The Pristine survey – I. Mining the Galaxy for the most metal-poor stars

Abstract

We present the Pristine survey, a new narrow-band photometric survey focused\non the metallicity-sensitive Ca H & K lines and conducted in the northern\nhemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii\nTelescope (CFHT). This paper reviews our overall survey strategy and discusses\nthe data processing and metallicity calibration. Additionally we review the\napplication of these data to the main aims of the survey, which are to gather a\nlarge sample of the most metal-poor stars in the Galaxy, to further\ncharacterise the faintest Milky Way satellites, and to map the (metal-poor)\nsubstructure in the Galactic halo. The current Pristine footprint comprises\nover 1,000 deg2 in the Galactic halo ranging from b~30 to 78 and covers many\nknown stellar substructures. We demonstrate that, for SDSS stellar objects, we\ncan calibrate the photometry at the 0.02-magnitude level. The comparison with\nexisting spectroscopic metallicities from SDSS/SEGUE and LAMOST shows that,\nwhen combined with SDSS broad-band g and i photometry, we can use the CaHK\nphotometry to infer photometric metallicities with an accuracy of ~0.2 dex from\n[Fe/H]=-0.5 down to the extremely metal-poor regime ([Fe/H]<-3.0). After the\nremoval of various contaminants, we can efficiently select metal-poor stars and\nbuild a very complete sample with high purity. The success rate of uncovering\n[Fe/H]SEGUE<-3.0 stars among [Fe/H]Pristine<-3.0 selected stars is 24% and 85%\nof the remaining candidates are still very metal poor ([Fe/H]<-2.0). We further\ndemonstrate that Pristine is well suited to identify the very rare and pristine\nGalactic stars with [Fe/H]<-4.0, which can teach us valuable lessons about the\nearly Universe.\n