vary arbitrarily with time and defining a time-like variable u by Eq. (8.17). Clayton (1985ab, 1987, 1988) has developed a very convenient series of models in which the inflow rate is... 

Fig. 8.41. Thin curves age-metallicity relation predicted by the two-inflow model of Chiappini, Matteucci and Gratton (1997). Thick curves sketch of suggested separate AMRs for the thick and thin disks. Data points are from Edvardsson et al. (1993). Adapted from Chiappini et al. (1997) by Pagel (2004).

Fig. 11.3 for the LMC. Figure 11.4 shows the resulting age-metallicity relations unlike the solar neighbourhood, there seems to be a reasonably well-defined relationship, despite the lack of clusters (from which ages were mainly determined at the time) at intermediate ages. The SMC, which does not suffer such a gap, shows... 

Fig. 11.4. Age-metallicity relation for the LMC, according to the SFR models shown in Fig. 11.3. After Pagel and TautvaiSiene (1998).

Everything suggests that the Fe/H ratio can be taken as a kind of chronometer, or at least as an index of evolution. It defines the chemical history of the Galaxy, and cannot decrease. The accumulation of iron in the interstellar medium is such that the abundance of this element increases monotonically, although in a way that is far from linear. The Fe/H ratio can be calibrated as a function of time by jointly determining the iron content and age of a great many stars selected from distinct generations. This then constitutes the basis of the age-metallicity relation. 

Fig. 8.2. Iron content as a function of the age of stars in the neighbourhood of the Solar System (age-metallicity relation). Age determinations are a dehcate matter and somewhat uncertain. This explains the wide error bars and scatter of the data. Type la supernovas must be included to reproduce the observed iron evolution.

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