We present \mathrm { \alpha } -element abundances of Mg , Si , and Ti for a large sample of field stars in two outer fields of the Fornax dwarf spheroidal galaxy ( dSph ) , obtained with VLT/GIRAFFE ( R \sim 16 , 000 ) .
Due to the large fraction of metal-poor stars in our sample , we are able to follow the \mathrm { \alpha } -element evolution from \mathrm { [ Fe / H ] } \approx - 2.5 continuously to \mathrm { [ Fe / H ] } \approx - 0.7 .
For the first time we are able to resolve the turnover from the Type II supernovae ( SNe ) dominated , \mathrm { \alpha } -enhanced plateau down to subsolar \mathrm { [ \alpha / Fe ] } values due to the onset of SNe Ia , and thus to trace the chemical enrichment efficiency of the galaxy .
Our data support the general concept of an \mathrm { \alpha } -enhanced plateau at early epochs , followed by a well-defined “ knee ” , caused by the onset of SNe Ia , and finally a second plateau with sub-solar \mathrm { [ \alpha / Fe ] } values .
We find the position of this knee to be at \mathrm { [ Fe / H ] } \approx - 1.9 and therefore significantly more metal-poor than expected from comparison with other dSphs and standard evolutionary models .
Surprisingly , this value is rather comparable to the knee in Sculptor , a dSph \sim 10 times less luminous than Fornax .
Using chemical evolution models , we find that both the position of the knee as well as the subsequent plateau at sub-solar level can hardly be explained unless the galaxy experienced several discrete star formation events with a drastic variation in star formation efficiency , while a uniform star formation can be ruled out .
One possible evolutionary scenario is that Fornax experienced one or several major accretion events from gas-rich systems in the past , so that its current stellar mass is not indicative of the chemical evolution environment at ancient times .
If Fornax is the product of several smaller buildings blocks , this may also have implications of the understanding on the formation process of dSphs in general .