Context : Individual stars in dwarf spheroidal galaxies around the Milky Way Galaxy have been studied both photometrically and spectroscopically .
Extremely metal-poor stars among them are very valuable because they should record the early enrichment in the Local Group .
However , our understanding of these stars is very limited because detailed chemical abundance measurements are needed from high resolution spectroscopy .

Aims : To constrain the formation and chemical evolution of dwarf galaxies , metallicity and chemical composition of extremely metal-poor stars are investigated .

Methods : Chemical abundances of six extremely metal-poor ( [ Fe/H ] < -2.5 ) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy ( R = 40 , 000 ) with the Subaru Telescope High Dispersion Spectrograph .

Results : ( 1 ) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra .
The lack of stars with [ Fe/H ] \lesssim - 3 in Sextans , found by previous estimates from the Ca triplet , is confirmed by our measurements , although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity .
( 2 ) While one object shows an overabundance of Mg ( similar to Galactic halo stars ) , the Mg/Fe ratios of the remaining five stars are similar to the solar value .
This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy .
No evidence for over-abundances of Ca and Ti are found in these five stars , though the measurements for these elements are less certain .
Possible mechanisms to produce low Mg/Fe ratios , with respect to that of Galactic halo stars , are discussed .
( 3 ) Ba is under-abundant in four objects , while the remaining two stars exhibit large and moderate excesses of this element .
The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo , indicating that the enrichment of heavy elements , probably by the r-process , started at metallicities [ Fe/H ] \leq - 2.5 , as found in the Galactic halo .

Conclusions :