Sofia Feltzing

A sample of 229 nearby thick disk stars has been used to investigate theexistence of an age-metallicity relation (AMR) in the Galactic thickdisk. The results indicate that that there is indeed an age-metallicityrelation present in the thick disk. By dividing the stellar sample intosub-groups, separated by 0.1 dex in metallicity, we show that the medianage decreases by about 5-7 Gyr when going from [Fe/H] ≈ -0.8 to[Fe/H] ≈ -0.1. Combining our results with our newly publishedα-element trends for a local sample of thick disk stars that showsignatures from supernovae type Ia (SN Ia), we draw the conclusion thatthe time-scale for the peak of the SN Ia rate is of the order of 3-4 Gyrin the thick disk. The tentative evidence for a thick disk AMR that wepresent here also has implications for the thick disk formationscenario; star-formation must have been an ongoing process for severalbillion years. This appears to strengthen the hypothesis that the thickdisk originated from a merger event with a companion galaxy that puffedup a pre-existing thin disk.Based on data obtained with the Hipparcos satellite.