A possible age-metallicity relation in the Galactic thick disk? ^*

Lund Observatory, Box 43, 221 00 Lund, Sweden e-mail: [sofia;ingemar]@astro.lu.se

Corresponding author: T. Bensby, thomas@astro.lu.se

Received: 30 December 2003
Accepted: 21 March 2004

Abstract

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]  to [Fe/H] . Combining our results with our newly published α-element trends for a local sample of thick disk stars that show signatures from supernovae type Ia (SN Ia), we draw the conclusion that the time-scale for the peak of the SN Ia rate is of the order of 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 appears to strengthen the hypothesis that the thick disk originated from a merger event with a companion galaxy that puffed up a pre-existing thin disk.