We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky .
In particular , we outline our photometric selection procedures and describe the low-resolution ( R \approx 3000 ) spectroscopic follow-up observations that are used to provide estimates of effective temperature , surface gravity and metallicity ( [ Fe/H ] ) for the candidates .
The selection process is very efficient : of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to –2.0 , 41 % have [ Fe/H ] \leq –2.75 and only \sim 7 % have [ Fe/H ] > –2.0 dex .
The most metal-poor candidate in the sample has [ Fe/H ] < –4.75 and is notably carbon-rich .
Except at the lowest metallicities ( [ Fe/H ] < –4 ) , the stars observed spectroscopically are dominated by a ‘ carbon-normal ’ population with [ C/Fe ] _ { 1 D,LTE } \leq +1 dex .
Consideration of the A ( C ) _ { 1 D,LTE } versus [ Fe/H ] _ { 1 D,LTE } diagram suggests that the current selection process is strongly biased against stars with A ( C ) _ { 1 D,LTE } > 7.3 ( predominantly CEMP- s ) while any bias against stars with A ( C ) _ { 1 D,LTE } < 7.3 and [ C/Fe ] _ { LTE } > +1 ( predominantly CEMP-no ) is not readily quantifiable given the uncertainty in the SkyMapper v -band DR1.1 photometry .
We find that the metallicity distribution function of the observed sample has a power-law slope of \Delta ( Log N ) / \Delta [ Fe/H ] = 1.5 \pm 0.1 dex per dex for –4.0 \leq [ Fe/H ] \leq –2.75 , but appears to drop abruptly at [ Fe/H ] \approx –4.2 , in line with previous studies .