VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy

Context. Fornax is one of the most massive dwarf spheroidal galaxies inthe Local Group. The Fornax field star population is dominated byintermediate age stars but star formation was going on over almost itsentire history. It has been proposed that Fornax experienced a minormerger event. Aims: Despite recent progress, only the highmetallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has beensampled in larger number via high resolution spectroscopy. We want tobetter understand the full chemical evolution of this galaxy by bettersampling the whole metallicity range, including more metal poor stars. Methods: We use the VLT-FLAMES multi-fibre spectrograph inhigh-resolution mode to determine the abundances of several α,iron-peak and neutron-capture elements in a sample of 47 individual redgiant branch stars in the Fornax dwarf spheroidal galaxy. We combinethese abundances with accurate age estimates derived from the ageprobability distribution from the colour-magnitude diagram of Fornax. Results: Similar to other dwarf spheroidal galaxies, the old,metal-poor stars of Fornax are typically α-rich while the youngmetal-rich stars are α-poor. In the classical scenario of the timedelay between Type II (SNe II) and Type Ia Supernovae (SNe Ia), weconfirm that SNe Ia started to contribute to the chemical enrichment at[Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia tookplace between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflectthe influence of SNe Ia and AGB stars in the abundance pattern of theyounger stellar population of Fornax. Conclusions: Our findingsof low [α/Fe] and enhanced [Eu/Mg] are compatible with an initialmass function that lacks the most massive stars and with star formationthat kept going on throughout the whole history of Fornax. We find thatmassive stars kept enriching the interstellar medium inα-elements, although they were not the main contributor to theiron enrichment.Based on FLAMES observations collected at the European SouthernObservatory, proposal number 080.B-0784.Table 7 is only available at theCDS via anonymous ftp to http://cdsarc.u-strasbg.fr(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A88Appendicesare available in electronic form at http://www.aanda.or