A sample of 229 nearby thick disk stars has been used to investigate the existence of an age-metallicity relation ( AMR ) in the Galactic thick disk .
The results indicate that that there is indeed an age-metallicity relation present in the thick disk .
By dividing the stellar sample into sub-groups , separated by 0.1 dex in metallicity , we show that the median age decreases by about 5–7 Gyr when going from [ Fe/H ] \approx - 0.8 to [ Fe/H ] \approx - 0.1 .
Combining our results with our newly published \alpha -element trends for a local sample of thick disk stars , that show signatures from supernovae type Ia ( SN Ia ) , we can here draw the conclusion that the time-scale for the peak of the SN Ia rate is of the order 3–4 Gyr in the thick disk .
The tentative evidence for a thick disk AMR that we present here also has implications for the thick disk formation scenario ; star-formation must have been an ongoing process for several billion years .
This is further discussed here and appear to strengthen the hypothesis that the thick disk originates from a merger event with a companion galaxy that puffed up a pre-existing thin disk .